To RAPIER or not to RAPIER?

[panzer1b]

So anyways, quick question to those from the community that have more experience with these things, are the RAPIER engines better in any category outside of being used as WEP (war emergency power) to save ur skin from a mistimed landing brake burn or in combat (once DMP makes that one viable), compared to just a turbojet coupled with a higher isp rocket?

I have used both RAPIERS and turbojets, and i will say the turbojets make ascent MUCH easier, with them also having some power at higher altitudes, and they feel like they use less fuel to get up to orbit, but it may just be a feeling, no hard stats there. We all know the rocket mode (closed cycle) is very bad unless you absolutely need the acceleration (its similar ISP to a LVT-30/45, a tad worse but also lighter engine, with a hair less thrust to boot). So could someone here come up with say a chart or some tips as to what craft if any actually benefit from rapier use (im talking about efficiency). Or is it best to stick with a jet and a 909/nuke dual engine setup? RAPIERS ofc make more sense for SSTOs that have no intention of flying in kerbin/laythe atmo much, as then they arent 100% dead weight, and do help landing on some planets that have too much gravity for say a single nuke to cut it (jets are useless outside of kerbin/laythe).

Edited November 24, 2014 by panzer1b

[jaredkzr]

The main advantage to the RAPIER engines is that you only have to put 1 engine on rather than 2. This saves weight and can help smaller craft reach orbital speeds. If your building larger SSTOs for interplanetary travel then dual nukes is probably the way to go.

[Alshain]

I just design what I feel like. A lot of my design is for the aesthetics, I like planes that function well but also look good. If I can fit 3+ engines on it, I'll use turbojets, but sometimes I just need more compact. It really depends on you.

[panzer1b]

I just design what I feel like. A lot of my design is for the aesthetics, I like planes that function well but also look good. If I can fit 3+ engines on it, I'll use turbojets, but sometimes I just need more compact. It really depends on you.

Guess someone here is also a "for looks" designer....

I just stuck 3 of them on my latest SSTO as they look good, and it looks so amazing when you blast full power with 3 rapiers and a nuke in the middle, so much blue fire coming outta engine!

But anyways, i primarily added them as ive had trouble landing occasionally (mistime landing by even a few seconds, and you wont have enough altitude to brake 15 tons using just a nuke). And i like the utility, albeit i COULD get away with turbojets, and older versions of this did use those, before i ofc got into landing on duna ect, since outside of landing on planets that arent kerbin/laythe, the jets are just better. Gotta try a minimalistic design though, as ive recently learned that low isp isnt always that bad (a super small vessel will actually have less dV using nuke then say 1 lv909, provided its so lightweight that the nuke is like 1/2 the total mass ect.

[Alshain]

Well you can check out all my plane designs in my sig, but this was one of the biggest examples where a RAPIER was the only choice. The main design idea came from watching The Next Space Race on Netflix and it showed mock ups of the Dream Chaser docking. The whole point was the rear docking port and I just built the ship out from there.

[tsotha]

I've had pretty much the same experience as OP. Every time I design a space plane for a purpose it always turns out cleaner if I mix jets and rockets instead of using Rapiers.

I don't care about aesthetics at all, though. You might be able to make better looking ships with rapiers.

[Raptor9]

If you want a complete evaluation and comparison between different propulsion schemes for SSTO's, look at this thread: http://forum.kerbalspaceprogram.com/threads/68030-RAPIERs-don-t-suck!-A-complete-performance-evaluation

This guy does a very good job on the pros and cons of running with just RAPIERs versus a compound propulsion system. The main selling point I took away from it is how efficiently it uses intake air compared to a turbine engine. You're able to achieve a much higher airspeed/altitude energy state before switching to rocket power. It's not the best rocket engine, nor is it the most powerful air-breather, but for a craft with a main function to get from the surface to orbit, it's the most efficient powerplant choice. The rest is up to designing a good airframe for the mission as well.

Biggest drawback: no alternator, so you'll need enough battery power to last you until you can put out some solar panels in orbit. (unless you're launching in the daytime with OX-STATs) If you're looking for ideas on a spaceplane design, my most successful layouts were SR-71-like in appearance.

Shameless plug:

http://www.youtube.com/watch?v=965HnZvKp4s

[GoSlash27]

Honestly... nerfing the RAPIER has helped, but in my experience the combination of turbojets and 48-7S still yields a larger payload fraction overall.

While the RAPIER would seem to provide lower engine mass and simplified construction over a hybrid system, in practice it's performance hit requires more fuel and structure to do the same job.

It's closer than it was, but not quite there yet.

Best,

-Slashy

[zekes]

No - it's more efficient to use 1 jet engine and 2 litttle .5m engines, it's lighter and more efficient.

[Jouni]

This guy does a very good job on the pros and cons of running with just RAPIERs versus a compound propulsion system. The main selling point I took away from it is how efficiently it uses intake air compared to a turbine engine. You're able to achieve a much higher airspeed/altitude energy state before switching to rocket power. It's not the best rocket engine, nor is it the most powerful air-breather, but for a craft with a main function to get from the surface to orbit, it's the most efficient powerplant choice. The rest is up to designing a good airframe for the mission as well.

That discussion is based on the old RAPIER before the recent rebalancing.

The main difference between the turbojet and the RAPIER in airbreathing mode is thrust: a turbojet generates 225 kN, while the RAPIER generates just 190 kN. As a result, you need 18.4% more jet engines in a RAPIER-powered craft than in a turbojet-powered craft. The turbojet requires 15.6% more intake air at 100% throttle than the RAPIER, but if you throttle the turbojet down to 84.4%, the engines are essentially the same. Both engines have the same Isp at the same altitude, and the velocity curves (the ratios of actual thrust to nominal thrust) are also the same until 2000 m/s. Above that speed, the RAPIER loses thrust faster.

If your intakes/mass ratio is low enough that you have to switch to rockets before reaching 2000 m/s (surface speed), the turbojet is asymptotically more efficient in reaching orbit than the RAPIER, if there are at most 0.22 tonnes of rocket engines per turbojet. This assumes that the vacuum Isp of the rocket engine is 360 s. Changing the specific impulse shifts the crossover point slightly. With more intakes, the situation becomes more favorable to the turbojet.

Of course, actual spacecraft use an integral number of engines. If the craft can fly with the same amount of turbojets and RAPIERs, the RAPIER can be more efficient, because it doesn't need separate rocket engines. On the other hand, if the craft needs more RAPIERs than turbojets, the situation can be more favorable to the turbojet than in the asymptotic case.

[Empiro]

Put me firmly into the RAPIER camp. If you also use FAR and B9, then they're pretty much required.

Reducing the number of engines you need make things far simpler when designing crafts. Their slim profile means that tail-strikes when lifting off are reduced. Plus, in the latest version, they have the same mass as a turbojet, so you'll almost always have a lighter design when using just RAPIERs.

[Wanderfound]

Put me firmly into the RAPIER camp. If you also use FAR and B9, then they're pretty much required.

Reducing the number of engines you need make things far simpler when designing crafts. Their slim profile means that tail-strikes when lifting off are reduced. Plus, in the latest version, they have the same mass as a turbojet, so you'll almost always have a lighter design when using just RAPIERs.

They also have a better speed/thrust curve in FAR. Turbojets accelerate faster, but RAPIERs have a better top speed.