Aerospikes?

@deepfriedfrenchtoast after consulting your link, Wikipedia and ChatGPT, I reckon a full-flow cycle would be best for our purposes. Dual expanders tend to use hydro lox, but we use methalox for Neptune, though, on Nexus, which still runs on hydrolox, we’ll consider using a dual-expander cycle if it can still provide sufficient performance. It’s a great idea, and is simpler and easier to maintain, but FFSC is just better with methane, more scalable and is decently higher performance. In theory, we could afford to use DEC, we have enough performance, but it’s not as compatible and easy to work with as FFSC when it comes to methane. If you want, you can read this. (my ChatGPT conversation on the subject) It provides some interesting points and compelling arguments. https://chatgpt.com/c/ca7633b3-0612-43b9-bcdb-b50b7885340c

:)

@deepfriedfrenchtoast you’ve hooked me now lol, I’ll look into it a lot as soon as I can

@Fyrem0th Carbon fiber just doesnt have a melting point high enough to be used in the regenerative heat shield face. You'd need to pump a ridiculous amount of fuel infront of the rocket for the resin to not melt. You really can only use a high temp metal or ceramic for the job. I think a metalic heat shield is ultimately better than a regen one since, again, it could be lighter, more resistant, more durable, and easier to replace than a ceramic one. Starship also demonstrated that for a large vehicle like itself or Neptune that a regen heat shield would be heavier than a ceramic one. A metalic heatshield would give great characteristics while being lighter than the other 2 options.

@deepfriedfrenchtoast I’ll do a lot more research on the carbon composite. Maybe we could reinforce it with something, but worst case, we can just make the engines out of a more heat (and cold) resistant alloy that is good under pressure, but I want to make most of the rocket carbon comp, cause it’s just so light and can be made cheaply and rapidly if done right.

I did a bit of a whoopsie. When I said the whole rocket would be regeneratively cooled, I meant only the Ferry and engines. We were never planning on regeneratively cooling the booster, but that’s my fault for not clarifying that 💀

The reason we went for regenerative cooling is because we can make the pipes out of carbon composite, so long as we don’t pump the prop through with too much pressure. We also picked it because it can be easily 3D printed with the vehicle with minimal parts and manual labour. A TUFROC heat shield would be somewhat impractical and require much more manual labour, we’ll do more research into metallic heat shields, they sound like they could be 3D printed on and require less replacements than TUFROC.

@Fyrem0th I still dont think that carbon fiber could be used in a cryogenic rocket engine. There have been cyrogenic fuel tanks made out of carbon fiber, like on the Electron rocket, but those only have to store fuel at about 4 bar of pressure, and the turbopumps of rocket engines need to have a pressure that is higher than the pressure in the combustion chamber, which carbon fiber just cant reasonably handle at cryogenic temperatures.

Having a regenerative setup for the booster return is likely overkill, instead wraping the hot parts of the booster with thermal protection wrap or thermal insulation paint would proabably be enough. Using a regenerative heat shield for the second stage might work, but with a high temperature alloy insead of carbon fiber. Starship had a regeneratively cooled heat shield for a while, though switched to a TUFROC ceramic heat shield because it was ultimatly lighter than having a regenerative cooling setup. Stoke Space is also developing a regenerative heat shield for their Nova rocket, though im intrested how their regenerative heat shield stacks up to a regular ceramic heat shield. Its hard for me to imagine that their regen shield with the weight of the plumbing and the extra fuel needed would be lighter than standard TUFROC, which isnt very heavy, only about 354.4 kg/m3. A TUFROC shield for the Nova wouldnt be hard to replace due to its small size as well. You can also potentualy use a more advanced metalic protection system for the second stage, like the one developed for the VentureStar. A metalic tile heatshield would potentually last longer, be easier to replace, and be a lot more durable than a ceramic tile heatshield.

@deepfriedfrenchtoast thanks for the really detailed answer! I’ve been thinking a bit about the dual-expander, but haven’t got round to really cracking down and studying in-depth. Thanks for the link! However, due to the engines being rapidly 3D printed, we can hopefully make the cooling and plumbing work no problem, but I’ll keep studying and tweaking. I can’t believe I forgor about the XRS-2200, it was literally my desktop background for like, 4 months LMAO. Though the gas generator cycle Aerospikes pump a little bit of gas generator exhaust through holes in the wedge thing (for linear Aerospikes) or a small hole in the very tip of the spike (for toroidal Aerospikes), but with a full flow staged combustion cycle, I’m confident we can achieve ~1000kN ASL and ~1300 in Vac. I think we could make carbon comps work at cryogenic temps cause rocket lab has been able to, and using something similar, we could. For the high temperatures, the entire rocket, including the engines with have methalox pumped through its walls, actively and regeneratively cooling it. I could well be wrong, but worst case, we can just use something heavier and more heat-resistant for the engines, due to the rest of the rocket being lighter.

Aerospikes could potentually be used, I think the challenges with weight and cooling could be solved if the goal of using aerospikes is strong enough to carry development far enough. In some cases the cooling problems of aerospikes could be turned into a bonus, like using the extra heat generated by an aerospike to drive a expansion cycle turbopump that is larger than a standerd rocket nozzle could support. There have even been a few studies done on taking advantage of an aerospike's heat in real life, like the Dual-Expander Aerospike consept.

There have been aerospikes tested that were in the ~100 ton thrust range. Specifically  the XRS-2200, which was designed to give 1,192kN or ~121 tons of thrust. Also as a side note, the XRS-2200 was able to use a simple gas generator cycle and still have preformance similar to the Staged combustion cycle of the RS-25 because of its aerospike nozzle.

You mention that carbon composites could be used in the construction of the engine, which is unrealistic. A simple composite like carbon fibers cast in resin can only be used in close to room temperature (depends but something like -50 to 300 celsius.) So the resin would vaporize in the combustion chamber, or crack and shatter in the cryogenic turbopumps. Using carbon fibers with other materials gets really complicated and is not well researched. Using something like molten steal to cast the fibers probably wouldnt work since the carbon fibers would disolve into the steel at that temp.