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Show me the Isp figures on these proposed nuclear thermal propulsion designs. Project Rho (the best source for this kind of stuff) is listing various nuclear t
by CydeWeys 6y ago
Show me the Isp figures on these proposed nuclear thermal propulsion designs.
Project Rho (the best source for this kind of stuff) is listing various nuclear thermal designs with high 3 digit Isps, e.g. 900s for this design: http://www.projectrho.com/public_html/rocket/enginelist2.php#ntrsolidcore http://www.projectrho.com/public_html/rocket/enginelist2.php...
If that's realistic, it'ss certainly an improvement over the 348s of the Falcon 9 second stage (and that figure is representative generally of chemical rockets), but it's not a sea change.
- JoeAltmaier 6y agoNuclear rocket may be able to fire continuously, e.g. accelerating halfway to Mars then braking the 2nd half. Chemical rockets can never do that.
- baq 6y agothis requires Isp in the high 10-100k, likely sub-1M range (think particle accelerators) instead of sub-1k (think very high temperature nuclear gas heaters).
- ericd 6y agoDo you know where current ion thrusters sit in terms of isp?
- CydeWeys 6y agoSome of the better ones that are actually in use get in the range of low thousands, like ~3k or so, but keep in mind that the thrust is so low that even just getting to the Moon takes months. They're only suitable for unmanned craft, and even then, only after you get into orbit and if you don't care about taking potentially many years to get somewhere outside the local SoI.
- ericd 6y agoAh, thought it was higher, thanks!
- JoeAltmaier 6y agoOh I get it. Thanks!
- bryanlarsen 6y agoNuclear-thermal rockets still require a "fuel tank". They heat up a reaction mass (typically LH2) and then throw it out the back. So they can't burn continuously, they'd run out of reaction mass. You're thinking of nuclear-electric, which uses an ion engine just like solar does. It can burn continuously, but it has incredibly low thrust. edit: (Note that I'm basically saying the same thing as baq, just in a different way) edit 2: except for the part where I'm wrong.
- penagwin 6y agoAnd ion thrusters still need "fuel" usually in the form of xeon. Okay it's not normal "fuel" but you still have a limited supply that can be exhausted. EDIT: The word is "propellant"! Thanks for the correction!
- CydeWeys 6y agoThe word you're looking for is propellant. In chemical rockets, the fuel and propellant are the same thing (to be precise, the combustion byproducts of the former comprise the latter). In nuclear fusion and ion thrusters, they're different. In all cases, Newton's second law means that you are using up propellant to generate an equal and opposite reaction in the direction you want to go, and thus the amount of "going" (delta-v) you can do is limited.
- penagwin 6y ago"Propellant" - Thank you! I knew it wasn't technically fuel, but it's still being expelled (as you mention, if you don't expel mass with a different velocity relative to the rocket, then you won't be able to accelerate a rocket)
- bryanlarsen 6y agoYah, I should have deleted my comment rather than editing it.
- DennisP 6y agoMaybe not but it's pretty significant. From 350 to 900 takes the payload mass fraction to LEO from 6% to 34%, so SSTO would be practical (though I wouldn't expect to see this for launch). Or, with a 10% mass fraction, the delta-v goes from 7.9 km/s to 20 km/s. Convenient calculator: http://www.quantumg.net/rocketeq.html http://www.quantumg.net/rocketeq.html
- greesil 6y agoOr just use a bunch of Hall thrusters, and not lob fissile material into Earth's atmosphere.
- ashtonkem 6y agoThe problem is that Hall-effect thrusters provide basically no thrust. Most Hall-effect thrusters provide millinewtons of thrust, with the best lab model producing 5.4N and the planned AEPS motor producing a theoretical max of 2.356N. A single Merlin 1D motor produces 690kN of thrust at sea level. You would need over 290,000 AEPS engines to match one Merlin 1D engine. At 40kW per engine, this would use 11.7 gigawatts to run, so you’d also have to add in the weight for a dozen or so nuclear power plants. Keep in mind that the falcon 9 uses nine Merlin 1D motors for the first stage.
