For a science fiction novel I'm currently writing...

The spacecraft in question is based in the asteroid belt and is not designed to approach a planet. The ship has an offensive/defensive capability and would need to be able to manoeuvre effectively in combat.

The timeframe is within the next two hundred years.

Additionally, it would need to be able to refuel easily within the asteroid belt.

With this in mind, I aim to equip the ship with magnetoplasmadynamic ion drives. My understanding is that they can operate using a hydrogen or ammonia fuel, which should be easily replenishable in the asteroid belt, and would be powered by a nuclear power source.

But an MPD ion drive does not produce much acceleration.

My question: What kind of drive might I use to provide rapid acceleration (in combination with an MPD drive) for combat situations, or to evade an enemy?

Ideally using hydrogen as a fuel (or something else obtainable in the asteroid belt without needing much refinement) and powered by electricity.

I know that chemical engines would be able to produce the acceleration, but use lots of fuel. If I need to use them, I will. But an alternative would be very welcome.

I don't have much of a background in science, but would like to accurately portray the drive system of the ship (in broad brush strokes at least).

By all means, comments on MPD drives are welcome too.

  • $\begingroup$ Are you looking for realistic space combat, where ships have realistic relative distances and velocities, or are you looking for a more traditional story-time space combat which includes much shorter distances and dogfighting. With the latter, you may find combat adapts to the engines, weapons, and "terrain" available. With the latter, no existing engine can possibly keep up with the fantasy of the typical sci-fi space battle. $\endgroup$ – Cort Ammon Dec 15 '15 at 17:33
  • 1
    $\begingroup$ @CortAmmon For those last two sentences, I think the former latter was supposed to be a former rather than a latter. $\endgroup$ – Samuel Dec 15 '15 at 18:26
  • $\begingroup$ @Samuel You're right. Man that gets messy when you mess up "former" and "latter" doesn't it! $\endgroup$ – Cort Ammon Dec 15 '15 at 20:20
  • $\begingroup$ @CortAmmon Absolutely, your former/latter comment is what I'm looking for. Realistic combat strategies and tactics as defined by the distances and velocities involved. I want to tell an adventure story, but with as much realism as I can fit within the constraints of poetic license. $\endgroup$ – Smoj Dec 16 '15 at 7:42

What you likely will need is a hybrid solution, using a high acceleration drive for your combat manoeuvres, switching to a more effective drive when you are no longer in a hurry.
It is the high acceleration part that is most limiting to your design, it does not really help you in a combat being somewhat manoeuvrable if the enemy’s ship is better. That said, no one have any real experience with how wars in space work, so here you just have to use your imagination.
Back to high trust, chemical engines is one of the few engine designs really offering good acceleration. They are easy to maintain, come in any size, and have technological readiness level.

But if you want higher performance you can still get it without having to sacrifice much of the high thrust capabilities. Nuclear thermal engines have a ISP twice that of chemical ones, and that is often more than a halving of your propellant need. Actually working prototypes have been built.
Assuming your 200 year time frame, it is likely that even more capable reactor design like liquid core or gas core have been developed. NTRs are most famous for running on hydrogen propellant, available in the asteroid belt, but work great with other propellants like methane, ammonia or water as well.
Hydrogen and oxygen, either for chemical or nuclear engines, are produced using electrolysis of water. One important difference is that a NTR can run water straight, allowing your requirement for fast refuelling.

For the low acceleration cruising part of the system there are a lot of great options using different ion engines, offering a performance tens of times or better than chemical rockets.

Combining the capabilities of all the three different types of engines are of course possible.

