House rules

GURPS Vehicles fans should be aware that, despite being marketed as mere simplifications, the vehicle‐design systems presented in GURPS WWII, GURPS WWII: Motor Pool, GURPS WWII: Weird War II, GURPS Steampunk, and even GURPS Vehicles Lite actually contain important refinements to the original system. For example, Vehicles As Cargo (WWII p. 137), Long‐Term Travel Speeds (id. p. 148), Electric Motors (Motor Pool p. 13), and Ground Speed and Ground Acceleration: Extra Detail (Lite p. 38) can be backported to Vehicles with little to no difficulty. Weird War II p. 85 and WWII p. 127 provide two important optional rules (regarding the HT of flying wings (contradicting a similar rule in Vehicles Expansion 1) and regarding the powering of turret and open‐mount rotation, respectively) that don't even receive their own headings!

50 % of the volume of each crew station or passenger seat is space. (See Vehicles Expansion 1 pp. 6 and 21. Out of a cramped seat's 20 ft³, for example, the seat's material takes up 10 ft³, and intra-component space an additional 10 ft³. The typical passenger (with gear) uses 4 ft³ (200 lb ÷ 50 lb/ft³) of that space.)

This space is designed for sitting, not for storage—so, generally, only half of it (one-quarter of the seat's total volume) can be used as cargo space if the seat is not folding. (For example, an unoccupied cramped seat can hold 5 ft³ of cargo, and the occupant of a roomy seat (capable of holding 10 ft³ of cargo when unoccupied) can reduce his seat's comfort to that of a cramped seat if he accepts 5 ft³ of additional cargo (on his lap, in his footwell, etc.).) The GM may allow the entirety of this space to be used for cargo on a successful roll vs. Freight Handling (see Everyman Tasks in Pyramid vol. 3 no. 65)—but, on a failed roll, the portion of the cargo that exceeds the normal half-of-space limit comes loose mid-journey and may damage other components or inhabitants of the vehicle, become damaged itself, or (if it was stored in an exposed crew station or passenger seat) fall out of the vehicle entirely.

The vanilla rules say that the HP total of an arm, rotor, or wheel subassembly should be twice that of most other subassemblies (e. g., leg, wing, or track). I can't think of any rationale that could possibly justify this discrepancy (if anything, Unliving wheel and rotor subassemblies should have half as many Hit Points as almost-totally-Homogenous bodies, wings, and superstructures of the same mass—see Basic Set pp. 557–558), so I recommend reverting it. (Note, however, that the tires of a wheel subassembly designed above TL4 effectively double that subassembly's HP—see Vehicles p. 182.)

A pod-borne powered propulsion system or power plant does not require access space (Vehicles p. 15) only if no other component in the pod has volume. (Empty space or frame volume does not count as a component. The rotation space of a turret or the retraction space of a retractable motive subassembly does count as a component. Cargo space does count as a component, even when not containing cargo, due to the reinforcement that cargo spaces typically have.)

The blurriness of the line between short-occupancy and long-occupancy vehicles (Vehicles p. 75) is exemplified by commercial airliners, which routinely undergo 24-hour flights while providing quarters to crew but not to passengers. A long-occupancy vehicle is not required to include quarters, but the GM should inflict FP penalties for sleeping elsewhere than in a bed (Dungeon Fantasy: Wilderness Adventures p. 24).

Generally, a component requires double access space (Vehicles p. 15) only if it is expected to undergo maintenance while it is operating. This usually is true if the vehicle's maintenance interval (id. p. 146) is shorter than its usual mission duration, and always is true if the vehicle's power plants require at least one onboard mechanic (id. p. 75).

The interior of a superstructure is accessible from the interiors of subassemblies that are attached to it, but the interior of a pod is not (Vehicles p. 9). Therefore, in order to conduct maintenance on a pod, a mechanic must be able to access the exterior of the vehicle and open up the pod's armor (negating the vehicle's streamlining and potentially allowing attacks to ignore DR). For this reason, a designer should think carefully before adding a pod to a long-occupancy vehicle, as conducting maintenance on such a pod while the vehicle is in round-the-clock operation may be difficult or impossible.

Aerial propellers (Vehicles p. 33) can be located in wings. (This is meant to prevent the absurd situation of being forced to place a high‐TL, electric motor–powered propeller within a pod that has zero volume (if you aren't using the rules for separate electric motors on WWII: Motor Pool p. 13).)

Before approximately TL8, the complexities of transmitting mechanical energy over long distances (and of converting energy between mechanical and electrical forms) generally make placing a propeller (or a rotor in a set of MMRs) outside the subassembly that contains the power plant or energy bank that powers it (or placing an energy bank outside the subassembly that contains the power plant that powers it) very difficult. (See Steampunk pp. 71–72 for discussion of this issue at TL5 and TL(5+1).) The GM should decide whether or not such a design is feasible.

No more than one propeller shaft can be located in the same subassembly, unless that subassembly is a wing. Up to two propellers may be installed on each shaft—either one at the front and one at the rear (tandem), or both on either the front or the rear (contra‐rotating, like a set of coaxial rotors). (Splitting a shaft into two halves and assigning a separate power plant or energy bank to each half‐shaft does not enable the designer to put two contra‐rotating propellers on each half‐shaft.) Based on the considerations described in the previous paragraph, the GM should decide whether or not running tandem propellers from a single power plant on a large subassembly (i. e., transmitting large amounts of power along the entire length of the subassembly) before TL8 is reasonable.

Passenger seats (Vehicles p. 76) that are in the same subassembly can be temporarily combined to form a smaller number of more-comfortable seats, at no cost. (For example, the three cramped seats in the family car on Vehicles p. 140 can be reconfigured for use as two normal seats.)

