R.A.P.I.E.R. engines... do they suck or am I using them wrong?

[chrise6102]

Hey guys!

So I finally got round to building an SSTO rocket today, I got through a few failed attempts first though, naturally!

My first attempt used these R.A.P.I.E.R. engines, I figured they would be a good way to go as they combined air-breathing and rocket engines. They were fine in the airbreathing stage but as soon as they switched to the rocket stage they burned through my oxidiser in no time flat, very inefficiently.

My next design used an equal number of aerospikes and these performed far, far better.

So R.A.P.I.E.R.s seem to be a bit crap for use in SSTO's which begs the question...

What are they good for? Has anyone found a good use for these?

[SpaceSphereOfDeath]

They're good for easy and quick designs, i.e no0bz they are quite inefficient in rocket mode (320 isp IIRC) but you save on weight by not needing 2 jets and one rocket or 2 rockets and one jet (unless your really crazy and build an asymmetric SSTO with one rocket and jet) as well as R.A.P.I.E.Rs have short nozzles so less chance to clip the ground.

[Renegrade]

I dunno, I haven't had any trouble with Rapiers. They do have worse efficiency than an aerospike (made up partly by being lighter though with the same thrust), and their air breathing mode is inferior to the TurboJet's (the thrust/velocity curve isn't visible in the VAB/SPH, but it favors the TurboJet considerably), but they basically let you have both in the same package, at the cost of needing to carry a bit more fuel.

They're also quite handy for smaller spaceplanes in that you only need one engine. In stock, that's kinda six-of-one, half-dozen-of-the-other (vs say having a pair of 48-7Ses to circularize), but in FAR/NEAR or any future stock aero, it means less area to push through the air...

It's kinda funny as there were complaints before about it being 'OP' and making TJ/aero planes obsolete heh.

@SpaceSphereOfDeath - it's 320 at sea level, 360 in vacuum. At 18km+ altitude, it's pretty much going to be an 8% difference.

[MartGonzo]

Hey guys!

So I finally got round to building an SSTO rocket today, I got through a few failed attempts first though, naturally!

My first attempt used these R.A.P.I.E.R. engines, I figured they would be a good way to go as they combined air-breathing and rocket engines. They were fine in the airbreathing stage but as soon as they switched to the rocket stage they burned through my oxidiser in no time flat, very inefficiently.

My next design used an equal number of aerospikes and these performed far, far better.

So R.A.P.I.E.R.s seem to be a bit crap for use in SSTO's which begs the question...

What are they good for? Has anyone found a good use for these?

Stay in airbreathing mode as long as possible at 25-30,000 M (depending on how many intakes you have), you can almost get up to orbital velocity (If you're using deadly reentry this can be an issue for parts without ablative shielding) then pull back switch to closed cycle and get your Ap up to around 80,000M quickly then cut engines, you will lose some from your Ap as you're still passing through the thick atmosphere.

if necessary give it another little burst as you get high in the atmosphere.

yep they do seem to munch through the oxidiser a bit quickly, but they're fun.

[Wanderfound]

I'm afraid it's probably you.

The trick with spaceplanes is to get as much speed and altitude as possible out of the air-breathers before you start burning oxidiser. It's mostly about piloting. Do it right, and it takes very little fuel. Do it wrong, and You Will Not Go To Space Today.

Wanderfound's Spaceplane Piloting Guide

(written with FAR/NEAR in mind, but the technique in stock is basically the same)

1) Get to 20,000m however you like. Around a 45 degree climb is probably most fuel efficient, but jet engines use so little fuel that it doesn't matter much. If the plane has enough power, I usually climb at 75 degrees or so just to get it done quickly.

2) When you get to 20,000m, level off and build some speed. You want to pile on as much horizontal velocity as possible while you make a slow ascent to 30,000m. Keep your angle of attack (the angle between where your nose is pointing and the direction in which the plane is actually moving, shown by the prograde marker when in surface mode) and climb rate low; by the time you hit 30,000m, they should both be around 10 or so. A low angle of attack reduces drag and helps your intakes work better. The low angle makes you climb slower, but that's okay; you need that time to get up to speed. As you go faster, the angle of attack required to maintain a given climb rate reduces, but as you go higher, the thinner air means that the angle of attack required to maintain a given climb rate increases. If you do it right, these two factors will roughly balance each other out and you should gain the necessary speed and altitude in a single smooth climb. However, a plane with some aerodynamic or piloting flaws may need to bounce up and down between 20,000 and 30,000m a couple of times while building speed before the final push.

3) Somewhere between 20,000m and 35,000m (exactly when depends on both plane and piloting), you'll start to run short of air. Don't switch to rockets immediately. If you've got multiple engines going, shut some down to concentrate the available oxygen into the ones you keep running. If you've already shut down as many as you can, throttle back a bit. You can dramatically increase your jet-only altitude by doing this, and once you get up to serious height the thin atmosphere means that you only need a tiny amount of thrust to accelerate.

