Please assume the following:

  • structural integrity is not a problem, accelaration will not damage the ship/ the passengers
  • There is a source of abundant energy on the ship, consider it enough for almost any purposes.
  • FTL is not possible.
  • All used technologies should be extrapolations of current technology or plausible inventions within the next millenium. Basically everything that does not break the current understanding of natures laws. And yes I am aware that 1. and 2. are breaking these, at least probably.
  • Collision avoidance and all other problems with very high velocities can be neglected.

Under these conditions, what would be the fastest spaceship engine or the one with the quickest acceleration? What could bring that sleek ship up to a significant fraction of c and bring it to (relative) halt again?


closed as too broad by JBH, sphennings, L.Dutch - Reinstate Monica, Azuaron, Amadeus-Reinstate-Monica Sep 6 '17 at 21:22

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  • $\begingroup$ When you say Structural Integrity is not a problem, does that mean that no matter how I choose to accelerate my ship, the ship will not sustain any damage? Are we to assume that the ship is made of unobtainium? $\endgroup$ – Aify Jul 31 '15 at 21:05
  • $\begingroup$ Yes, basically. Its a technology called 'stasisfield' and is used to avoid dealing with these effects :) $\endgroup$ – user6415 Jul 31 '15 at 22:37
  • $\begingroup$ believe it or not your spaceship doesn't have to move at all except tumbling with excitement, instead your vessel erects a wrap bubble and ride it across space. However since you stated that FTL is not possible there will be some major issues lol. $\endgroup$ – user6760 Aug 1 '15 at 0:14
  • $\begingroup$ Why don't you just use the "source of abundant energy" directly for thrust, possibly adding extra propellant to the exhaust as an "afterburner"? $\endgroup$ – DOS4004 Apr 1 '17 at 22:28
  • $\begingroup$ 40 generations of development time makes this far too broad a question, in the real world that takes us from the iron age to the space age and we seem to be following Dyson's predictions of accelerating acceleration of development I don't think anyone can predict where we'll be by 3000A.D. $\endgroup$ – Ash Sep 6 '17 at 14:13


Just throw them out the back and hang on.

If you don't need to worry about acceleration, structural integrity, or energy storage then you should just ride the shockwave of successive nuclear detonations. See Project Orion, and more generally nuclear pulse propulsion, for more information.

  • 2
    $\begingroup$ It never ceases to amaze me how often the answer to the "make a ship go fast" question is Project Orion. Every. Time. :) $\endgroup$ – Green Jul 31 '15 at 23:37
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    $\begingroup$ @Green Yeah, it's one of those things where people walk into this community and we're like "What do you mean you don't know about Project Orion? Everyone knows about Project Orion". $\endgroup$ – Samuel Aug 1 '15 at 0:07
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    $\begingroup$ Scott Manley does such a good intro to that project. youtu.be/vwrLR2kv5KA $\endgroup$ – Green Aug 1 '15 at 0:46

New answer:


AKA Nuclear or Antimatter Photonic Rockets

If you don't have any structural integrity or energy storage, you can use powerful nuclear reactors to generate intense temperatures, to the point where blackbody radiation will generate thrust.

An antimatter-matter powered photon rocket would obtain the maximum c specific impulse; An antimatter-matter annihilation version could be used for space flight.

However, note that the acceleration is practically garbage for this method. It does, however, theoretically have the highest top speed, even if acceleration and deceleration is bad.

If the acceleration is really that important to you, please see my "old answer" (Or Samuels).

Old answer below:


AKA Nuclear pulse rockets.

Given your listed requirements, I believe that a Nuclear Pulse Rocket would be the answer to your question. You can use nuclear explosions to generate the thrust required.

EDIT: Samuel beat me to posting it, but as an alternative:

Consider the Antimatter version of the NPR. When applied to the same theory, you should be able to attain even higher accelerations and speeds.