Similarly, passenger seats that are in the same subassembly also can be temporarily split to form a larger number of less-comfortable seats. (For example, a roomy seat can be reconfigured for use as two cramped seats.) However, this doubles the FP cost of using those seats over what it would normally be (Vehicles p. 143—e. g., a base of 1 h/FP for a cramped seat in the example, rather than the usual 2 h/FP).

This rule also is applicable to crew stations. However, the number of crew stations cannot be increased by splitting, since no new control interfaces (e. g., steering wheels) are created. For example, if a roomy crew station is split in two, the result is a cramped crew station and a cramped passenger seat, not two cramped crew stations.

The GM should adjudicate whether or not safety equipment (Vehicles pp. 79 and 160, and Basic Set p. 432) works properly (or at all!) on passengers in reconfigured seats. The GM also may find it necessary to rule that certain combinations of seats are not possible, given the internal layout of the vehicle. (For example, the roomy crew station and the roomy seat in the above-referenced family car realistically are separated by the center console.)

If exposed and non-exposed seats are combined, treat the combined seat as exposed.

The Fourth Edition ST score of a vehicle designed in Third Edition can be determined by setting the vehicle's Basic Lift equal to the load above a base weight that reduces the vehicle's acceleration to 80 % of its base-weight acceleration (i. e., the load that constitutes Light encumbrance for the vehicle), and then calculating ST from Basic Lift: ST = √(5 lb⁻¹ × BL), rounded to the nearest integer.

Which weight should be used as the base weight is a matter of preference. The simplest option, and the one that matches the official rule for determination of the vehicle's Hit Points (Basic Set p. 558), is to use the vehicle's empty weight. However, it may be preferable to use the curb weight (empty weight plus the weight of consumables required for normal operation) instead, since it doesn't make sense to measure the strength of a vehicle that has no fuel and therefore cannot actually exert itself. A third option is to use the loaded weight: vehicles that can carry items within themselves normally use the Payload advantage (Template Toolkit 2 p. 7), and weight carried with that advantage doesn't count as encumbrance (Basic Set p. 74), so a vehicle's ST score really is relevant only for attacking and for carrying extra cargo on its exterior. I personally prefer to use curb weight as base weight for this house rule, and to house-rule the Payload advantage so that weight carried internally still counts as encumbrance.

For example: The family car on Vehicles p. 140 has a curb weight of 3152.9 lb + 15 gal × 6 lb/gal = 3242.9 lb and a curb-weight acceleration of 0.8 ((mi/h)/s)/(kW/t)^0.5 × √(95 kW ÷ 3242.9 lb × 2000 lb/t) = 6.124 (mi/h)/s. To determine the load past curb weight at which acceleration is reduced to 80 % of its original value, solve the equation 0.8√(95 ÷ (3242.9 + x) × 2000) = 0.8 × 6.124 for x. This yields a value of 810.1 lb for Basic Lift, so ST = √(5 × 810.1) = 64. This is a bit higher than the estimate of ST 59 given by the formula on Basic Set p. 558.

If any exposed facing of a subassembly has no armor (or has only open-frame armor), that subassembly cannot provide aerial or aquatic lift or thrust, as it cannot redirect air or water without a smooth surface. (See also the note on mandatory rotor armor located at Vehicles p. 23.)

Retractable wheels and retractable skids (Vehicles p. 7) no longer are separate subassembly types. Instead, any motive subassembly can be made retractable, starting at TL6.

The retraction space of a retractable motive subassembly is represented, not as a multiplier to body (or wing) volume, but as a component with volume equal to 1.5 times the volume of the retractable subassembly. (This may require recursion.) If the retractable subassembly retracts into multiple other subassemblies (e. g., into both the body and the wings), divide this volume evenly between the other subassemblies—or, alternatively, divide the volume in rough proportion to the weight that each part of the retractable subassembly supports. For example, a large airplane might have two wheels supporting the body and one supporting each wing, if the body is about as heavy as both wings combined when the airplane is fully loaded.

If the Different Structures for Subassemblies optional rule (Vehicles p. 19) is used, no subassembly can have a frame strength higher than the frame strength of any subassembly that supports it. For example, if a pod is attached to the body of a car, and the body (which obviously supports the pod) has medium strength, the pod cannot be heavy—it must be medium, light, extra-light, or super-light.

The support of this rule is not necessarily the same support described on Vehicles p. 9, which emanates invariably from the body. Rather, it is a common-sense, physics-based relationship that may vary based on the vehicle's mode of operation. For example, in a flying car consisting of two wings and a set of wheels attached to the body: When the car is on the ground, the set of wheels supports the body and the body supports the wings. When the car is in the air, however, the wings support the body and the body supports the set of wheels—both relationships are reversed. Therefore, the entire vehicle must have a uniform frame strength. However, if a pod were attached to the body, it would be supported by the body regardless of the vehicle's current mode of operation.

In a vehicle whose chain of support is ambiguous—for example, an airplane with one wheel supporting the body and one wheel supporting each wing, where whether the body supports the wings or vice-versa (while the plane is on the ground) is uncertain—either direction can be chosen.

(This rule is meant to disallow designs in which large amounts of weight are borne indirectly by an impossibly flimsy subassembly—e. g., an extra-heavy tank body sitting on a super-light set of small wheels or flying on super-light wings.)

