________________________________________ GURPS Vehicles 2nd Edition Additions MA Lloyd (malloy00@io.com) 9 August 1998 Modifications and Additions to Chapter 12: Vehicle Action ________________________________________ p144 Piloting. Piloting(Aerospace): Replace 'winged flight and reentry' with 'controlled launch, reentry or aerobraking'. Pilot(High Performance Spacecraft): Flip the defaults to Aerospace-2, Low Performance Spacecraft-4. Piloting(Ornithopter): Use this skill for all vehicles flying via Ornithopter drivetrains. Defaults to Helicopter-3, Vertol-3, other Piloting-5 Piloting(Warp Drive): Use this skill for all vehicles flying faster than light in something analogous to normal space. p146 Maintenance. When finding maintenance intervals ignore the cost of components that don't suffer wear - armor, fuel tanks, self destruct systems. Do not exclude structural cost though, frame maintenance is necessary for most vehicles. p147 G-Force Table. The formula is 0.000796 x speed x degrees. p148* Hazard Control Rolls. Damage. The threshold is for damage from a single event or damage roll, not simply 5 points of damage in a turn. p149 Roll. Treat each turn of rolling as a 20 mph collision for purposes of damaging the occupants (cf p.160). p151 G-Forces and Realism. An intermediate step, allow maneuvers over 2xMR, but make an HT roll to see if the stresses cause a breakdown. p153 Light Woods. The terrain is covered by light vegetation. Size 5 and larger vehicles with masts or unfolded wings or rotors are immobilized. Otherwise the only effect is to obscure sight lines. p153 Riding Outside Vehicles. There is room for 1 person per 7 square feet of deck, not 9. The surface area/20 includes the top. p154 Currents. The Gulf Stream runs at 2-3 mph. Rivers run 0-20 mph, and contrary to common expectation are often slower in 'rapids' than downstream, all that turbulence wastes energy that isn't going into speed of the flow. p154*Running Aground. exceeds draft --> less than draft. p154*Sinking rate should be 1 yd/sec, to agree with p187, not 1d yd/sec. p154 Sinking. Unsealed vehicles flood 1d6% of their hit points every minute. p154 GLOC. G-suits and G-seats reduce the felt gravity by 3G, instead of halving it, womb tanks still halve felt gravity. p154 GLOC. Modifiers. -2 if standing. p154 GLOC. For accelerations that build up slowly, roll each time a g boundary is crossed, but allow the -3G for a long acceleration. p154 Sidebar. Last line than --> then p156 Stalling. Stalling cannot exceed the Climbing and Diving limits on p155. An aircraft that stalls while climbing more than 50% climbs 10% next turn, flies level the second, dives a maximum of 50% on the third and reaches (1d+2)x10% only on the fourth if the die roll is 4+. p156 Last sentence, see addenda for p134. p157 Falling. If you know the terminal velocity on Earth, you can compute it for another world by multiplying by the square root of (surface gravity/atmosphere density). p158 Collisions. To prevent low speed impacts from vaporizing small objects base collision damage on the body hit points of the lowest hit point object involved in the collision. p159 Collision Aftermath. Instead of the flat breakpoints and fractions modify this to agree with Hitting Obstacles: For head on collisions both vehicles slow by (other vehicle hp/damage inflicted) x closing speed. For T-bone, both slow by (other vehicle hp/damage inflicted) x original speed. Make any necessary control rolls for Hazardous Deceleration. p159 The Sound Barrier. The speed of sound at STP is 740 mph, not 760 mph. p160 High Altitudes. Also divide all heights by the planet's surface gravity. p162 Ejecting. If ejection seats collide with something - say the roof if ejecting indoors or the ground if ejecting while upside-down, damage is 6dx5. p162 Ejection Capsules. If the capsule isn't properly designed (see The Sound Barrier on p159) it probably breaks up too. Drop the 'or in space', a vehicle in space will not break up, though it might burn up if it tried to reenter without the capsule. p164 Long Distance Space Travel. The formula is sometimes more useful in the form t = 68.7 hours * square root of (D(AU)/a(G)). p164 Ground to Space Capability. For vehicles using a lift system the 'operate at speed' restriction is important; it rules out the use of rotors or gasbags on all but the lowest escape velocity worlds. If using wings the space capable engines alone must have enough thrust to keep the vehicle above stall speed, or it will stall before it