# Fission Fragment Rockets **URL:** \ **Category:** Factual Questions\ **Created:** [June 23, 2026, 6:09pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139 "2026-06-23T18:09:27Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![spacecadet86](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@spacecadet86](https://boards.straightdope.com/u/spacecadet86)\ **Post date:** [June 23, 2026, 6:09pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/1 "2026-06-23T18:09:27Z")
Just how practical would fission fragment rockets be in the distant future for reaching 550AU or greater distances in human lifetimes? Would fission fragment reactors ever be a practical source of energy generation on land? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 23, 2026, 6:10pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/2 "2026-06-23T18:10:23Z")
I don’t know; what’s a fission fragment rocket? ---
**Author:** ![spacecadet86](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@spacecadet86](https://boards.straightdope.com/u/spacecadet86)\ **Post date:** [June 23, 2026, 6:11pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/3 "2026-06-23T18:11:21Z")
> **[Fission-fragment rocket](https://en.wikipedia.org/wiki/Fission-fragment_rocket)** > > The fission-fragment rocket is a rocket engine design that directly harnesses hot nuclear fission products for thrust, as opposed to using a separate fluid as working mass. The design can, in theory, produce very high specific impulse while still being well within the abilities of current technologies. > In traditional nuclear thermal rocket and related designs, the nuclear energy is generated in some form of reactor and used to heat a working fluid to generate thrust. This limits the designs to t... Frrankly, I’m not sure myself. ---
**Author:** ![Horatius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/horatius/32/3572_2.png) [@Horatius](https://boards.straightdope.com/u/Horatius)\ **Post date:** [June 23, 2026, 6:29pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/4 "2026-06-23T18:29:28Z")
> [@spacecadet86](#): > > Would fission fragment reactors ever be a practical source of energy generation on land? I feel like a power reactor that spews out fission fragments as a matter of its design would be a non-starter. We’ve got lots of new possible reactors that would be far safer, so that even if they were less efficient, we’d still prefer them. ---
**Author:** ![spacecadet86](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@spacecadet86](https://boards.straightdope.com/u/spacecadet86)\ **Post date:** [June 23, 2026, 10:06pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/5 "2026-06-23T22:06:19Z")
How do you feel about it as a form space propulsion? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 23, 2026, 10:17pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/6 "2026-06-23T22:17:13Z")
Using a nuclear reaction to heat something up and spew it out the back is a reasonable near-future technology idea. But there’s no reason why the stuff you spew out should be the spent fuel itself. In fact, it’s a fairly bad choice. The efficiency of a rocket depends on the speed of the exhaust. The temperature of a substance is, basically, the amount of energy per particle. So for any given temperature, heavier particles will be moving slower than lighter particles. Fission byproducts tend to be fairly heavy, as atoms go, so they’ll be going slower than, say, hydrogen. So it’d make more sense to use the fission reaction to heat up hydrogen, and spew it out the back. And for a power-generation reactor, you probably don’t want to be spewing anything. We can already make fission reactors that work just fine for power generation. The only problem with them is that people are irrationally scared of nuclear reactors, and that would apply at least as much to a fission-fragment reactor. ---
**Author:** ![Horatius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/horatius/32/3572_2.png) [@Horatius](https://boards.straightdope.com/u/Horatius)\ **Post date:** [June 23, 2026, 10:24pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/7 "2026-06-23T22:24:24Z")
> [@Chronos](#): > > So for any given temperature, heavier particles will be moving slower than lighter particles Yes, but they’re claiming a specific impluse (Isp) that’s really high. That’s a measure that takes into consideration all those factors. I guess the real question is how long they can maintain that, as compared to other nuclear engines. The science seems sound, but as usual, the engineering might screw us. ---
**Author:** ![Ponderoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponderoid/32/19221_2.png) [@Ponderoid](https://boards.straightdope.com/u/Ponderoid)\ **Post date:** [June 23, 2026, 10:48pm UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/8 "2026-06-23T22:48:10Z")
> [@spacecadet86](#): > > 550AU or greater distances Are you by chance thinking about putting a space telescope out that far and using the Sun for gravitational lensing? ---
**Author:** ![spacecadet86](https://avatars.discourse-cdn.com/v4/letter/s/e9a140/32.png) [@spacecadet86](https://boards.straightdope.com/u/spacecadet86)\ **Post date:** [June 24, 2026, 12:23am UTC](https://boards.straightdope.com/t/fission-fragment-rockets/1031139/9 "2026-06-24T00:23:15Z")
Yes. I am sad none of us alive today will ever be able to experience the discoveries of such far out missions.