In addition to the vehicle-wide HT score, calculate a separate Health score for each subassembly (except the body), using the formula on Vehicles p. 26 but substituting the subassembly's HP for the body's and the subassembly's loaded weight for the vehicle's. If any subassembly's HT is lower than the HT calculated for the entire vehicle, use the subassembly's HT score for the entire vehicle. (Alternatively: Whenever the vehicle succeeds on a roll vs. HT, the roll counts as a failure for any subassembly for which (vehicle HT minus subassembly HT) is less than the roll's margin of success. This may have catastrophic consequences for the vehicle!)

(This rule is meant to disallow designs in which large amounts of weight are borne directly by an impossibly flimsy subassembly—e. g., heavy fuel tanks in a super-light pod or super-light wings.)

It may be useful to calculate SM as a non-integer number equal to 2 × log₁₀(volume ÷ ft³) − 1.5, prior to rounding it to the nearest integer.

If you're using the Structural Weight and Cost optional rule (Vehicles Expansion 1 p. 5), don't simply choose the larger of the vehicle's structural surface area and the vehicle's structural volume. Instead, for each subassembly that normally is included in structural surface area, choose the larger of that subassembly's area and volume, and include that number as part of the effective surface area for purposes of the calculation of structural weight and cost.

Taking the transport aircraft on Vehicles p. 140 as an example:

Access space (Vehicles pp. 14–15 and 74–75, and Vehicles Expansion 1 p. 23) can be used as cargo space (Vehicles p. 15) or standing room (Vehicles p. 76) if the component with which the access space is associated is not in use.

In certain cases, access space can be used as cargo space or standing room even when the component is in use. For example, cargo or passengers can be packed like sardines into the aisle of a subway car or (if properly protected from heat) the engine compartment of a locomotive. However, such usage may make (1) traversing the access space (Basic Set p. 387), and (2) conducting maintenance (or stoking) on the associated component, difficult or impossible.

The number of wheels chosen on Vehicles p. 7 is not necessarily the same as the number of wheels used in determining gMR and gSR on id. p. 129. Up to two wheels may be placed on the same end of an axle if they are combined into a wheel pair, and several wheels or wheel pairs may be combined into a single wheel group. Each wheel pair or wheel group counts as only a single wheel for the purpose of determining Ground Maneuver Rating and Ground Stability Rating. For example, an eighteen-wheeler truck really has only six wheel groups for gMR and gSR: four two-pair groups and two individual wheels. Similarly, a large airplane may have dozens of individual wheels gathered into just three or four large groups.

Using many smaller wheels rather than just a few larger wheels is a good idea in a campaign (usually at or above TL6) where smaller wheels (and tires!) are mass-produced, but larger wheels can be purchased only at limited-production prices, or cannot be purchased at all and must be built from scratch (Vehicles p. 199). Additionally, many jurisdictions have per-axle weight limits on the wheeled vehicles that legally can travel on roads maintained by those jurisdictions, so a vehicle with many wheels can bypass those restrictions—though such a vehicle may still fall afoul of restrictions on maximum length, width, or height!

(Per-axle weight restrictions exist because spreading weight over several wheels or wheel pairs is significantly less damaging to a road than is concentrating that weight on just a few wheels or wheel pairs. Think of it as being based on per-wheel or per-wheel-pair (but not per-wheel-group) ground pressure (Vehicles p. 130).)

According to Vehicles p. 26, Size Modifer is equal to 2 × log₁₀(volume_ft³) − 1.5, rounded to the nearest integer. According to Basic Set p. 550, Size Modifier is equal to 6 × log₁₀(length_yd) − 1.5 + N, rounded to the nearest integer, where N is 2 for box-, sphere-, and blob-shaped objects, 1 for elongated boxes, like most ground vehicles, or 0 otherwise. If these expressions are set equal to each other, length_yd = ∛(volume_ft³) ÷ 10^{N ÷ 6} and volume_ft³ = (length_yd)³ × 10^{N ÷ 2}—or, in units that match, length = 3 × ∛(volume) ÷ 10^{N ÷ 6} and volume = (length ÷ 3)³ × 10^{N ÷ 2}.

If you desire more detail: Set N to 3 for a cube, 2 for a sphere or a 1∶1∶1.5 box, 1.5 for a 1∶1∶2 box, 1 for a 1∶1∶3 box, or 0 for a 1∶1∶5 box.

If you desire even more detail: For a W∶W∶L box, N is equal to 2 × log₁₀(27 × (W ÷ L)²) and W ÷ L = √(10^{N ÷ 2} ÷ 27).

Historically, tailless aircraft (Vehicles Expansion 2 p. 5) that achieved stability without computerized controls were commercially available and/or thoroughly prototyped in TL6 (Dunne and the Hortens). Accordingly, tailless aircraft can be designed at TL6 and without controlled instability.

This house rule makes tailless designs superior to conventional designs in many, if not most, situations. The GM may wish to consider imposing other restrictions on tailless designs, with the justification that, even though many components are listed as suitable for being placed in wings in theory, in practice the flat shape of a wing can cause a lot of interior volume to be wasted. For example, the GM might say that, even though a wing's volume is 40 ft³, its height is insufficient to accommodate a roomy passenger seat.

For a ground vehicle traveling up, down, or across a slope (Vehicles p. 153), or an air vehicle climbing or diving (id. p. 155), recalculate top speed and acceleration (and, for an air vehicle, Maneuver Rating and deceleration—or, for a ground vehicle, ground pressure), including an additional gravitational thrust equal to −(vehicle's weight) × sin(climb angle). For a ground vehicle, multiply ground motive power and ground pressure by cos(ground angle). For an air vehicle, recalculate stall speed, multiplying lift area (and static lift from ornithopter wings and non-vectored propellers, fans, and engines) by cos(climb angle) and including additional static lift equal to thrust × sin(climb angle). (A vehicle pulls out of a dive by taking a bend maneuver, not by going faster.)