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    $\begingroup$ Water, available from comets and probably many asteroids makes both hydrogen and oxygen, meaning "plain old" chemical engines would work fine. The nuclear option could provide even more boost combining with them but while thrust of that would be limited by the reactor power output, the chemical thrust is only limited by engine size and durability. Also, the reactor would be useful in powering ion engines. So, "travel" on ion propulsion, "maneuver" on nuclear+chemical capped to maximum power output of the nuclear engine, and "overdrive" in which nuclear propulsion is overshadowed by chemical. $\endgroup$ – SF. Dec 15 '15 at 15:52
  • $\begingroup$ @Hohmanfan Exactly what I am looking for. A hybrid system, which will allow steady acceleration for cruising, and rapid acceleration for escape or evasion. $\endgroup$ – Smoj Dec 16 '15 at 7:47
  • $\begingroup$ @SF Yes, I figure that water/ammonia/methane is readily available from a number of sources in the asteroid belt, if not from many of Jupiter's moons. I'm assuming trade and industry as far out as Jupiter, with many of the resources making their way through the belt on their way back to Earth. $\endgroup$ – Smoj Dec 16 '15 at 7:49
  • $\begingroup$ @Smoj: Lots of trade/industry would be based on solar sails as they are very cheap - a tiny tug with enormous solar sail dragging big chunks raw metal tacking against the Sun to approach Earth. While one cargo would take many years to reach Earth, an uninterrupted stream of such barges could provide a constant supply line, and once technologies for making solar sails in space are developed, they could become the primary means of bulk transport of non-perishables. $\endgroup$ – SF. Dec 16 '15 at 8:52
  • $\begingroup$ @SF For sure. I think that will be the most efficient way to do it. Unmanned solar sail vehicles. another option would be to stockpile mined resources on an asteroid with high eccentricity, and wait for it to approach earth orbit, then jettison the stockpile and recollect it when the earth is near enough. Trickier, slower, and much more dependant on a timetable but cheaper for the huge masses of material that I anticipate coming from the belt once industry gets out there. $\endgroup$ – Smoj Dec 16 '15 at 10:13

This is a use that cries out for the Orion Drive. Propulsion by chucking nukes out the back and riding the blast wave (more or less). No chemical system needed, and arguably no ion drive, either. Since it's intended to operate entirely in space, fallout should not be a problem. The fact that you're carrying nukes probably isn't a problem either.

A note of caution. With OD ships, you might be tempted to invoke a defensive tactic of keeping the pusher plate between you and the enemy - it is, after all, an effective nuke stopper by definition. However, it needs to be almost perfectly aligned, and a slight attitude alignment error will probably result in damage to the recoil absorption mechanism, which will disable your drive system.

  • $\begingroup$ Are there a lot of nukes to be found in the asteroid belt for refueling? $\endgroup$ – Samuel Dec 15 '15 at 20:32
  • $\begingroup$ @Samuel - Refuel? We don' need no steenking refuel! $\endgroup$ – WhatRoughBeast Dec 15 '15 at 20:34
  • $\begingroup$ Alright, you are the one with the nukes... Still +1 because this was going to be mentioned no matter what other requirements were there. $\endgroup$ – Samuel Dec 15 '15 at 20:35
  • $\begingroup$ @Samuel - And don't you forget it. $\endgroup$ – WhatRoughBeast Dec 15 '15 at 20:36
  • $\begingroup$ Don't forget that you can mount lasing rods around the edge of your pusher plate and take advantage of the drive nuke to pump the rods too. $\endgroup$ – Monty Wild Dec 15 '15 at 22:01

A fusion drive would fit your requirements, and produce plenty of thrust.

One of the early designs estimated that the VISTA spacecraft could deliver a 100-tonne payload to Mars orbit and return to Earth in 130 days, which is pretty fast.

The reason this hasn't been pursued much in the last few decades is that we didn't really know how to make controlled fusion very well.
This is something that is beginning to change, and could be a definite possibility in the next 200 years. It would most likely use helium-3 and deuterium as a fuel which is pretty abundant on the moon because of the solar wind.
It's likely that it would be found on asteroids as well.

A side benefit of these is that the fusion torch could be used as a weapon if needed. If aliens ever entered the solar system, we'd have a way to stop them.