completely clears the atmosphere. The burn time formula given assumes constant weight, which works for reactionless thrusters, but prevents realistic rockets from making orbit. To find the fuel requirements and burn times for a reaction engine use the rules under Specific Impulse (p136a), with delta-v equal to the orbital (or escape) velocity. p164 Space to Ground Capability. Streamlining matters, add to paragraph 3: 'Vehicles with less than excellent streamlining suffer more damage: x2 if superior, x4 if very good, x10 if good, x20 if fair, x50 if none' In paragraph 4: speed after aerobraking is equal to terminal velocity unless the vehicle has a higher top air speed and turns on its engines to maintain the higher speed. p165*Advanced Hex Grid Movement. Under Suggested Scale, change 100 miles to "1,000 miles (1,760,000 yards)" and change "10 space hexes per combat round" to "2 space hexes per combat round". p165 Extreme Temperature. Gasoline engines require heating below -50F. ________________________________________ Vehicle Action p154a Icebergs _________________________________________ Rescued from the playtest draft, one of few bits there that didn't make the final cut: SB Icebergs! For ships and boats, pack ice can only be penetrated by specialized icebreaking vessels with DR 50 or higher front armor and only very slowly. Near the Arctic and Antarctic circles, icebergs are a hazard to surface ships as well as to submarines cruising on or just below the surface. The chance of encountering an iceberg varies greatly depending on how far north or south a ship travels. The main danger is meeting one at night or in bad weather: if the GM decides that a vessel is on a collision course with an iceberg, he should allow one each vision, sonar and radar roll to detect it, rolling against the skill or IQ of best lookout or radar operator on duty, with modifiers for the quality of the equipment and the alertness of the crew. Making a radar or sonar roll allows detection with plenty of time to avoid the iceberg. Otherwise, a Seamanship roll is required to avert a collision. No roll is allowed to avoid it if both sensors and lookouts fail. Collision with an iceberg is like collision with any other large immovable object. ________________________________________ Vehicle Action p164a Aerobraking ________________________________________ Aerobraking is the use of an atmosphere to slow a vehicle. It is commonly used for Space to Ground re-entry, but it is not necessary for the vehicle to finish the aerobraking maneuver by landing. Aerobraking can be used as part of an orbit lowering maneuver, or to slow down after interplanetary trip. If using the simple calculation method for an interplanetary voyage, you can used the full delta-v to determine trip speed, rather than saving half to slow back down, but you must aerobrake at the end of the flight from a speed equal to the delta-v spent. If you want to land, you can reenter one orbit after the aerobrake, or you can add the reentry velocity to your arrival velocity and land immediately if you have enough DR. For simplicity aerobraking allows you to shed 1500 mph per minute and requires a Pilot (Aerospace) roll. It causes 1 point of fire damage per 375 mph shed. Aerobraking can bring the vehicle down to its terminal velocity; to slow more than that will require some other braking method. Aerobraking assumes a gradual entry into the outer atmosphere. If you are colliding with a dense region more suddenly, decelerations are much higher than 1500 mph/min (1.15 G). Typically (speed/terminal velocity)^2 times 1 G (1315 mph/min). Remember to use terminal velocity in that media. Aerobraking Shields A separate subassembly can be added to protect an aerobraking spacecraft, this is a common feature in some kinds of early interplanetary missions, as it can be detached and left in orbit for use at multiple times during the mission. The shield is a large streamlined shell held in front of the vehicle. Its only statistic is surface area, which must be equal at least half the combined area of the vehicle. The shield weight is the weight of that area of armor of the DR needed for the aerobrake, plus 10% for structural members and attachment points. It counts as all the necessary armor, plus radical streamlining for the aerobrake, but gives a -1 to the Piloting roll. ________________________________________ Vehicle Action p164 Cinematic Space Travel ________________________________________ Cinematic spacecraft often behave like aircraft or