For a ground vehicle, weight × cos(ground angle) − (static lift) must be at least 0.1 × weight. If the vehicle violates that condition, it automatically loses control. (The condition is violated automatically if cos(ground angle) is less than 0.1—i. e., if the ground is steeper than ±84 ° or ±9.9∶1.)

Note that ground angle is different from climb angle! A ground vehicle traveling sideways across a steep slope (neither climbing nor descending) still is affected by formulas referencing ground angle. Also, climb angle is a negative number for a descending vehicle.

(This rule replaces the rules cited in the first paragraph.)

If (1) one vehicle is towed by another vehicle (Vehicles pp. 95–96), (2) the towed vehicle takes up at least half the towing vehicle's towing capacity, and (3) both vehicles are using wheels, tracks, or halftracks, then the combination enjoys the benefits of articulation (Vehicles Expansion 2 p. 4).

It's readily apparent from the formulas given in the Performance chapter of Vehicles that top speed and acceleration vary based on fuel consumption, as laid out in the following table.

For example: The family car on Vehicles p. 140 has a top speed of 100 mi/h at full power output. If the driver chooses to save gasoline by traveling at half throttle, the car's speed falls to 100 mi/h × √(0.5) = 71 mi/h. Conversely, if the driver chooses to abide by a speed limit of 70 mi/h, he must reduce his throttle to (70 mi/h ÷ 100 mi/h)² = 49 % of maximum output.

(Make note that this calculation relies on multiplying the top speed or acceleration before the application of any cap due to streamlining, helicopter rotors, etc.)

If you happen to be designing a vehicle without a calculator (or a slide rule), you can use this table to determine approximate roots and powers.

Instead of typing X^Y into a calculator:

Similarly, instead of typing ^Y√X (which is the same as X^1/Y) into a calculator:

If a number or a row is outside the range of the table, multiplying a number by 10 is equivalent to adding 216 to its corresponding row, and dividing a number by 10 is equivalent to subtracting 216 from its corresponding row.

For example, if you want to calculate the cube root of 200: 200 ≈ 1.99952 × 10 × 10; R₁ = 65 + 216 + 216 = 497; R₂ = 497 ÷ 3 = 166; and ³√200 ≈ 5.86838. (A calculator returns the value 5.84804, so the error of this result is less than 0.35 %.)

Likewise, for 300^2/3: 300 ≈ 2.99814 × 10 × 10; R₁ = 103 + 216 + 216 = 535; R₂ = 535 × 2/3 ≈ 357 = 141 + 216; and 300^2/3 ≈ 4.49550 × 10 = 44.9550. (A calculator returns the value 44.8140, so the error of this result is less than 0.32 %.)

The result of any such calculation is guaranteed to have an error smaller than 1.1 %. (Specifically, (1.00536 minus an infinitesimal positive number)² is 1.00000 when calculated with this table, but a calculator returns 1.01075.)

Results are guaranteed to be within ±0.54 % of the true value.

(This rule replaces the Cube Roots appendix on Vehicles p. 138.)

A set of off-road wheels has a basic Speed Factor of 14, rather than 16.

(According to Vehicles, off-road wheels are superior to heavy wheels in every way, and there's no reason to use heavy wheels after TL5. However, Alternate Spaceships (in Pyramid vol. 3 no. 34) gives to the top speed of an off-road wheeled drivetrain a flat penalty of 5 yd/s (10 mi/h). This penalty ranges from 6.7 % to 18 % of the unmodified top speeds possible for wheeled vehicles in Alternate Spaceships, and that's equivalent to a Vehicles Speed Factor penalty ranging from −1 to −3 applied to the normal value of 16 for wheeled vehicles above TL5. 14 also is a nice intermediate number between the 12 of pre-TL6 no-pneumatic-tire wheels and the 16 of post-TL5 pneumatic-tire wheels.)

The vanilla rules multiply the top speed of a streamlined ground vehicle by a bonus factor only if the vehicle's top speed before the bonus is at least 50 mi/h. This creates a rather distasteful discontinuity as the pre‐multiplication top speed of a streamlined vehicle rises from 49.9 to 50.1 mi/h. The obvious solution is to apply the bonus factor regardless of pre‐bonus top speed.

However, this causes streamlining to grant a bonus even at low speeds, which may be undesirable. An alternative, somewhat more complicated solution is: top speed = 50 mi/h + max(pre‐bonus top speed − 50 mi/h, 0 mi/h) × bonus factor.

Both the vanilla rule and the house rules listed above share a different flaw: a basic top‐speed value is calculated, and then is increased to account for streamlining. This house rests on a foundation of sand, since in reality every vehicle starts at the top speed that it theoretically could achieve in vacuum, and then has that speed reduced by drag, or reduced by a smaller amount if the vehicle is streamlined. In order to follow this theory, add a factor of 1.1 to the normal ground‐speed equation, and then another factor of 1 for good streamlining, 0.955 (i. e., 1.05 ÷ 1.1) for fair streamlining, or 0.909 (i. e., 1 ÷ 1.1) for no streamlining. If you wish to use this paragraph with the previous paragraph: iteration 1 of top speed = the vanilla calculation × 1.1; and, if iteration 1 > 50 mi/h, iteration 2 = iteration 1 − (iteration 1 − 50 mi/h) × 0, 0.05, or 0.1. (Vehicles Lite p. 38 provides a third iteration that can be applied on top of iteration 2.)

