Spike engines aren't that great not because of cost, but because their supposed benefit is largely eaten by added weight and cooling complexity in most designs, so the entire thing isn't worth it, usually. Meanwhile, things like nozzle extensions can provide 80% of their benefits for 20% effort. The article is light on information on whether they solved typical issues, and what exactly they traded for the efficiency.
A favorite of mine is Thrust Augmented Nozzle or TAN, where, at low altitudes, extra propellant is injected into the nozzle (not the chamber) to prevent flow separation and to increase thrust, albeit at lower specific impulse. Think of it as a bit of an afterburner for a rocket engine.
>A favorite of mine is Thrust Augmented Nozzle or TAN, where, at low altitudes, extra propellant is injected into the nozzle (not the chamber)
and that gets close to my favorite - air-augmenting regular rocket engine. Very simple and bumps up your thrust and ISP up to 2-3x http://www.astronautix.com/g/gnom.html
A similar, but much more useful thing for the future seems to be LANTR, a.k.a. the only practical nuclear rocket engine we know of. At the very least it doesn't waste all oxygen from water mined on other bodies of our solar system like traditional pure-hydrogen NTRs do. And its average impulse density of propellant is way higher, of course.
This is solving a different issue. Aerospike engines deal with the fact that rocket engines need to be different in an atmosphere vs in vacuum. Nuclear thermal rockets (NTRs) are not really envisioned for being used in the atmosphere, the risk of a blowup is too high. I consider them a wonderful technology which could really open up new possibilities, but we first need to nail down the challenge of affordably getting to orbit.
Except LANTR solves a very similar problem, which is the requirement for different performance parameters at the beginning and at the end of a rocket flight (high thrust and propellant density in first stage, low thrust and high Isp in the second stage) in a single stage vehicle. And I mentioned it because its way of operation is very similar to the thing describe above, i.e., injecting something extra into the nozzle, so it reminded me of it. Not because I consider it comparable to using aerospikes on Earth (I'm not convinced that's useful at all) but because this is one of the few such variable Isp options that appears to provide any useful benefits at least somewhere (even if it's not on Earth).
The problem with NTRs in the atmosphere isn't the risk of blowup, it's that their thrust/weight ratios are just terrible. And in the atmosphere, it's early in the launch, so Isp is not important.
I don't think it necessary true that NTRs have terrible trust/weight. Depending on what your 'fuel'. You potentially lose ISP of course, but your Thrust would go up.
It also depends on the engine configuration, you could do much better in terms of weight and density.
Thrust of a rocket is a function of nozzle throat and exit areas and thrust chamber pressure, and is (ideally) independent of the density of the propellant.
Engines used on a first stage typically have high thrust, but that's because they're designed with large throats, not because of the propellant they use.
A dense propellant does allow the pumps to be smaller. But I don't think that's going to help much in a NTR, since the reactor itself is heavy. The problem with an NTR is the difficulty of getting high mass flow past very thin fuel elements (the thinness necessary for high heat transfer) without destruction of the fuel elements by that flow. The thin fuel elements also ensure some fission products will escape and be carried off in the propellant.
I had to look that up. This was the nicest explanation I found, but if you recommend a different one, please post. I also found stuff on Kerbal, but I'm not clear how real that is.
Umm, could you help educate us. How do you know/state this so confidently?
The way I see it, there are roughly infinite previously non-constructible parts that by definition we as a society couldn't test. Now that we can construct these new parts, shouldn't we test them and then use them if they are better/cheaper?
There have been rocket engines built by stacking carefully made thin metal slices (with holes cut for coolant channels) and then using diffusion bonding to join them into a single monolith. I could see 3D printing being used for this instead, and perhaps more cheaply.
I see, you just don’t believe that additive manufacturing is a truly new paradigm and that there are parts that can be constructed now that were previously just non-constructible.
Why? Even if a additive manufacturing is slower than traditional manufacturing, 3d printers can be mass-produced (unlike assembly lines and skilled engineers).
I assume that all of the support gear (turbopumps, etc.) are not 3D printed. I'd be super impressed if the fuel and oxidizer injectors were printed.
Anyway, there's a bunch of stuff they're not including in this statement; it's like "we can print your car's engine block" but glosses over the additional whirly-sparky stuff that turns a lump of metal -- even if it is in the right shape -- into a functional engine.