  • $\begingroup$ It looks like NAPR only has a specific impulse of c/1000, or am I reading that wrong? The maximum of 0.1mm/s^2 is pretty awful acceleration :) For NPR, as far as I understand it the antimatter version's only difference is that is allows for smaller charges to be used, not that it allows for "even higher accelerations and speeds". Was there a different method you were referring to? $\endgroup$ – Samuel Jul 31 '15 at 22:35
  • $\begingroup$ @Samuel I did mention that the acceleration is bad for NAPR. The win situation with the NAPR is that you have A LOT of "fuel" since you're just using blackbody radiation, and since there's no size or energy constraint you can have ludicrously massive power generators (nuclear OR antimatter). With the NPR, antimatter explosions creates a lot more energy, thus it should accelerate faster (Assuming the charge size remained the same). $\endgroup$ – Aify Jul 31 '15 at 22:43
  • $\begingroup$ @Samuel However, do correct me if my logic is wrong. $\endgroup$ – Aify Jul 31 '15 at 22:44
  • $\begingroup$ As you say, no size or energy constraint, so having "a lot of fuel" is a given in either case. That is, efficiency doesn't really come into it. How do you get more energy than E= mc^2? You're again just talking about efficiency by limiting the charges to the same size. I'm not saying it won't work, I'm just saying it's not better/faster. $\endgroup$ – Samuel Jul 31 '15 at 22:53
  • $\begingroup$ @Samuel Is it really not? I'm not sure I understand what you mean (But I'm no science major so I'm probably wrong). The NAPR has the highest top speed because an ideal photon rocket (which we can assume, because of stated requirements), is dependent on the ratio final and initial mass - super high initial mass with a very low final mass would allow you to reach speeds extremely close to the speed of light. The 19 to 31 km/s exhaust velocity of the NPR isn't even close to the speed of light exhaust the NAPR has, and IIRC you can't go faster than your exhaust velocity. $\endgroup$ – Aify Jul 31 '15 at 22:59

If there is truly unlimited energy, I wouldn't use a brute-force approach like nuclear pulse engines. The limiting factor will be reaction mass or fuel, so the exhaust speed of the engine needs to be as high as possible.

Particle accelerators to bring your reaction mass close to lightspeed?

  • $\begingroup$ It will probably take lots of room and be slow to accelerate though. And also magnetic monopoles might be helpful if the OP's scifi has it. $\endgroup$ – Mathmagician Aug 26 '17 at 5:56

I'm going to assume that the thrust we're talking about involves throwing large amounts of reaction mass out the back as hard as possible. If you can thrust at 1g continuously, IIRC, you will reach an appreciable fraction of c within a matter of weeks.

Given unlimited energy to accelerate that mass, the question then becomes where that reaction mass comes from. Carrying it with you isn't practical solution because you have to accelerate everything you carry with you as well. But I think there are two solutions that stretch physics without breaking it entirely.

1) the Bussard Ramjet. Carry enough reaction mass to accelerate to critical velocity and decelerate again, and collect interstellar hydrogen for the rest. The problem as I see it is that the hydrogen will already be moving "backwards" so your gains will diminish over time.

2) Assume mass is a vector, not a scalar, and use handwavium to rotate it into the complex plane. If inertia is a function of the real component you can carry extra mass and push it with real mass. Yes, this violates conservation of kinetic energy. But since physicists are now saying that the expansion of space may violate conservation of energy, I'm okay with that.

For an energy source, consider using a microsingularity. Nukes only convert a tiny fraction of matter into energy. A black hole converts much more. Also, black hole physics are less well known than physics in flat space, so you have more room for handwavium.


One of the fastest known Propulsion Technologies is "Magneto Plasma Dynamic" propulsion. But it is not powerfull. It is more or less the opposite to Chemical Rockets, which are powerfull, but not fast. I think it could be possible to combine these 2, espacally under your circumstances


The next millennium? That's 40 generations, you're probably talking about multiple paradigm shifts there, almost anything you've every read in a hard science fiction novel is up for grabs and possibly some of the stuff that seems really soft too, that's not really useful.

To give you some kind of answer I'll concentrate on what I think of as the top end and incidentally a piece of tech that answers the points on collision, structural integrity, acceleration and energy: Gravity manipulation, in David Brin's Earth "Cavitronics" the fictional science, based in current ongoing research (which is only in the theory/math phase admittedly), of manipulating the fabric of space-time directly to create artificial gravitational singularities with virtual or real mass. The applications of this piece of technology are vast and varied, although the book pretty much only uses the powerplant side of it, Isaac Arthur does a really good video about that use of artificial black holes here and discusses some weird propulsion ideas using them here.

I'm going to go a different way; if you can create huge virtual masses you can create huge and very real gravity fields, you use one field to keep the ship together and stop everything in it from going squish a couple of laterally displaced fields to catch potential impactors, and a really large forward displaced field to create accelerations similar to those seen in gravity slingshots except vastly larger. If you can fully compensate for acceleration stresses, and you probably can, you could go from a standing start to the brink of C instantly, and back again.