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(image borrowed from Nyrath at Atomic Rockets)
Section A.) Reaction Mass and other limits.
Phase World uses Contragravitic drive almost exclusively, which is a reactionless drive. this means that it generates thrust without requiring the ejection of material out the back of the ship. This material is properly referred to as reaction mass, usually shortened to remass, although it is often called "fuel" by the modern media, largely due to the fact that nearly all our real life rockets burn chemicals to generate energy that causes the waste products to be ejected out the back of the rocket at high speed. thus their fuel is also their reaction mass. but chemical rockets are inefficient and not very good for use by real spacecraft, so any reaction drive (as such are often known as) will usually have it's remass separate from whatever it uses to generate power. the term reaction in these names is not a reference to chemical reactions, but rather a referance to the third law of motion, "any action has an equal and opposite reaction." eject some mass away from your ship, and you ship will move in a direction 180 degrees from that direction. both the ship and the mass would have the same amount of energy behind it, though the ship, generally having more mass, will not gain as much velocity. so a reaction drive is any drive that uses remass propelled opposite the desired direction of travel to propel a ship. a reactionless drive is a drive able to propel a ship without using remass. Robotech, Aliens Unlimited, and Mutants in Orbit all use reaction drives as their main means of propulsion, and thus have to worry about remass. Ships using reaction drives need to track the amount of remass that have available. combined with the acceleration value, this generates what scientists call Delta-V, the total amount of velocity you can obtain using the ships supply of remass. for simplicities sake, these values are left mainly for roleplay use, and players and gamemasters merely have to track the "thrust points" available to the vehicle. one thrust point is equal to one mach of velocity. so a ship that accelerates at 5 mach per melee marks off 5 thrust points each melee. a ship that generates 5 mach per day would mark off 5 thrust points after each day of thrust. etc. each ship would have a specific number of Thrust points available in Delta-V available to them. once the ship runs out of thrust points, it cannot accelerate or decelerate any more, though all space craft have small maneuvering thrusters able to let it spin in place. these thrusters are never powerful enough to alter the ships velocity or course on their own. so a ship that runs out of thrust points (delta-V) is dead in the water, coasting along on inertia at whatever velocity it had last obtained.
sadly, palladium doesn't provide us with information about how long any of its reaction drive ships can travel under drive. only in mutants in orbit is information given, and that is not in a form easily used in an RPG. Game masters should decide for themselves how many thrust points a ship has available to them. the best way to assign these is to decide how long the ships can operate at one gravity of acceleration, in melees, and divide that in half (as 1 mach per melee is 2 gravities). that would generate the number of thrust points required to pull that off. it is a good idea to check the orbital velocity and travel time notes in sections 8 and 9 when deciding, as these can give you a minimum value needed to acheive whatever mission the ship has to perform (such as being able to reach orbit, or travel to mars under constant thrust.) Mutants in Orbit does provide a duration limit in it's ship construction system entry for fuel tanks, i recommend using this to determine your thrust points. the best way would be to determine how many melees the listed time equals, and multiply those by 5 (which is the highest mach number any MiO ship can do) this gives the ships sufficient remass to travel around the solar system as described, although for most trips ships would need to coast most of the way.Section A1.) Reaction mass use while attaining orbit.
Official palladium canon is that to reach orbit, one needs a vessel able to exceed mach 5 within an atmosphere. This is a good rule of thumb for aerodyne space craft (spacecraft designed to operate as airplanes within the atmosphere.) any craft able to reach mach 5 within an atmosphere will be capable of reaching the upper edges of the atmosphere, where it merely requires switching to a space drive or drive mode to accelerate to orbital speeds, a process which only requires 5x the normal thrust points of space acceleration. however, a craft attempting to reach orbit purely utilizing a space drive, like the real world space shuttle and cargo rockets, will not only require acceleration of 1 mach per melee or greater, but will expend 10x the normal thrust points while within the atmosphere, the result of friction, atmospheric pressure, and overcoming rest inertia. even an aerodyne craft that cannot reach mach 5, but is capable of operating in space, will expend these additional thrust points. Some lower technology spacecraft (such as the aforementioned Space shuttle and cargo rockets), overcome this issue by using powerful additional drives to power the initial acceleration, usually in discard-able stages which are dropped when spent to reduce mass.