- CydeWeys 6y ago> You would need over 290,000 AEPS engines to match one Merlin 1D engine. Even worse, even an infinite number of AEPS engines would not match a single chemical rocket engine of any type, because Hall effect thrusters have a thrust-to-weight ratio of significantly less than 1. So it can't even lift itself off the pad, let alone anything else.
- ashtonkem 6y agoIf I’m doing my math right, the 25kg AEPS engine would need to produce 245N to even hover without any payload. So it’s theoretical max is about 0.9% of what’s needed to even lift itself.
- Gravityloss 6y agoYeah, with the high dead weight of nuclear propulsion, it's not clear cut where it's advantageous and how much.
- Gravityloss 6y agoIt's true. Specific impulse is only one part of the equation. Tankage mass is really bad with pure hydrogen, which is usually the reaction mass of choice. Hydroden's density is abysmally low. And the reactor weighs a lot too, compared to chemical rocket engines which have very high power densities and thrust to weight ratios. So nuclear thermal would tend to only win over chemical propulsion for long duration low thrust large delta vee missions. But then, at the top end, it's squeezed by solar electric propulsion which has much higher specific impulses (but even worse thrust to weight ratio). Solar cells and power electronics have improved a lot since the sixties...
- jjoonathan 6y agohttps://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA (actually built and tested -- vids on youtube -- nixed in 1973) Vac Isp: 841s Sea Isp: 710s
- CydeWeys 6y agoDo we have any concrete figures on an entire rocket designed to use this engine? Like, mass fraction, etc.? Because if you look at the engine in isolation it seems good, but that's ignoring all the shielding you'll need and the huge fuel tanks because your propellant density is so low.
- jjoonathan 6y agoRight. I haven't read enough to have an informed opinion on the matter. Hopefully someone here has.
- CydeWeys 6y agoIt may not be something we know yet -- based on reading the available materials it doesn't seem like we got past the stage of designing the engine on its own.
- stqism 6y agoI don’t know if any real rockets were built, but according to this old video we’d see a reduction of weight due to the absence of heavy chemical fuel. https://youtu.be/eDNX65d-FBY https://youtu.be/eDNX65d-FBY
- CydeWeys 6y agoThe fuel mass is already incorporated into Isp. Hydrogen being less dense causes a problem (not solves one) because the fuel tanks need to be much larger for the same propellant mass, and thus you're adding a lot of structural mass.
- sq_ 6y ago
- EthanHeilman 6y agoYou seem pretty knowledgable about this stuff. Wouldn't an increase of a factor of 3 in ISP, result in a non-linear increase in performance (deltaV) due to less fuel requirements? How important is it that nuclear thermal propulsion can use water as most of its reaction mass for in situ utilization (ISRU)?
- sudosysgen 6y agoIf the engine had the same mass, it would. But these engines require incredibly heavy shielding.
- CydeWeys 6y agoYup, that's exactly the problem. Once you have it incorporated into an overall rocket (and aren't just looking at the engine in isolation), the gains are smaller. And the engine and fuel are significantly more costly and risky. The fuel to fill up a Falcon 9 costs only a few hundred thousand bucks. The fuel to fill up a nuclear thermal rocket ... would be astronomical in cost.
- EthanHeilman 6y ago>The fuel to fill up a nuclear thermal rocket ... would be astronomical in cost. Perhaps I have it wrong, but my basic understanding of a nuclear thermal rocket is that it uses a reactor to energize a reaction mass like water. The water should be cheap, and the nuclear fuel should be expensive, but how much of the nuclear fuel would be reusable? Would the total lifetime cost be less than a Falcon 9 assuming you replaced the reaction mass?
- CydeWeys 6y agoNuclear fuel is very expensive to process and manufacture, and is not reusable. In particular, for these nuclear rockets, you'd never even be returning it to the Earth, so it is definitely not reusable for that reason alone. The reactor itself is going to be very expensive too; doing anything nuclear just fundamentally ramps up the cost a lot vs building e.g. a Merlin engine. The hydrogen propellant is cheap enough, sure. I'm specifically talking about the nuclear fuel itself as being expensive.
- csours 6y agoWhat ISP is: https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse
- deleted 6y ago[deleted]