I am wondering what type of engine would most likely be available in the next 80 that can constantly accelerate a spacecraft at 1G. Preferably, it could accelerate it for 4 weeks.

The engine could be fission, fusion or ion. If there's another one, please do mention it!

My biggest gripe is the fuel mass requirements. If we use the rocket equation below: $$ {\Delta} v = I_{sp}g_0ln{\frac{m_0}{m_f}} $$ we can calculate the fuel mass requirement for a rocket with an isp of 900, acceleration of 1g (9.80665 ms-2), dry mass (aka $m_f$) of 120 metric tonnes (or 120,000 kg) and a ${\Delta}v$ of 23724247.68 ms-1 (or about 7.91% of the speed of light), calculated with ${\Delta}v = a{\times}t = 9.80665 {\times}(4{\times}7{\times}24{\times}3600) = 23724247.68 ms^{-1}$.

Rearranging the equation for $m_0$, we get: $$ m_0 = m_f {\times}e^{\frac{{\Delta}v}{I_{sp}g_0}} $$ Plugging in the values we got above, we get this huge number: $$ m_0 = 120000 {\times}e^{\frac{23724247.68}{900 {\times} 9.80665}} = 120000 {\times} e^{2688} $$

Anyone who knows something about exponents knows that $e^{2688}$ is a huge number. This means that the fuel mass requirements for an engine with an isp of 900 (I used a theoretical number for a nuclear thermal engine) for a craft constantly accelerating at 1G for 4 weeks is astronomical (pun slightly intended).

I know that a more efficient engine (e.g. one with an isp of 1,000,000, which gives $m_0 = 120000 {\times} e^{2.4192} = 120000 {\times} 11.2368... = 1348423.947$ kg) would be far better.

I know in The Expanse, the engines are fusion, but:

  1. I don't know if we'd have fusion tech like that within the next 80 years
  2. what fuel would an engine like that need?

So, would fission, fusion or ion engines be better, and which would be able to produce the thrust needed for 1G of constant acceleration for 4 weeks? Also, would these engines' fuel mass requirements be calculated with the rocket equation?

Thank you all for your time!

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    $\begingroup$ No one can predict what will happen in technology and engineering in 80 years. You are asking for opinions, which is off-topic here. $\endgroup$ Commented Jan 30 at 12:52
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    $\begingroup$ @OrganicMarble maybe one can write about technologies which are coming and more importantly, which ones could cover the requirements needed. Ofcourse we do not know when they will exist. We cannot know if it will take 50 years, 80 years or 100 years. $\endgroup$ Commented Jan 30 at 13:17
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    $\begingroup$ The engines from Expanse are powered by plot (per authors 'powered by efficiency not fusion') and most likely nothing like them can exist. Which is fortunate since an engine that can accelerate a vehicle at 1G for 4 weeks is also a civilisation ending kinetic kill weapon. $\endgroup$ Commented Jan 30 at 13:21
  • $\begingroup$ Yeah, this is a thing we need to consider. Maybe we should instead just try and figure out what type of engine we'd need. $\endgroup$
    – Tom
    Commented Jan 30 at 13:21
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    $\begingroup$ This question would probably get a better answer on Worldbuilding.SE with the "Hard Science" Tag. $\endgroup$
    – codeMonkey
    Commented Jan 30 at 16:09

2 Answers 2


"Realistically", the only engine type that might be able to achieve performance levels near this and which currently has a non-zero TRL, is probably Project Orion style nuclear propulsion.

It basically works by detonating shaped nuclear charges against a large plate that pushes the spacecraft. It does so continuously: it needs a lot of nuclear charges to operate, but it is both high-thrust and very high ISP (theoretically).

This concept was rather worked on extensively during the cold war era when serious high-level USA military leadership thought there was a decent chance that the conflict against the Soviets would continue and eventually escalate into an actual shooting war in space or that the space race would continue onwards.

Plans were drawn up and there were even actual tests done with conventional explosives that showed controlled flight of the test article; unclassified video here.

The issue is that it has many issues like:

  • Highly nuclear-resistant plate
  • Thermal buildup in the ship
  • The political reality of having a ship that has literally thousands if not more of nuclear weapons in orbit
  • Nuclear Backsplash
  • ...and many more

While I am reasonably confident that the engineering problems like the blast plate or the thermal dissipation system could be solved within the next 80 years because there are no true "physics dealbreakers" in the proposal, predicting the political reality and the "motivational reality" is much more difficult:

With the current global stance towards anything nuclear being what it is, I suspect it will be quite a long time or something really significant needs to happen before any nation (or world-government) signs off on producing tens of thousands of nukes for an expensive science project. We'll either need to be far more utopian and have the cash to burn, or far more dystopian and have a cavalier attitude towards nukes.