Section B.) Drives as Weapons: (AKA "The Kzinti Lesson")at these speeds, even a bolt or nail can be a weapon of mass destruction. ships even more so. as a general rule, an object moving at 3kilometers a second does damage equal to it's mass in TNT. what this means is that for every 10 mach of velocity, an object does 1D6x10md per ton of mass. this damage is applied to both objects in the collision.
powerful reaction drives, such as fusion, antimatter photon drives, and torch drives, are highly destructive. they generate plumes of high temperature gases which provide the propulsive force, and these plumes can cut through ships like a knife through butter. for these drives, the drive plume extends 1 kilometer per mach of acceleration, and inflicts 1D6x10md per mach of accel for every 1000 ton of mass to anything that enters the plume. lower technology races often used fusion drive or photon drive ships as weapons to defeat more advanced enemies in Space Fiction Novels, by making use of this fact.
this sort of combat was popularized by Larry Niven in his "Known Space" and "Man-Kzin Wars" settings. The Kzinti, a race of imperialistic felinoids that had gravity drives, heat rays, and powerful warships, encountered mankind, which had developed into a pacifistic, fairly ignorant race with little concept of weapons or war. needless to say, we proceeded to kick the Kzinti's rear ends all over space, because while we had forgotten war, we had the smarts to turn the 'common' fusion drive spacecraft into really big cutting torches, communications lasers into ultra-long range laser cannons, and generally out thought the kzinti in every war, always inventing new ways to fight. thus the origins of "the kzinti lesson", which is "the effectiveness of a reaction drive as a weapon is directly proportional to its effectiveness as a drive." or in short, the more powerful the drive, the potent it is as a weapon.
[a cropped page from the webcomic Nip and Tuck, showcasing the use of applied physics. (part of the in comic Rebel Cry scifi movie)]
At its most extreme:
Even reactionless drives can be weapons. the most simple weapon in the human arsenal has always been a thrown rock. the bigger the rock, or the faster the rock is going, the harder it hits. and when a rock is moving very fast and is very big? The result is something called a Relativistic Kinetic Impactor. a fancy name for what is basically just a rock (or ship) moving at really high speeds. how high? well, the fastest anything can move in normal space is just below the speed of light. so pretty darn fast. imagine a thought experiment. we take a chunk of rock from an asteroid belt. say a thousand tons. fairly tiny. we then tow it up to 60% of lightspeed using a reactionless drive, and aim it at something really big. the resulting impact would be enough to kill everything on a planet. hundreds of times more lethal than the rock that killed the dinosaurs. this is a fairly important fact, since every starship, from the smallest fighter up to the largest dreadnaughts, could be employed to cause damage on a global scale if used as a kamikaze. even the fairly tiny Scorpion starfighter, at 20 tons, could slam into the planet fast enough to destroy entire regions if given a few days to accelerate.
Most groups in the Three Galaxies aren't insane enough to employ such tactics, because most worlds will surrender when their space forces are defeated. Planets provide immense political and economic power to those who hold them, so destroying entire planets just isn't something worth doing. at worst, a fleet that is looking to force a world to surrender might drop really tiny rocks (a few tons each) from orbit to hit military bases or cities. such Oortillery (orbital artillery) usually scares the planet into submission before any real damage is done.
Section C.) Notes on Travel times:
when traveling between two points using constant acceleration, the time spent in transit can be very brief. it is a function of distance, acceleration, and a complex formula. thankfully, Instead of equations, I can just give a list. this was created by my friend Cray over at Classicbattletech.com
With 1G of acceleration available, you can travel...
from Earth to the moon or Earth-moon Lagrange points (~400,000km) in 3.5 hours.
0.3 AU in 37 hours (Venus-Earth closest approach)
0.5 AU in 49 hours (Mars-Earth closest approach)
0.6 AU in 53 hours (Mercury-Earth closest approach)
1.5 AU in 84 hours (Asteroid Belt-Earth closest approach)
2.5 AU in 108 hours (Mars-Earth farthest opposition)
4 AU in 138 hours (Jupiter-Earth closest approach)
10 AU in 217 hours (Saturn-Earth almost closest approach, or Earth-Sol Jump Points)
40 AU in 434 hours (Pluto-Earth closest approach)
G's do not linearly decrease travel time. it's a function of square roots. That means if you travel somewhere at 2Gs, you do not divide the 1G travel time by 2. You divide by the square root of 2. If you want to get somewhere in half the time as at 1G, you need to travel at 4Gs. It gets progressively worse. To get somewhere in 1/3 the time, you need to travel at 9Gs. at 10g you can go 1.219277 AU in 1 day. the speed of light, or rather .999...C, at 1G, requires 365 days of acceleration. most vessels can never achieve this velocity, due to relativity.
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