Under the vanilla rules, ground top speed is calculated in three iterations. First, iteration 1 is determined based on the power‐to‐weight ratio and a drivetrain‐based factor (Vehicles p. 128). Then, if iteration 1 exceeds 50 mi/h, it is multiplied by a streamlining‐based factor to obtain iteration 2 (ibid.). Finally, if iteration 2 exceeds a streamlining‐based threshold, for iteration 3 it is manipulated so that, beyond that threshold, top speed increases with the fourth root, rather than with the square root, of power‐to‐weight ratio—i. e., more slowly (Vehicles Lite p. 38).

If you look at the graph of all these iterations of top speed as a function of power‐to‐weight ratio and of streamlining, there are two obvious problems with the vanilla method of calculating iteration 2, both taking place at the point where iteration 1 exceeds 50 mi/h (if the vehicle has any streamlining). In the first place, there's a discontinuity in iteration 2. (This is caused by multiplying iteration 1 in its entirety by a factor, rather than modifying only the part of iteration 1 that exceeds the threshold of 50 mi/h.) And, in the second place, the slope of iteration 2 increases (even if the first problem is corrected by the method suggested above), causing the graph to nonsensically wiggle back and forth rather than being consistently concave downward. (This is caused by starting with a smaller number for iteration 1 and increasing it by a factor to get iteration 2, rather than starting with a larger number for iteration 1 and decreasing it by a factor to get iteration 2.)

The way to fix these problems is twofold. First: To obtain iteration 1, multiply the result of the vanilla equation by an additional factor of 1.1. Second: If iteration 1 exceeds 50 mi/h, then iteration 2 = 50 mi/h + (iteration 1 − 50 mi/h) × S ÷ 1.1, where S is 1.1 for Good streamlining, 1.05 for Fair streamlining, 1.03 for a fairing (Vehicles Lite p. 38), or 1 for no streamlining. (The process for determining iteration 3 remains unchanged. Acceleration always is based on iteration 1.)

It may be useful to design retractable subassemblies (or removable subassemblies on a multi-section vehicle) with a lower level of streamlining than that of the rest of the vehicle. (For example, the retractable wheels of many airplanes are unarmored and therefore cannot be streamlined. Similarly, it's conceivable that an airplane whose typical mission involves reaching a target area at high speed, loitering at low speed over that area, and finally leaving the area at high speed might have unstreamlined pop turrets.)

If a vehicle contains subassemblies that have multiple levels of streamlining, in any given calculation always use whichever level of streamlining (among the levels of all non-retracted subassemblies) produces the worst result. For example, on an airplane that has good streamlining except for a set of unstreamlined wheels (retractable but currently extended), ground top speed and air top speed should be calculated with no streamlining, but stall speed should be calculated with good streamlining, because a higher level of streamlining produces a worse (higher) stall speed.

For a subassembly designed under this rule, determine the structural cost by consulting the Vehicle Structure Table (Vehicles p. 19) as usual, but using the subassembly's streamlining rather than the body's streamlining.

The cost multiplier labeled if Wings or Rotors in the Vehicle Structure Table (Vehicles p. 19) should be used for any vehicle that flies with the aid of aerodynamic (as opposed to aerostatic) lift. This includes not only wings and rotors but also the lifting-body feature. (See Vehicles p. 133.)

Base a vehicle's required number of mechanics on the power requirement of all simultaneously-active power-using propulsion and lift systems or on the power output of all simultaneously-active power-producing systems, whichever is larger.

Always base a vehicle's required number of stokers on the power output of all simultaneously-active wood- and coal-burning power plants. Never use power requirements for determining this number.

If a power plant in a short-occupancy vehicle requires stokers, it also requires access space as if it were in a long-occupancy vehicle, so that the stokers have room to work.

The relationship between frame strength and armor DR given on Vehicles p. 20 is rather unhelpful. An alternative guideline is brought to mind by Spaceships and by Armor by Facing (in Alternate Spaceships in Pyramid vol. 3 no. 34): the weight of armor on an ordinary, non-military vehicle generally should be between one-twentieth and one-seventh of the vehicle's loaded weight (i. e., between one and three Spaceships systems), if the vehicle is armored at all. The designer then can decide with what kind of armor he wants to fill that weight allotment.

(For reference: The ratio of armor weight to loaded weight is around one-third for the roundship on Vehicles p. 139, one-fifth for the transport aircraft and family car on id. p. 140, and one-seventh for the utility helicopter on id. p. 141; and, according to Basic Set p. 17, a ST 10, 150-lb human carrying one-seventh of his loaded weight in armor has Light encumbrance.)

A simpler alternative to the limit stated on Vehicles p. 16 is that the body's volume must constitute at least half the volume of the entire vehicle.

As a replacement for HT Scores and Winged Vehicles (Vehicles Expansion 1 p. 6): Whenever (1) the body's HP is referenced by the rules and (2) the summed HP of all wings exceeds the body's HP, the summed HP of all wings should be used instead of the body's HP. Similarly, if the wings' total volume exceeds the body's volume, the wings' total volume should be used where the body's volume is called for. (For example, the volume of a wheels subassembly should be based on the total wing volume, and the wheels should be attached solely to the wings rather than to the original body.)

If both of these situations apply, the pair of wings generally can be considered the vehicle's effective body, while the original body (if it exists at all) is merely a combination of superstructure and pod: its interior is accessible from the interior of the wings, it can accept high levels of streamlining, and it can accept any component that is listed as needing to be placed in a superstructure or a pod (but not a component that must be placed in the body). If only one applies, the choice of whether the pair of wings or the original body is the effective body is up to the designer, but one of them must be chosen for all purposes.