surface ships. There is no point in fudging the space movement rules to duplicate this, most of the battle scenes using it are modeled on WWII footage, so simply build the spacecraft as TL6 surface ships or airplanes, call the guns lasers, the propellers thrusters, assume the fighter wings bite into the ether and go from there.... ________________________________________ Vehicle Action p165a *Really* Long-Distance Space Travel ________________________________________ The constant acceleration formula on p164 is a simple Newtonian one. Some ships, primarily those with ramscoops or reactionless thrusters, can accelerate long enough to reach relativistic velocities. The speed of light is about 354 G-days, after 50 G-days or so relativistic effects are worth worrying about. To determine the trip time aboard ship, as shortened by time dilation, use: t'(yrs) = (1.937/a') x cosh^-1 (1 + 0.516 x d x a') where d is the distance in light years in the rest frame, a' is the acceleration in g measured aboard ship and cosh^-1 is the inverse hyperbolic cosine (most scientific calculators have this function). For outside observers the trip takes longer, the elapsed time in the rest frame is: t(yrs) = (1.937/a') x sinh(0.516 x a' x t'(yrs)) Incidentally, to obtain the relativistic rocket equation multiply the desired delta-v by 1/[(1-(v/c)^2)^0.5] and measure the mass ratio in the rest frame. ________________________________________ Vehicle Action p165a Extreme Environments ________________________________________ *Flotation in Other Liquids* Changing flotation to a flat 62.5 lb/cf (p18a) also allows a simple rule: To design a vehicle that floats in something other than water replace the density of water (62.5 lb/cf) in the design chapters with the density of the other liquid. Some typical densities: Fresh water 62.5, Salt Water (average modern ocean) 64, Liquid ethane 35.8, Liquid methane 26.4, Liquid ammonia 42.6, Concentrated sulfuric acid 115.1, Molten sulfur 119, Molten lava 165-210. *Gravity* Changes in gravity alter the weight of the vehicle, but most statistics based on weight are really functions of mass or relative densities and unchanged. Some exceptions are: * Thrust based lift directly counters weight. If weight changes the vehicle may no longer be able to fly. * Aerodynamic lift directly counters weight, recompute stall speed with the new effective Lwt (essentially multiply by the square root of the local gravity). * Movement penalties in hilly or mountainous terrain are multiplied by the local gravity. Ground pressure also changes. * Ground traction is limited by gravity, multiply gMR by the local gravity. * Climbing speed losses and diving speed gains are multiplied by gravity * Damage from falls is multiplied by gravity. *Meteors* Any spacecraft runs the risk of colliding with a meteor. The chance something will punch a hole in the hull is Surface Area/[500 * DR^2] percent per day. Perhaps 1 in 100 such hits will require more than a small patch. In regions with a lot of debris - Trojan points, ring systems, low Earth orbits full of old junk, but *not* realistic asteroid belts - the risk may go up several orders of magnitude. If your velocity is higher than typical insystem speeds you will be punctured more often. At high speeds divide effective DR by (heliocentric velocity/40,000 mph). *Space Radiation* Most vehicular shields are intended for spacecraft. For simplicity assume the interplanetary background is about 200 rads/year and a solar flare delivers about 2000 rads cumulative. A long occupancy spacecraft above LEO needs at least PF50. ________________________________________ Vehicle Action p166a Air Density ________________________________________ The density of an atmosphere depends on its average molecular weight (u in g/mol), pressure (P in atmospheres) and temperature (T in kelvins); and on the surface gravity (g in gees). Average molecular weight is the weighted average of the molecular weights of the component gases; for example Jupiter is about 90% hydrogen, 10% helium and 0.1% everything else, so the molecular weight of its air is about 0.9x(2.02) + 0.1x(4.00) + 0.001x (~30), or 2.2 g/mol. The density at the surface is 0.76 lb/cf x [P x u /T], about 0.080 lb/cf for the Earth. Pressure and density fall as you go up with a scale height (H) of 133 miles/(u x g). Density at height z is surface density x [e^(-z/H)]. Sensible atmosphere on Earth begins about 50 miles up; for other worlds call it [10 + ln(relative surface density)] scale heights.