GDW's House Rules
Fire, Fusion & Steel
Chapter 9: Sublight (Maneuver) Drives
Index
Æther Propellers (Space 1889)
Bussard Hydrogen Ram
HEPlaR
Ion Drives
Maneuver Drive Design
Propellers
Realistic Thrusters
Self-Contained Thrusters
Solar Sails
Maneuver Drive Design
Propellers
HEPlaR
Self-Contained Thrusters
Realistic Thrusters
Ion Drives
Over 30 years ago, John W. Campbell popularized a supposed invention by a fellow named Dean. His invention, the Dean Drive, was a simple device which converted rotational momentum to linear momentum. Never mind that that violated the laws of conservation of momentum, he claimed he could do it. He wasn't able to convince the patent office, however, and never could provide a convincing public demonstration of the device, and so interest faded.
But had the drive worked, it would have had an extraordinary effect on travel, and is a perfect substitute for both CG and all thrusters. It is a reactionless means of efficiently converting electricity to thrust. The electric motor (or any other sort of engine) spins a dense shaft, thus creating rotary momentum. The Dean Drive (or, for purposes of the game, the Dean Converter) then changes this rotary momentum to linear momentum, or thrust. The standard illustration of a Dean Drive at work shows a cargo truck unloading at a second-story window, its Dean Drive converting the momentum of its drive shaft to upward momentum.
Very nice. But why does this violate the conservation of momentum? Think of the solar system as a closed system spinning with a fixed average velocity. Everything in it is spinning, although at different velocities, and each of the individual bodies has angular momentum. The sum of the angular momentum of all of the individual components of the solar system equals the total angular momentum of the system.
Now, suppose something changes velocity. Assuming that its mass stays the same, its momentum changes, since momentum is a combination of mass and velocity. Does this change the total momentum of the system? No, it doesn't, because the only way things can change velocity within the system is to change the velocity of something else at the same time. Remember, "for every action there is an equal and opposite reaction." So, if a rocket accelerates in one direction, reaction mass (the rocket exhaust) is accelerating in the opposite direction, and with equal force, so the net change in momentum in the system overall is exactly zero. The sum of the momentum of all the bodies in the system is still equal to exactly the same total.
But that is not true with a Dean Drive. The Dean Drive manages to change the momentum of the object it accelerates without changing the momentum of any other component of the solar system (or the galaxy, or the universe), and thus alters the total momentum in the system, hence violating the law of conservation of momentum. For the Dean Drive to work, our current understanding of the physical universe has to be seriously flawed.
On the other hand, who is to say, especially in a science-fiction game, that our understanding of the physical universe isn't seriously flawed? Certainly not us, and in that spirit we offer the Dean Converter as an alternative to conventional thrusters.
Design: The Dean Converter is an extremely efficient reactionless drive which requires only electric energy to operate. Each cubic meter of Dean Converter machinery converts 1 megawatt of power to 100 metric tonnes of thrust. There is no lower or upper limit on the size of the Dean Converter machinery. Each cubic meter of Dean Converter masses 2 metric tonnes and costs MCr0.1. Tech level of initial availability is left entirely up to the referee.
Maneuver drives in previous editions of Traveller were explained as related to the same body of theoretical physics which allowed artificial gravity and damper fields, which is to say manipulation of gravitational force and the strong nuclear force. Artificial gravity was defined as a force which could either push or pull and which acted on the gravitational field of a mass. Clearly, this would not be an efficient means of travel outside a gravity well, and so a further advance was postulated which allowed the force generated by the drive to push on the actual thruster plates of the ship itself, propelling it through space and achieving a true reactionless drive.
The problem with this approach is that it runs into the same wall as does the Dean Drive. A drive can be reactionless without violating the law of conservation of momentum if it moves a craft around without changing its velocity. ("Huh?" No, this is not a typo. Consider the jump drive. It moves a ship from point A to point B, and does so without pushing it through the intervening space. It is a reactionless drive, but when the ship arrives at point B it has exactly the same momentum in the form of a vector as it had when it left point A. Thus, its velocity has not changed, nor has the momentum in the system overall been altered.) But a reactionless drive which changes the physical velocity of the craft shatters the law of conservation of momentum.
As indicated in the discussion of the Dean Drive, referees of science-fiction games should not be too shy about breaking the laws of physics. The fiction in the genre's name gives you license to do so. But the science in the genre's name requires you to at least know what you are doing and have a pretty good reason for doing it. In the case of reactionless drives in general, the best reason is that the sort of campaign you are running requires a cheap, efficient, ho-hassle way of getting from here to there. It is best suited to campaigns with a strong space-opera flavor. (Don't sneer; those can be the most fun sometimes.)
Design: Thruster plate technology becomes available around tech level 11 (due to the fact that it is tied closely to many of the other theoretical breakthroughs that occur at about the same time). Each cubic meter of installed thruster plate drive generates 40 metric tonnes of thrust, masses 2 tonnes, requires 1 MW of power, and cost MCr1. Surface area (used by spacecraft only) in square meters is equal to thrust in tonnes ÷ 200.