(Note that this is contradicted by Weird War II p. 85, which was published a year after Vehicles Expansion 1.)

In determining the ground top speed and ground acceleration (but not the ground deceleration, ground Maneuver Rating, or ground Stability Rating) of a ground vehicle that uses only propulsion systems that push on the air rather than on the ground (e. g., a propeller-driven airplane or a jet-powered car), the formulas given in Air Performance should be used, even if the vehicle cannot leave the ground.

(Taking the transport aircraft on Vehicles p. 140 as an example: Under the vanilla ruleset, ground top speed is √(5 370 ÷ 4 ÷ 14.9) × 16 × 1.05 = 160 mi/h, and ground acceleration is 160 ÷ 16 × 0.8 = 8 (mi/h)/s. Under this ruleset, however, ground top speed is √(7 500 × 5 370 ÷ 1 000) = 200 mi/h (the same as air top speed), and ground acceleration is 5 370 ÷ 29 734.5 × 20 = 3.6 (mi/h)/s (the same as air acceleration).)

If a vehicle has propulsion systems of both types (pushing on the air and pushing on the ground), its ground top speed and ground acceleration should be calculated with both sets of formulas, and the worse value should be chosen as the final value. For the purpose of the formulas, assume that 1 kW of ground motive power is equivalent to 4 lb of air motive thrust.

Additionally, for any ground vehicle, always calculate ground acceleration from the value determined for ground speed before that speed is capped or multiplied due to streamlining.

(It obviously makes no sense that the transport aircraft sample vehicle on Vehicles p. 140 has ground acceleration that is literally twice its air acceleration.)

Realistically, wood armor should be considered ablative.

Starting at TL1, armor can be fashioned out of stone. Stone armor weighs 1.2 (lb/ft²)/DR, and (with Armor Volume) has density of 200 lb/ft³. It is ablative and fireproof.

Stone armor costs 0.75 $/lb on any facing of a building except the top, or 12.5 $/lb on the top facing of a building or any facing of a vehicle. The extra cost represents physical reinforcement (such as wire-connected scales) and/or advanced architectural techniques (such as arches, domes, and vaults), which are necessary in applications that involve large tensile stresses (such as the roof of a building or the armor of a vehicle).

(See Low-Tech Armor Design (in Pyramid vol. 3 no. 52) and Low-Tech Companion 3 pp. 33–34. It should be noted, however, that that article's numbers for wood armor vary greatly from those given for wood armor in Vehicles, so the article may be working from different assumptions than Vehicles does, despite its explicit protestation to the contrary.)

(See also concrete armor on WWII: Motor Pool pp. 11–12.)

The designer should keep track of the volume of components or fractions of components even if they are on the surface of, rather than in the interior of, a subassembly. For example, if cargo with a volume of 190 ft³ is stored in a 100-ft³ open cargo space, the extra 90 ft³ of cargo doesn't just disappear—it should contribute to the vehicle's final volume and Size Modifier, even though it need not be accounted for in determining the weight and cost of the vehicle's frame. Likewise, a bicycle that's carrying a human rider may have significantly worse maneuverability than a bicycle that has the same subassembly volume but is piloted by a compact remote-control mechanism. See also: external mounts and vehicles stored in them; sails; and non‐concealed weapons.

For each subassembly, calculate, not only volume without external components, but also volume with armor (if you're using the Armor Volume rule from Vehicles Expansion 2 p. 5) and volume with all external components (including, not just armor, but also aerial propellers, the upper halves of humans in exposed seats, the entirety of humans in cycle seats, airplanes carried on decks, etc.). The weight and cost of a subassembly's frame are based on the volume without external components, but Size Modifier and Maneuver Rating are based on volume with all external components, and the volume (without external components) of wings, wheels, etc. is based on the body's volume with all external components.

Likewise, calculate, not only area without external components (6 × (volume without external components)^2/3 × factor, as usual; see Vehicles p. 18), but also area with armor (6 × (volume with armor)^2/3 × factor) and area with all external components (area with armor + Σ(area of non-armor external component)). The weight, cost, and Hit Points of a subassembly's frame are based on area without external components, but lift area is based on area with armor, and aerodynamic drag is based on area with all external components. (Note that, for the motive subassemblies listed under Wheelguards and Armored Skirts (Vehicles p. 23), part of the subassembly's underside must be unarmored in order for the subassembly to function, so ground contact area is based on the area of the subassembly without armor. For example, armor applied to a TL5+ wheels subassembly does not protect its pneumatic tires (Vehicles p. 182). However, a skids subassembly's ground contact area still is based on area with armor.) (In calculating aerodynamic drag (Vehicles p. 134), do not use the D variable, since that increase should already be included in area with external components.)

To determine the volume of a propeller, use the same density as for a ducted fan. The volume of a set of sails can be determined with Armor Volume (Vehicles Expansion 2 p. 5). The volume of a typical human with his equipment can be taken as 4 ft³ (see Vehicles pp. 25–26 and 80); assume that half of that volume extends outside the vehicle if the human is in an exposed seat, or all of that volume if the human is in a cycle seat, a harness, or standing room.

As an extension of the three varieties of vehicle weight prescribed on Vehicles pp. 25–26 (empty weight, loaded weight, and loaded weight with hardpoints), it can be useful to think of the vehicle as existing in even more states, and to calculate weights, volumes, costs, and even performance statistics for all those states separately.

