AmpCat

@Nertea, this actually brings up another question: Is it possible in KSP to have say a LH2 + Atm engine running at something like 2000 ISP, then as the Atm lowers, just that portion of the ISP drops off? So when you get to vacuum, rather than switching modes, you're just running at LH2 ISPs. I'm imagining if I have propellant ratios at 1 and 1, and ISP to 2000, as the Atm pressure lowers, will ISP slowly drop down to 1000. Can I set the flameout percentage to 0% or something, and it just run fine without Atm and only the LH2? No idea if the game allows this or if you have to switch to another mode.

I had guessed a lot of this is 'what feels right', but looking at the conversion of nuclear engines from other sources to use LH2 instead of LF, you did have some reasoning behind the numbers, since they were consistent and seemed rather specific. I guess what I'm really after is a nuclear Broadsword, running either LF or LH2 and Atm in air-breathing and just LF/LH2 in closed cycle. And closer in diameter to 3.75m or Mk3 parts. I just love the looks of the models from Kerbal Atomics! But then again, I like the low-profile of the Broadsword. I just hate throwing off huge chunks of rocket for my missions. Feels.. wasteful. Inelegant. Well, if you ever need ideas for a new class of part, I'd love some Atm-breathing, LH2 (or LF) burning, multi-mode atomic engines! Basically, a nuclear-assisted internal aerospike engine. I'd love to help where I can. While I've done modeling before, used Unity, and am a software guy by profession, I confess I've never tried to mod before. Maybe I can take this as an opportunity to get back into modeling after all these years and make a spin-off atomic engines mod. I won't delude myself into thinking I can make comparable models, however. P.S. How come no one else uses turbo-ramjet assisted vertical launch rockets? I can easily get one to 80 or even 100km apoapsis before the engine runs out of air at 25-30km altitude, then use rockets to circularize. Just have to be careful not to overheat my nosecone and those LH2 tanks. I've even got mine landing vertically again from orbit onto Kerbin.

@Nertea and others, I've been tinkering with SSTOs of the vertical lift variety and been having a lot of fun using the Kerbal Atomics engines; in particular the high ISP Liberator. However, I have one last challenge I'm trying to tackle, and that's getting one that works decently in non-oxygen atmospheres. For now I just make huge ships with many Liberators, but that only goes so far, and it's inelegant. There's several atmosphere breathing fission ramjets I've seen, using simply a reactor and the ATM resource, but they're heavy and low thrust. While I'm not trying to be terribly scientifically accurate, it stands to reason you could have something like an atmosphere-augmented version of the Kerbal Atomics engines, that in atmosphere they would burn LqdHydrogen and IntakeAtm for additional thrust like a ramjet. Or, perhaps using a combination of LqdHydrogen, Oxidizer and IntakeAtm. The goal for me, is nuclear landers/rockets/spaceplanes that can comfortably operate in non-oxygen atmospheres. So, to this end I started tinkering with making duplicate versions of these engines and tweaking them to use IntakeAtm as a fuel. I've got them (and a variant of the Mark IV Broadsword as well) in a useful state, but now I wanted to try and make these at least somewhat balanced. Right now I'm trying to keep ISP, thrust and weight similar to the parent engines, with tweaks towards heavier, lower thrust, and little lower ISP in closed cycle mode. In air-breathing mode, ISP is still similar to a jet (in the 2000-3000 range). I've noticed @Nertea seems to be using some method to the madness of stats like ISP, thrust, weight, cost, etc. Any thoughts on how I could try and balance engines like this to fit in? Would this be something you might want to fold into to the Kerbal Atomics mod itself? Being a fan of standards, I'd love for this concept to be pulled into this mod, but I know it's a lot of work, so I'll be happy with something that at least feels standard on my own computer, too. I'd gladly post the changes I made to the files if it helps.