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I am trying to design a single seat fighter capable of achieving (or getting close to) hypersonic velocities at high altitude. In this universe I am assuming compact fusion is a common technology. The aircraft would use some manner of compact inertial aneutronic fusion generator to create heat and electricity for the craft, an intake system would direct compressed air around a heat exchanger like a high intensity electric arc (or the reactor walls itself) superheating it and expel it out the back. A magnetic duct system would be used to direct the exhaust so that no plasma would come in contact with the aircraft itself.

I am by no means a physicist or aerospace engineer so I would appreciate it if some could let me know if this would actually work and achieve any kind of advantage over conventional fossil fueled aircraft.

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    $\begingroup$ You should ask specific question, otherwise you'd draw wrath of moderators. Anyway, if you want to go nuclear and make it simple I'd suggest something like: en.wikipedia.org/wiki/Project_Pluto $\endgroup$
    – Shadow1024
    Jul 1, 2019 at 15:28
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    $\begingroup$ As noted, please propose a specific technology that we can 'reality-check' for you. Otherwise, this is just pretty open ended, and this site discourages discussion. I am not aware of any proposals for a fusion aircraft reaction, I think you would be much wiser going with an aircraft fission reactor. $\endgroup$
    – kingledion
    Jul 1, 2019 at 15:34
  • $\begingroup$ Here is another link that might be helpful. $\endgroup$
    – Bilbo Baggins
    Jul 1, 2019 at 15:42
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    $\begingroup$ When this fighter loses a battle, will the falling debris consist of hot-enough-to-start-fires and radioactive fragments? $\endgroup$
    – user535733
    Jul 1, 2019 at 16:40
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    $\begingroup$ Voting to close as unclear. At this time, this question is not ready to be answered (which is why the one answer you have isn't actually an answer). Aircraft dynamics are all about thrust vs. weight, so if your fusion plant is light enough, of course your plane can fly. Do you need help inventing the rules concerning your fusion plant? Do you need help devising an engine that a fusion plant could drive? What, really, is your question? $\endgroup$
    – JBH
    Jul 1, 2019 at 16:53

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Not a Physicist here, but I do actually know a few things about jets...

So lets assume your fighter is designed aerodynamically to be able to handle supersonic speeds, that's pretty easy compared to your alternative power supply, since we have had supersonic jets for 60+ years.

As stated in the Question, that kind of propulsion system is theoretically possible but wildly impractical. For one thing, if this is a fighter and you are worried about Plasma level heat in the exhaust, no level of chaff will help you evade heat seeking missiles. What you have to do to to shield from the plasma is going to add significantly to the weight, which will slow you down. But it is not totally impossible to power a very fast jet with a small fusion engine rather than with lots of dangerously volatile fuel.

The big problem you are running into is Thrust. Can you get enough air/fuel/whatever into the aircraft and expel it out of the back end fast enough the Sir Isaac Newton gives you a kick in the pants Via his 3rd law. Currently this is done by what Aircraft Mechanics like to call Suck, Squeeze, Bang, and Blow. Jet Engine Sucks in a large Volume of air with a big fan (SUCK), uses the rotational energy of the fan to agitate the air and compresses it (SQUEEZE), Adds a generous amount of fuel and ignite it to add even more energy to the mix (BANG), and then let it all out the back end to create thrust (BLOW). Here is a bit about that: Suck, squeeze, bang, blow There are many private aircraft around the world that regularly cruise at .8 Mach and the new Bombardier Global 7500 which can get up to .92 Mach using engines that rely on that technology.

That's pretty much how a modern jet works. Now your little nuclear reactor would probably work best to supply the "Bang" Phase. It would be able to super heat the air at that phase rather than relying on a fast chemical reaction. You might be able to do that without the necessity of a plasma arc and all the attendant problems that go with playing with stuff that can reduce things to their component atoms.