Space includes cargo, access, empty, and intra-component space. Intra-component space is half the nominal volume of a seat (Vehicles Expansion 1 p. 21) or the nominal volume of a tank minus the volume of its frame (id. p. 24). Collapsible space includes: (1) all cargo space, access space, and intra-seat/-station space in a multi-section vehicle (regardless of whether the seat/station is folding); and (2) intra-component space in a folding seat/station or a collapsible tank (regardless of whether the vehicle is multi-section). If a vehicle neither is multi-section nor contains any folding seats/stations or collapsible tanks, it does not have any collapsible space, so its folded statistics are the same as its empty statistics.

Vehicles (pp. 87–88) gives no guidance on how quickly energy banks can be charged or discharged. In reality, however, these limits do exist for batteries of some types.

Very generally: An advanced battery or a lead–acid battery has a maximum power input equal to 1 kW for every 3.6 MJ of capacity: charging takes at least 3600 seconds (one hour). Likewise, an advanced battery or a lead–acid battery has a maximum sustained power output equal to 1 kW for every 3.6 MJ of capacity: it cannot be fully discharged in less than one hour. These guidelines are absolute minimums and will drastically shorten the life of most batteries if used for sustained intervals, so the designer is advised to use a larger ratio—say, 18 or even 36 MJ/kW (five or ten hours)—for a vehicle whose battery is expected to last for a long time.

Additionally, a lead–acid battery (but not an advanced battery) can tolerate power output of 1 kW for every 180 kJ of capacity during non-sustained intervals—e. g., for ten seconds, in order to power the starter motor of a diesel engine (Vehicles p. 83).

(See High-Tech: Electricity and Electronics pp. 16–18, and look up the C ratings of some real-life batteries. A rating of 1C is equivalent to a charge or discharge time of 1 hour, while a rating of 0.2C is equivalent to a charge or discharge time of five hours. This rule is meant primarily to prevent advanced batteries from being used inappropriately. For example: The Family Car sample vehicle on Vehicles p. 140 has top speed of 100 mi/h and acceleration of 5 (mi/h)/s. A clever designer might think of replacing part of the engine with a small advanced battery, in order to have two separate sets of ground-performance statistics—e. g., a gasoline-only maximum speed of only 85 mi/h, but enough additional battery capacity for a short burst of 5-(mi/h)/s acceleration every few minutes. In reality, however, a small advanced battery may not be able to deliver enough power for this application, leaving a large advanced battery, a flywheel, and a larger lead-acid battery as the only alternatives.)

Vehicles implicitly assumes (pp. 87–88) that all energy banks have infinitely-variable input and output power: e. g., a 1-MJ energy bank can provide 1 MW for 1 s, 1 kW for 1 ks, or 1 W for 1 Ms, and can be recharged at the same rates. In reality, however, only clockwork, flywheels, and power cells/slugs can be charged at high levels of power. Lead–acid batteries and advanced batteries, on the other hand, can be damaged by such treatment. Charging them at inappropriately high rates may even cause them to explode!

As a rough guideline, the GM should limit the input power of lead–acid batteries to 1 kW for every 18 MJ (5 kWh) of capacity, and the input power of advanced batteries to 1 kW for every 3.6 MJ (1 kWh) of capacity. Charging

The maximum output power of an advanced battery may be assumed to match its maximum input power. The maximum output power of power cells/slugs and flywheels may be taken as 1 kW per 360 kJ and 1 kW per 36 kJ, respectively. Lead-acid batteries and power cartridges have no maximum output power.

For example: The TL7 Family Car sample vehicle uses a 95-kW gasoline engine. In order to match this power output, a battery-powered version must use a battery with a capacity of at least 95 kW × 3.6 MJ/kW = 342 MJ. A vehicle that uses a battery smaller than that may be unable to achieve sufficient acceleration for safe merging onto high-speed roadways, unless it also includes an auxiliary flywheel for short bursts or pulses of boosted operation.

The GM may also want to consider limitations on maximum discharge rate. Excessive discharge can cause an advanced battery to be damaged by overheating, due to the resistance that its internal chemistry offers to the electric current. A very tentative maximum discharge rate is 3.6 MW/kJ for advanced batteries—i. e., full discharge in not less than one hour. (Other energy banks have no maximum discharge rate.)

I can't emphasize enough that these guidelines are extremely hazy, and a GM is well within his rights to say that a high-quality advanced battery can achieve discharge rates literally dozens of times higher than 100 % per hour. It does, however, bear mentioning that regularly charging a battery in less than five hours will severely

(See High-Tech: Electricity and Electronics pp. 16–18. This rule is meant primarily to prevent advanced batteries from being used inappropriately. For example: The Family Car sample vehicle on Vehicles p. 140 has top speed of 100 mi/h and acceleration of 5 (mi/h)/s. A clever designer might think of replacing part of the engine with a small advanced battery, in order to have two separate sets of ground-performance statistics—a gasoline-only maximum speed of 85 mi/h, but enough additional battery capacity for a short burst of 5-(mi/h)/s acceleration every few minutes. In reality, however, a small advanced battery may not be able to deliver enough power for this application, leaving a large advanced battery, a flywheel, and a larger lead-acid battery as the only alternatives.)

If using the Armor Volume optional rule (Vehicles Expansion 2 p. 5), do not place armor inside a subassembly. Instead, treat it as an external component.

If the volume of a non-body subassembly normally is a certain percentage of the body's volume, that volume now is a percentage of the body's volume with external components. For example, if a body has volume 1600 ft³, and the armor attached to the body has volume 5 ft³, then the minimum volume of a set of standard wheels attached to the body is 160.5 ft³ (not 160 ft³). Any armor attached to the set of wheels is in addition to (not included in) that 160 ft³.