  • $\begingroup$ I'm tending towards a fission reactor onboard the ship, but as you say, the future is likely to provide an answer to the problems faced with fusion. Still an idea that I'm playing with. The fusion torch is something I'll look into more deeply as well. No aliens in this book, humans can be nasty enough. $\endgroup$ – Smoj Dec 16 '15 at 8:32
  • $\begingroup$ @Smoj the fusion torch would mostly be used for thrust more than for power, and fusion is a bit cleaner than fission. They just tested a fusion prototype last week with encouraging results: news.sciencemag.org/physics/2015/10/… $\endgroup$ – AndyD273 Dec 16 '15 at 12:21

From a practical POV, a dual cycle nuclear reactor will have to be at the heart of your spaceship. The reactor (for simplicity we will assume a solid core NERVA type design, but you can choose whatever is most suitable) provides the high thrust for boosting away from a planet or asteroid and whatever combat manoeuvres you contemplate. The reactor based NERVA design can provide ISP's ranging from 800 to 1200 seconds, so it can be 2 to 3 times as fuel efficient as the Space Shuttle Main Engine (ISP @ 450), although this still is pretty limiting in terms of just how much high thrust you will have on tap. For example, the SSMEs ran for a total of 8.5 min based on the amount of fuel in the tank. A design with 3 NERVA engines and the same amount of LH2 in the tanks could conceivably run for 25 min, which seems like a lot more, but remember that you have to accelerate, decelerate, accelerate back and decelerate again to make a round trip mission. Some of the calculations you will need can be found here: http://www.projectrho.com/public_html/rocket/enginelist.php

The second part of the equation is the high efficiency plasma drive. When the NERVA is shut down, the reactor is still quite hot (and by adjusting the neutron environment in the reactor through use of control rods or drums, the core can be kept running. In this case, the reaction mass can run through the core but the nozzle is closed off. Hot remass then is used to run a thermal generator (take your pick) to power the high ISP drive.

The basic issue is that there are very few high thrust/high ISP drives that are even theoretically possible. ORION nuclear pulse drive is possible with current technology, but the nuclear pulse units are only going to be available at supply depots. You might be able to get high thrust/high ISP by using antimatter to heat remass or initiate nuclear fusion reactions. If you go that route, you can use small amounts of antimatter to heat large amounts of remass to extreme temperatures and velocity, potentially reaching large fractions of light speed given enough antimatter, reaction mass and time.

The design of ships using dual cycle nuclear fission reactors will be interesting (don't forget that radiators are going to be needed, and everything needs to be both very light weight but also strong enough to withstand sudden applications of thrust from the NERVA engine), as well as interesting issues of thermal management (the reactor cannot simply be turned on and set to maximum thrust, nor suddenly shut down) and careful calculations of reaction mass so you don't run out in the middle of a fight.


It depends on how "realistic" you want to be. For something conceivable right now, probably the best thing would a combination of that Ion Drive for progressive acceleration as well as Hydrogen engines for reactive/dogfighting engines. If your protag is humanoid or humanish, they'll have a bunch of water lying around anyway, and the breakdown of hydrogen for fuel and oxygen for air would be useful.

If you'd like to get into slightly speculative, take a look at the Resonant cavity thruster, the which is something that's currently being studied because it breaks some of the laws of physics, but still seems to work. If your scope is a hop skip & jump into the future, you can have them just work on a much larger scale than they do (?) now. Alternative means of long range travel include the previously-mentioned Orion Drive, fusion drives or Solar Sails (that would be in lieu of an ion engine as they too provide progressive acceleration), or an exterior launch source, such as a massive railgun or Mass-Effect-Style Relay devices across the galaxy (launch the ship in a certain direction, then they can maneuver from there to the target).