Research here is also rather hampered because especially the closer it gets to literal bombs, the more taboo it gets and the more red tape is involved (probably for a good reason). Project Orion came about, in part, because everyone thought everything nuclear was really really cool: They had a new fancy nuclear hammer, and every problem looked like a nukable nail. Like, they made plans to do all sorts of wild things with nuclear bombs such as using them for land excavation or build nuclear trains / planes that could fly forever and would usher us into a new golden age of prosperity.

That said, despite the general current taboo about nuclear, there has been a recent resurgence in nuclear interest. NASA is actively working on nuclear-power solutions for next generation off-world bases and there are plans to revive work on nuclear-thermal rockets.

Unfortunately, while nuclear thermal rocket technologies would provide a huge leap forwards for our space capabilities since they'd let us get chemical-rocket levels of thrust at ISP values of >1000, this is nowhere close to the handwavium-powered drives of the Expanse universe.

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    $\begingroup$ Orion, with a specific impulse on the order of 30,000 m/s, gets nowhere near the performance level OP is asking about. A generous 10:1 mass ratio yields a ∆v of 69,000 m/s for a single stage, which works out to about 2 hours at 1G. $\endgroup$ Commented Jan 30 at 17:18
  • $\begingroup$ @RussellBorogove Several of Dyson's designs work out to tens of kilometers per second (galileo.phys.virginia.edu/classes/109.jvn.spring00/nuc_rocket/…). The trick is to build them really, really big. $\endgroup$
    Commented Jan 30 at 22:35
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    $\begingroup$ Don't ask me to take that paper seriously. How can you read nonsense like "Deuterium costs $100 per pound... This cost... is precisely what makes hydrogen bombs so uniquely efficient as weapons of mass murder" and not dismiss the rest out of hand? $\endgroup$ Commented Jan 30 at 23:01
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    $\begingroup$ lol. "weight of 3e5 bombs: 3e5 tons. total fuel cost of mission: 3e8 pounds deuterium, \$6e10" -- i.e. Dyson's positing that half the weight and half the cost of the bombs is deuterium. Over here in reality, thermonuclear warheads use a few grams of deuterium-tritium (approximate cost zero) in a package of 100-300kg costing on the order of \$10M. Even if you assume you can improve the mass ratio by a factor of 10 and the cost ratio by a factor of 100, Dyson's numbers are utter fantasy. $\endgroup$ Commented Jan 30 at 23:19
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    $\begingroup$ Ah, okay, my bad, the mass numbers are less silly then. The cost is still absurd. $\endgroup$ Commented Jan 31 at 0:40

A Pure Antimatter Rocket.

The "within 80 years" requirement makes a number of different technologies possible. For example, if we assume that by then we can manufacture a small army of space robots, we could use them to set up a mine on an asteroid. The mine could supply a solar panel factory. With that factory, the robots could create a very large solar panel array which could, in turn, power an antimatter factory. The antimatter factory could conceivably produce enough antimatter to power a space probe.

The reaction of 1 kg of antimatter with 1 kg of matter would produce 1.8E17 J (180 petajoules) of energy (ref).

A spacecraft that has been accelerated four weeks at 9.81 m/s would be traveling at...

$$v=at=9.81(4)(7)(24)(3600)=23732352 m/s$$

...which is 7.9% of the speed of light.

The kinetic energy per kg of that spacecraft would be...

$$KE/kg = 0.5 m v^2 = 0.5(1)(23732352^2)=2.81612E14J$$

Which would be about the equivalent of 0.001564513 kg or matter and 0.001564513 kg of matter. Of course, more than this would be needed after factoring in the efficiency of the antimatter rocket engine.

The nice thing about an antimatter-powered spacecraft (as opposed to, for example, a fusion powered spacecraft) is that you might not need a heavy and sophisticated reactor / rocket engine to convert your fuel into energy and then thrust. You really just need to invent something akin to a highly reflective parabolic mirror, and then arrange for the matter and antimatter to annihilate each other at the focal point.

While I do not want to trivialize the engineering challenges of containing and handling antimatter and the radiation produced by an antimatter reaction, it seems like there is a good chance that these problems could be solved within an 80-year timeframe.

  • $\begingroup$ Antimatter rockets are interesting due to their potential. One issue is that you need ALOT of energy to produce a small amount of antimatter. In 80 years, using technologies such as skyhooks, I bet we could get enough mirrors into orbit around the sun, to make a small Dyson swarm. Maybe that could provide the energy needed for the antimatter rockets. Isaac Arthur has a great video about it $\endgroup$ Commented Jan 31 at 6:01
  • $\begingroup$ Your antimatter drive still has to obey conservation of momentum. It not only has to provide KE for the ship, it also has to provide KE to the exhaust, even if that exhaust is pure photons. I have the (fully relativistic) calculations here: physics.stackexchange.com/a/345492/123208 At 7% of lightspeed, you can use the non-relativistic equation for KE. I have a graph that shows when the relativistic effects become substantial: physics.stackexchange.com/a/595175/123208 $\endgroup$
    – PM 2Ring
    Commented Jan 31 at 23:11

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