(Sketchy flowchart: Body → body with external components → other subassemblies → other subassemblies with external components → vehicle with external components.)

Lift area, drag area, and wheel contact area are based on area with external components. External components other than armor should have their area included separately, not as part of the 6x^2/3 formula. For example, if a subassembly has volume 120 ft³, is covered in armor with volume 5 ft³, and has an external propeller with volume 8 ft³, the area of the pod with external components is 6 × (120 ft³ + 5 ft³)^2/3 + 6 × (8 ft³)^2/3 = 174 ft².

(Giving volume to armor creates problems regardless of whether it is treated as an internal component or an external component. If armor is an internal component, how is a wheels subassembly that contains armor supposed to be sized vs. the body, when a wheels subassembly normally is forbidden from containing internal components? If armor is an external subassembly, doesn't the designer need to recalculate drag and lift area based on the total volume of subassemblies plus the armor that forms the actual surface of those subassemblies? In my opinion, making it an external component is the less-bad option.)

If a vehicle contains multiple possible sources of motive power, the designer should make explicit note of the details of the vehicle's drivetrain. For example, in a vehicle that contains a gasoline engine, a lead–acid battery, and a wheeled drivetrain (such as the Family Car on Vehicles p. 140), the drivetrain may be:

These details should be reflected in the vehicle's statistics, and may be important in a campaign. (For example: If the PCs in a TL7 campaign pick up a TL10 power cell from Area 51, adding it to a vehicle that already is a series hybrid or a parallel hybrid can be done quite easily (a minor repair), but adding the same battery to a vehicle whose old lead-acid battery isn't already capable of driving the wheels will require replacing the existing mechanical drivetrain with a drivetrain that includes electric motors (a major repair at best).)

Additionally, the GM may wish to make a parallel-hybrid drivetrain heavier and more expensive than a non-hybrid or series-hybrid drivetrain capable of transmitting the same amount of power. (A gasoline engine's drivetrain has a complicated automatic or manual transmission, while a battery's drivetrain needs much less gearing but does require electric motors.) See WWII: Motor Pool p. 13 for details.

As a supplement to Auto-Rotation and Helicopters (Vehicles Expansion 2 p. 31): Autogyros always are in autorotation, even when the engine is running. If the engine of an autogyro fails, the vehicle starts gliding automatically, and there is no need to make a hazard control roll.

Seven new frame strengths are added to the Vehicle Structure Table (Vehicles p. 19 and Vehicles Expansion 1 p. 6) and to the instructions under Hit Points (Vehicles p. 20).

Generally, each of these new frame strengths should be treated as the next-weaker vanilla frame strength for the purpose of any vanilla rule that references frame strength. For example, a very cheap 70-%-strength TL6 frame should be wooden, not metal (Vehicles p. 19).

(The widely-spaced vanilla frame strengths can be very annoying in forcing the designer to use either an overburdened frame or an overbuilt frame, with nothing in between. Conveniently, however, the pattern is very easy to interpolate.)

If a vehicle has a pair of biplane or triplane wing subassemblies, the designer may declare that the wings are arranged longitudinally, in a tandem configuration, rather than in the usual vertical stack. This adds a −1 penalty to the effective TL used in determining Aerial Maneuver Rating (Vehicles p. 135; compare top-and-tail and coaxial rotors vs. multiple main rotors), but removes the −1 penalty to Aerial Stability Rating for having biplane or triplane wings (id. p. 136).

The GM may wish to require spacecraft to be designed with radiators, from which they reject excess heat.

For every megawatt of power that a spacecraft's power plants can generate, the spacecraft must have at least 62.5 ft² of area dedicated to radiators. This area can be provided as radiator panels on the spacecraft's surface, or as radiator wings that project from the vehicle's surface.

Radiator panels have no weight, cost, or volume, and can be placed on any exposed surface of the vehicle, even over armor (but not over solar cells). To design a radiator wing, use the rules for designing solar panels on Vehicles p. 96, but cover the pseudosubassembly with radiator panels rather than with solar cells.

A radiator wing can be made capable of retracting into a subassembly. Use the rules for retractable solar panels on Vehicles p. 96, but weight and cost are increased by only 25 percent over the weight and cost of a non‐retractable radiator wing, and the volume occupied by the retracted radiator wing is increased to 1 ft³ for every 40 lb.

(This rule is backported from Transhuman Space pp. 186–187. For rules on overheating, see id. pp. 200–201. For the retraction, extension, and Hit Points of radiator panels, use the rules for solar cells on Vehicles p. 96. See also Spaceships 1 p. 31.)

Electric motors become available at TL6. In the absence of better information, use the rules and statistics given on WWII: Motor Pool p. 13 for electric motors at all Tech Levels from TL6 onward.

Alternatively, an intrepid GM may wish to imitate the weight progression that's used for leg and flexibody drivetrains on Vehicles p. 31 (along with a cost of 1 $/lb):

(The weights given for TL5 and TL(5+1) electric motors on Steampunk p. 72 are extremely low in comparison to the weights given for TL6 electric motors on WWII: Motor Pool p. 13. I would ignore Steampunk on this topic. Maybe the Steampunk numbers are meant exclusively for stationary motors, rather than for vehicular motors.)

The refinement given on WWII p. 142 is not presented in a very coherent fashion. As far as I can tell, the author meant to say, not that wing volume is divided by 5 (i. e., 0.02 × body volume per wing) and wing area is multiplied by 5^2/3, but that wing volume is divided by 5 only in calculating Size Modifier.

Tilt-rotor vehicles (Vehicles p. 34) can be created using any of these three combinations: