RAPIER boosters - useful?

[kelmv]

I've started playing with the RAPIER engine as a first stage engine to get a small rocket up through the atmosphere and I was wanting to know what other people thought of them for this purpose

[Clockwork_werewolf]

Interesting, I've played with turbo jets being boosters but they need lots of intakes and engines per fuel tank. How many intakes do you use per engine and how many do you need compared to normal boosters? I might have to try this idea.

[kelmv]

I'm currently testing 3 ramjet intakes and 4 nozzles. I'm just playing with them at the moment to see what I can get them to do. I'm mostly interested in their little ability to swap between air-breathing and rocket modes, but right now I'm having FPS issues for some reason, so testing is slow.

[helaeon]

I tried this the other day after seeing Scott Manley do it as I was looking for a similar solution to what he did... it didn't work very well. I turned around and built one using turbojet engines and some efficient rockets and that worked far better.

[kelmv]

Well, I can say this: 30k feet and ~1100m/s horizontal velocity off of 2 2xFL-T800. I think my ascent was too shallow, but it seemed to work alright. Might also help if I dumped the air intakes when I swapped over to a closed system to save drag and weight.

[kelmv]

Ok, after ditching the boosters I was just about 700m/s shy of orbit. Not too bad, all things considered.

[Jouni]

If you want to use RAPIERs, it's better to use them all the way to the orbit. Otherwise turbojets will be lighter and more powerful and allow you to go higher and faster in air-breathing mode.

[kelmv]

Another test. Less fuel (2x 1 FL-T800 with three engines and three intakes). With a 4.5 ton load (undrained FL-T800 tank at the tip) it reached orbit with 267 units of fuel left in the center. 4 FL-T800s put 4.5 tons into orbit. As I recall, that isn't too bad.

[SpaceSphereOfDeath]

If you want to use RAPIERs, it's better to use them all the way to the orbit. Otherwise turbojets will be lighter and more powerful and allow you to go higher and faster in air-breathing mode.

This is the reason I personally don't use them, jet engines are for air craft, and thus work better in atmosphere, RAPIERS work as SSTO engines, meaning they work at both lower ISP and thrust in atmosphere then jets, and lower ISP in a vacuum, but they work in both reasonably well, thus for in atmosphere boosters use jet engines, the air breathing mode of the RAPIERS have a max ISP of around 2000, I've seen jets go up to 2700, so yes you might as well just go all the way to orbit with them.

[Dispatcher]

Each engine seems to have its niche. If you compare the mass that the air breathers can lift, the RAPIER (air mode) outperforms the jets simply because it can switch to closed cycle. In vacuum mode (only) it compares with the LVT30 & 45 (but these latter can lift more mass). The aerospike can haul less mass to a much higher altitude. See comparisons links in my sig line.

Since the RAPIER is intended for SSTO use, their use as boosters only seems questionable given that the turbojet and rocket engines will yield better results.