In Short, use your special power supply in a way to replace a phase of the regular jet engine, and make sure you aren't making your pilots glow in the dark or dumping dangerous stuff everywhere you go. This is far more realistic than a method using heat exchanges or direct outflow of plasma

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  • $\begingroup$ The big advantage of arcjets is that they can reach extremely high temperatures. This is useful for rockets, as higher temperatures give you higher specific impulse aka fuel efficiency (at the cost of requiring more power for the same thrust). However, this isn't very useful in a jet engine, which is already temperature-limited by the need of not melting it. So more conventional methods are indeed more adapted. $\endgroup$
    – Eth
    Jul 1, 2019 at 16:57
  • $\begingroup$ This is a non-answer. The OP asked, "let me know if this would actually work," which you did not address (unless you were trying to express a frame challenge?). $\endgroup$
    – JBH
    Jul 1, 2019 at 16:57
  • $\begingroup$ @JBH Not really a non-answer to me, but I'm biased. My overall point is the OP wants to use a nuclear engine in a jet., I'm showing how something like that could "realistically work" within what we already have as modern technology. Apologies If I wasn't real clear. I'll try to think of how exactly to edit. $\endgroup$
    – Paul TIKI
    Jul 1, 2019 at 17:07
  • $\begingroup$ @Paul Tiki I think your answer was perfectly adequate. All I wanted to know is if the nuclear reactor could take the place of fuel and provide comparable or greater performance over conventional fuels given the reactor was light enough. I should have written my question more carefully, something I will keep in mind in the future as this was my first post. $\endgroup$
    – Boo Radley
    Jul 1, 2019 at 17:17
  • $\begingroup$ @BradleyKnauer Keep asking questions and pay attention to the comments. If you ask a question and it gets closed, don't take it personally. Good Quality questions generate really cool and creative answers. Welcome to the party. $\endgroup$
    – Paul TIKI
    Jul 1, 2019 at 17:26
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You want the good old Bussard ramjet.

https://en.wikipedia.org/wiki/Bussard_ramjet

Bussard[1] proposed a ramjet variant of a fusion rocket capable of reasonable interstellar travel, using enormous electromagnetic fields (ranging from kilometers to many thousands of kilometers in diameter) as a ram scoop to collect and compress hydrogen from the interstellar medium. High speeds force the reactive mass into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy as rocket exhaust opposite to the intended direction of travel, thereby accelerating the vessel.

Bussard proposed it for use in interstellar space but it would work even better in the high atmosphere because of more raw material to collect and throw back. Heating the intaked (intook?) materials to plasma would allow you to confine them magnetically as you and Bussard propose.

You would need to get up to speed with some other method, as is true for normal ramjets. Once up to speed you might adjust your energy delivery according to the density of the atmosphere you traversed, keeping the plasma a given consistency.

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  • $\begingroup$ I’d question whether you can get up to the requisite speeds for fusion due to compression to start. I was under the impression that Bussard ramjets required speeds on the order of tens of kilometres a second in order to operate or they just acted as big magnetic parachutes. $\endgroup$
    – Joe Bloggs
    Jul 3, 2019 at 9:23
  • $\begingroup$ @JoeBloggs - I think the Bussard ramjet does not produce fusion by compression because that would be serious, serous compression. Plus who knows how fusable the stuff you collect in your hopper might be. You have a fusion engine with a fuel source, just like a ramjet. The scoop is just to collect mass which you then heat up and hurl behind you. I suspect also Bussard needs more speed because of how thin the mass is up in space. Fusion ramjet is just a regular ramjet minus gaseous exhaust from burning jet fuel, $\endgroup$
    – Willk
    Jul 3, 2019 at 12:15
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    $\begingroup$ From your quote: “High speeds force the reactive mass (interstellar hydrogen) into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs.” If all the scoop is for is to collect reaction mass then in-atmosphere you may as well use a metal bucket, at which point it’s just a regular ramjet with a novel approach to superheating the air and bears no resemblance to a Bussard ramjet but the word ‘ramjet’... $\endgroup$
    – Joe Bloggs
    Jul 3, 2019 at 12:26
  • $\begingroup$ That will teach me to read my links! You are right. $\endgroup$
    – Willk
    Jul 3, 2019 at 13:12
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A feasible fusion method could be so-called "Focus Fusion", which is an aneutronic fusion process using hydrogen-boron fusion. It creates a very narrow (focused) plasma stream that goes through a "reverse particle accelerator", which generates electricity by decelerating the ions, leaving very little excess heat.

The process only works in a vacuum chamber, so you can't just use the plasma stream as exhaust, but the electricity could power jet engines by heating air and powering turbines.

So far, the method is only theoretical, but the theory seems to be sound, though critics say it will be very difficult to achieve the necessary precision.

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