Two hundred years is a surprisingly long time science-wise, so you can touch on other speculative endeavors as well (heck, Star Trek: Enterprise is based in 2151, and that's using a warp-bubble drive). An interesting idea if the target environment was an asteroid field may be a gravity wave type of Propulsion, where the controller could increase and decrease the mass of the vessel on the fly, allowing movement through gravity effects and slingshot maneuvers. There's also the very confusing Mustafa drive, allowing a craft to effectively swim through space

  • $\begingroup$ The 'up to two hundred years' came from the fact that when I was a kid, I thought that by now I'd have an apartment on the Moon and holidays on Mars in my hover car. Progress has been somewhat slower. But I'm tempering that timeframe with the notion that sooner rather than later, population will outstrip resources (especially water) and we will need to spread our wings with a little more urgency. These are all great responses! $\endgroup$ – Smoj Dec 16 '15 at 8:06


We've already started experimenting with these. The main hold back is that 1) it's not strong enough to be escape Earth gravity, and 2) it really requires a nuclear power source (fission or fusion) for a decently high propulsion. For some reason, nations are currently reluctant to put significant nuclear power sources on spacecraft.

Refueling is either with radioactives (fission), which you can find in the asteroid belt, or hydrogen (fusion), which you might need to mine from Jupiter's atmosphere. I'm not 100% sure that's reasonable, given your limited acceleration, although with fusion I'd bet you can get better acceleration. I think fission might be more likely and easier to refuel.

The other thing you need is propellant. Wikipedia suggests specific gases, however this site indicates those are simply more efficient - you can really use practically any non-volatile gas:

The VASIMR® engines are capable of using almost any gas or substance with a high vapor pressure as a propellant. However, some gases are better suited to forming plasma than others, and the gas’ mass also plays an important role in rocket performance. For missions near Low Earth Orbit (LEO) and the moon, argon and krypton gas have the highest performance for a 200 kW VASIMR®. However, for missions that are much farther away, e.g. Mars or Jupiter, lighter gases are more desirable and give VASIMR® an optimum performance in these cases.

I'd say that if you're already into space, this might be a technology we could use in the asteroid belt right now. 200 years will certainly allow for greater efficiencies and a mature design.

  • $\begingroup$ Sounds promising, but I'm not sure it would work for this particular application, for various reasons. Not the least would be the apparently high levels of RF thrown out, which would leave a signature that would be easy to detect, relative to the other drive types (not so good for stealth). Feel free to correct me if I'm wrong about that... $\endgroup$ – Smoj Dec 16 '15 at 8:17
  • $\begingroup$ @Smoj: Thrust = energy. Energy is going to be detectable, and sensors will be tuned toward whatever output known technology uses. So stealth will be roughly proportional to how fast you accelerate, regardless of the drive you use. $\endgroup$ – Dan Smolinske Dec 16 '15 at 8:50
  • $\begingroup$ I guess so. Just thinking ahead to the combat parts of the book that are coming later... In terms of stealth, the biggest help there will be the vast distances involved, and signal interference from asteroids and human activity. $\endgroup$ – Smoj Dec 16 '15 at 10:17
  • $\begingroup$ There is no stealth in space, and once a ship lights off a high thrust, low ISP engine there will be a huge thermal signature to follow. Even when the engine is shut down, you simply need to extrapolate the future position of the ship, since it will continue to move at a constant velocity until either another burst of thrust is applies, or you do something like a gravitational slingshot. $\endgroup$ – Thucydides Dec 16 '15 at 20:48
  • $\begingroup$ @Thucydides I guess stealth isn't quite the word I was after (in the B-2 bomber sense at least). The ship's on the wrong side of the law and is keeping a low profile as much as possible. Sure, any period of acceleration would need a comparable period of deceleration to stop. But I figure that a dumb projectile weapon (eg. rail gun) with a predictive targeting system might be evaded by random changes of vector and velocity. Evading smart missiles will require some form of ECM or decoys. With the distances involved, even lasers might be avoided by random movement to some degree. Cheers! $\endgroup$ – Smoj Dec 17 '15 at 1:47

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