Bussard Hydrogen Ram
Dædalus Thermonuclear Pulse Drive
The Dædalus drive was first proposed by the British Interplanetary Society in the mid-1970s, and in its simplest terms consists of a fuel tank and ignition chamber. Hydrogen fuel is fed into the ignition chamber and super-heated by lasers or high-energy electron beams. The resulting thermonuclear explosion pushes the ship forward.
Fuel is in the form of pellets. Each fuel pellet masses about 1.5 grams and consists of a mixture of deuterium (heavy hydrogen) and helium-3, all enclosed in a superconducting shell. The detonation of a single pellet produces between 15 and 20 tonne/seconds of thrust.
The ignition chamber and supporting electronics (electron guns and magnetic fields which protect the ship as well as directing thrust) give the engines a lower thrust-to-mass ratio than a conventional fusion rocket, but provide fuel efficiency about one order of magnitude higher. For long interstellar voyages, fuel efficiency is critical.
Design: Dædalus drives are available at tech level 9 and above. Each cubic meter of engine masses 1 tonne, generates 1.5 tonnes of thrust and consumes 0.0005 tonnes of fuel per hour. Price is MCr0.5 per cubic meter of engine. Surface area of the ignition chamber in square meters is equal to tonnes of thrust ÷ 200. Because of the exacting manufacturing requirements, fuel for the Dædalus drive costs 100 times as much as conventional hydrogen fuel.
Solar Sails
Æther Propellers (Space 1889)
Reactionless Thrusters (GURPS)
Reactionless thrusters directly transform energy into thrust through means inexplicable to 20th-century science, bypassing the need for reaction mass. In game terms, they function much the same way that rocket engines do, except that they do not need fuel. GMs of rigorously "hard science" campaigns should feel free to omit reactionless thrusters!
The TL 9 reactionless thrusters produce considerable waste heat discharge, as much as any rocket of equivalent thrust. TL 12 thrusters produce some waste heat discharge, similar to a jet engine. TL 14+ thrusters produce no waste heat and are invisible and silent. Most "grav" vehicles use TL 14+ reactionless thrusters for propulsion and anti-gravity for lift; perhaps UFOs do also.
Reactionless thrusters built at TL 12 and up come in both "standard" and "super" versions — the latter have more thrust for a given thruster weight, but are more expensive. At TL 16+, ultra-powerful "mega thrusters" may be available. Not all universes will have them — GM's option.
Reactionless thrusters can be used for propulsion by aircraft, spacecraft, watercraft or underwater vehicles, and by ground vehicles equipped with skid or wheeled subassemblies.
Reactionless thrusters are rated for their tonnes of motive thrust. Use the same suggested thrusts for reaction engines if you want fast vehicles with jet-aircraft speeds! However, at TL 15+, many contragrav vehicles use reactionless engines. Combined with contragrav, a typical flight pack worn by a person would only need about 10 kg of thrust, while a car might use 100 kg, and a grav truck or tank about 1 tonne.
The more thrust a vehicle has, the faster it will move and accelerate. As with rocket engines, vehicles may have more than on thruster for added safety; each is built individually, but they combine their thrusts. Decide what type of engine the vehicle will use, how many it will have and what each engine's motive thrust will be. For each engine the vehicle has, decide on its location and calculate its statistics as shown below:
| TL | Type | Thrust | MW | MCr |
| (3+1) | Standard Thruster | 2 | 2 | 0.02 |
| (3+1) | Super Thruster | 5 | 6 | 0.10 |
| 9 | Standard Thruster | 2 | 2 | 0.10 |
| 12 | Standard Thruster | 4 | 4 | 0.08 |
| 12 | Super Thruster | 10 | 10 | 0.40 |
| 14 | Standard Thruster | 40 | 4 | 0.08 |
| 14 | Super Thruster | 100 | 10 | 0.40 |
| 16+ | Mega Thruster | 2000 | 200 | 20.0 |
Values are per cubic meter. Reactionless thrusters mass 2 tonnes per cubic meter and have a surface area equal to thrust ÷ 200.
Steampunk (TL 3+1): If electric and magnetic fields are strains in the ether, then electricity and magnetism can exert force on the ether. The right sort of apparatus might be able to exert much more intense forces. This would create an equal and opposite force in the apparatus — it would be pushing against "empty space," which would make it a reactionless drive. Two models might be available and are listed on the table above.
Another application for reactionless thrust is a hollow sphere whose interior generates outward thrust. Thrust equal to atmospheric pressure could inflate a gasbab containing no gas at all, allowing static lift superior even to that of hydrogen: 215 kg of lift per cubic meter of envelope (3.01 tonnes per displacement ton of envelope). No gas or ballast is consumed in the process, but it does take energy to inflate the gasbag: 0.088 MW per kilogram of lift. The apparatus that generates the force weighs 1 kg and costs Cr3 per square meter of surface area.
Bibliography