Grav Drive Proposal for a GURPS Space STL-/FTL-Drive system (c)1996 by Stephan Aspridis Theory and specs: ----------------- The Grav Drive is both a reactionless STL and a FTL drive system. Basically, it operates by projecting a gravity field in front of the ship. To understand the the operating principle behind the Grav Drive, it is essential to consider some definitions from quantum mechanics: 1. Electromagnetic waves (such as light) display wave-like and particle-like behaviour. 2. Energy and mass are equavilent. ..which implies that matter can be defined as an electro- magnetic wave with a specific frequency. This implication becomes important when asking the following question: "If matter and electromagnetic waves are essentially the same thing, why does matter exerts gravity while electro- magnetic waves don't?" The answer is, that electromagnetic waves would exert gravity if their energy was high enough to do so. Matter has the highest energy density possible, and for an electromagnetic wave, this means frequency. At frequencies in this range, the radial velocity of the sinusoi- dal electromagnetic wave would exceed the universal velocity of free space (speed of light), which is not practicable. Since these excess energies cannot be accomodated by free space itself, they must "decelerate" somehow and do exactly this by displaying them- selves as gravity - the higher the excess energy of the particle, the higher the gravitational effect. This in fact very similar to the Cerenkov effect and forms the base for the whole scientific field of gravitics. All right, but how can this effect be used for propulsion? Using a high-strength gravity projector, you get a remarkable efficient dual-purpose drive. In STL mode, this is straight-forward: The gravity field projected in front of the ship is pulsed several billion times per second. The field forms, attracts the ship, is then canceled and formed again at a new point which has the same distance to the ship as the last. The net effect is that the ship seems to continously fall into a projected gravity well which is at a fixed distance to the ship. Note that this is the only way to circumvent Newton's law of conservation of momentum. A gravity field which would really be continous rather than pulsed would form a closed system and as such the total energy of the system would have to change in order to accelerate the ship. In FTL mode, the whole thing becomes more complicated: As mentioned above, excess energies of electromagnetic waves have to "decelerate" if they would otherwise exceed c. This is the reason why matter exerts gravity (the denser it is, the more gravity per volume unit). Of course, this goes for ships, too. The idea is to project a care- fully modulated gravity field in front of the ship which counteracts the gravity of any object (in this case the ship) entering it by phasing it out on a subatomic level. The result is that the ship is unable to "decelerate" it's excess energies which, as we know, also cannot be accomodated by free space. To avoid a paradoxon, the ship must leave our frame of reference and enters a non-spatial continuum known as "hyperspace". Practical implications: ----------------------- As the ship seems to continously fall into a gravity well, it moves in free fall - there are no acceleration effects. The only limitations are available space and power requirements. It is possible to switch from STL to FTL mode (and back) without stop, provided that the minimum safety limit for FTL mode is reached and that the storage banks for FTL mode have energy left. For obvious reasons, the FTL gravity field has not a fixed distance to the ship, as opposed to the STL field. So, if you switch from STL to FTL mode, it doesn't take long to enter the field. There is no such thing as a 'free ride'. The FTL gravity field is adjusted to the subatomic structure of the ship by which it is projected, other ships are not able to use it. Due to the fact that the ship is not pushed as in conventional drive systems but pulled instead, the projector must either be bow-mounted or have a free field for projection in front direction. The ship retains the same velocity it had while leaving normal space. The drive unit produces no waste heat. Visual effects: --------------- The gravity field for hyperspace entry can visually be perceived as a faint distortion in space. A ship that enters the field seems (to an outside observer) to blur and vanish. Inside the ship, space seems to blur and is replaced by an undulating dull gray nothingness. When emerging from hyperspace, this effect is reversed. Game stats: ----------- Drive type: combined reactionless STL and hyperdrive FTL. Treat as reactionless drive of appropriate TL, but double mass, size and cost. FTL Speed: thrust rating (in G)/10 = ly/hr (a 1G ship has a FTL rating of 0.1ly/hr, a 10G ship 1ly/hr and so on) STL Speed: maximum thrust is determined by engine size Fuel: cost, consumption etc.: energy requirements are the same as for thrusters of the appropriate TL. Energy must come from storage banks dedicated to the task (see below for details). Ease of FTL navigation: Complex 3-D, with a -1 to the Astrogation roll for each 50ly traveled and an addi- tional -2 if no information about the re- gion is available. An hour is required for calculating the course, hurried skips give a penalty: -2 for 30 minutes, -4 for 10 mi- nutes, -6 for 1 minute and -8 for no cal- culation at all. Engineering skill dificulty: average, as on p. S35 Obstacles to FTL travel: FTL gravity field cannot properly form within 50 diameters of planetary or stellar masses. Any attempt to engage the FTL drive within this zone invariably results in a misskip (roll 2d on the FTL error effect table; if the Astrogation roll was a failure, double the result). Ships also cannot leave hyper- space within 50 diameters of large masses. Any attempt to do so causes the ship to drop out of hyperspace at the 50 diameter border, usually without further effects. Time effects of FTL travel: no effect FTL side effects: no effect FTL error effects: depends on amount by which the Astrogation roll was failed: 1-2: Nothing happens. You engage the drive, but go nowhere. 3-4: Off-position (minor). You emerge at the right destination, but in a random location 3d AU away from the calculated reentry point. 5-6: Off-position (major). You emerge at a random location at the right distance. 7+ : Damage. Ship takes medium damage and goes nowhere. FTL special notes: it is impossible to make a mid-course correction while in hyperspace; once you set the coordinates before the skip and engage the drive, there is no way (short of disengaging the drive) to tell whe- ther the course is right or not. STL special note: as the ship is in free fall, there are no acceleration effects (even at several times native G). Limit for maximum thrust is the size of the engines. Drive reliability: checkup every year, or roll against Engineering at (year over checkup-1) after every use to avoid power loss by (roll was failed by)%. Maximum range: MW as for thrusters. The energy for FTL mode must come from storage banks dedicated to this task (treat as capacitors for appropriate TL, but release the energy continuously instead in a single burst). Example: A TL11 1000 ton ship with 1G acceleration and a FTL rating of 0.1ly/hr uses 100MW for the drive. It's storage bank has a capacity of 10000MW/h. Thus, the ship can remain in FTL mode for 100 hours maximum. After that, the ship must drop out of hyperspace and recharge the storage bank. Designer's note: ---------------- While it would be possible to design the STL- and FTL-part of the drive separately, this would impose some problems: If the FTL gravity field would be incorporated in a (for example) 'hyperdrive grid' around the ship, you could use a standard hyper- drive unit as described in GURPS Space. In this case, it would be necessary to set the drive maximum speed arbitrarely instead of calculating it based on the G-rating of the ship. Furthermore, the ship would be able to be very long in hyperspace, because of the low energy requirements of a standard hyperdrive unit. As for the STL-part, you could either use reactionless drives (in which case there is no reason to split the drive principle) or - and this would be (in my opinion) the only reason to do it, rely on reaction drives instead for a different campaign flavour. In summary, the Grav Drive is nothing more than a combined reaction- less drive/hyperdrive using the design specs of reactionless drives in GURPS Space. The only advantage the STL-part has over the drives described in GURPS Space is the lack of acceleration effects. So you can build your drive as large (and strong) as you like without having to resort to grav-compensators (you will probably notice very fast that this is not imbalancing because drives with very high G-ratings are not practical - I think about 1G on TL9, 2-10G on TL10 and around 20-100 on TL11+. 1G should be the minimum for ships intended for plane- tary landings, however, otherwise you would face some problems during take off).