4) Keep this going for as long as your plane and your patience can tolerate. A well-built and -flown plane should be able to get over Mach 4.5 and 30,000m in a single attempt on jets alone. Once you've wrung as much speed and altitude out of the jets as possible (you want at least Mach 4 and 30,000m), force the nose up to 45 degrees and light the rockets. If you have both jets and rockets, don't shut down the jets immediately; the thrust of the rockets will drive a ram-air effect that kicks the jets back into life for a while. Keep the rockets burning until your apoapsis exceeds 70,000m, then shut off and coast until it's time to circularise. Point prograde and close your intakes while coasting to minimise drag

A good plane and pilot should be able to get the apoapsis to 70,000m with less than a minute of rocket power. Done properly, it requires very little fuel. But if you try to brute-force it from lower speeds and altitudes, the atmospheric drag is going to drain your oxidiser tanks before you get anywhere near orbit.

If you're having trouble with design rather than piloting, have a poke at the designs linked in my .sig. They're all tuned for FAR (and if you're getting into spaceplanes, I highly recommend that you give FAR/NEAR a try; stock aero is a joke), but the Benchmark is certainly able to reach orbit in stock with tanks better than half-full. Just try to keep the angle of attack below 20° or you'll spin out.

[Laie]

Rapiers are pretty good. On small vessels (say 8 tons or less), where the extra weight of an LV-909 would cost more fuel than the efficiency saves, Rapiers are the superior solution. The bigger your craft gets, the more worthwhile it becomes to use dedicated jet and rocket engines. The difference can be quite significant, but not insanely huge (say, 10-20% rocket fuel saved). Rapiers are still allright-ish even for large vessels, where they make for simpler designs.

If your aerospike design outperforms the rapiers by so much, I suspect the reason lies elsewhere. Wanderfound above adresses what would be my first guess as well.

Edited August 31, 2014 by Laie

[dryer_lint]

I have the same problem. I built a plane with RAPIERs and it could hardly take off, then I built one with aerospikes and jets and it worked great.

[chrise6102]

Thanks for the replies guys, I was making a relatively weighty craft (about 30t) so I guess that explains some. I've built SSTO spaceplanes using both jets and rockets before so I know to get it as fast as possible while still in the atmosphere, I think it was more of a design flaw than a piloting flaw! I'll give it another go at some point.

[Wanderfound]

RAPIERs can handle bulk loads just fine. This one is 48 tons wet, powered by four RAPIERs and two Aerospikes.

Note the Mach number and periapsis altitude.

As well as piloting, low-drag aerodynamics matter.

Edited August 31, 2014 by Wanderfound

[Rhomphaia]

Rapiers are pretty good. On small vessels (say 8 tons or less), where the extra weight of an LV-909 would cost more fuel than the efficiency saves, Rapiers are the superior solution. The bigger your craft gets, the more worthwhile it becomes to use dedicated jet and rocket engines. The difference can be quite significant, but not insanely huge (say, 10-20% rocket fuel saved). Rapiers are still allright-ish even for large vessels, where they make for simpler designs.

If your aerospike design outperforms the rapiers by so much, I suspect the reason lies elsewhere. Wanderfound above adresses what would be my first guess as well.

For me the RAPIER is at its worst on smaller spaceplanes. The problem is that its lower max velocity in air-breathing mode means you need 500m/s more from the rocket just to get into orbit, when a turbojet can practically get there on its own with just a tiny push from rockets to circularize. 48-7S's are the way to go.

The RAPIER really comes into its own on larger spaceplanes when used alongside turbojets.

[GreeningGalaxy]

I like the RAPIER mainly because it opens up possibilities for single- or dual-engine SSTOs, which were impossible or at least nearly so before its release. I mean, yes, most of my SSTOs are big KSPI nuclear-powered monsters or at least quad-engine RAPIER affairs, but there are some times when I just want to let Jeb take a vacation to LKO and don't have much else I want to haul up with him.

[Wanderfound]

I like the RAPIER mainly because it opens up possibilities for single- or dual-engine SSTOs, which were impossible or at least nearly so before its release. I mean, yes, most of my SSTOs are big KSPI nuclear-powered monsters or at least quad-engine RAPIER affairs, but there are some times when I just want to let Jeb take a vacation to LKO and don't have much else I want to haul up with him.

Also fun for sportsplanes in general. Stick as many RAPIERs as you can fit on the back of the smallest airframe that can carry them, give it just enough wing to take off and land, and you get something that is ludicrously quick in both air and space. If you're using Deadly Reentry, torching your plane on the way up is a definite possibility.

[Zephram Kerman]

Seems to me the Rapier engine is a trade-off from perfect efficiency to simplicity. It really pays off for very small craft with room for only a single engine. But it also works well enough for larger designs. The SP+ parts inspired me to finally attempt a Kerbal-ized version of Skylon.

[AlexanderB]

I found that straigt-up replacing a rapier with a TJ and two 24-77's (or 48-7s's) halved the consumed fuel to orbit, to the point where the same craft that could orbit and land once with a few % fuel to spare, (after engine-swap) could take off, orbit, land, take off, orbit and land again before running out of fuel.

That also means I could take off half the fuel tanks, and replace them with usefull cargo/passenger space, improving my SSTO capability from single pilot to orbit, to tons of cargo and dozens of passengers.

The rapier just cuts out in air breathing mode at a lower altitude and speed, even with manual switching set and throttling down, while the TJ keeps going (at very low thrust and also reduced throttle) even when the air intake bar shows 0.00. (not sure if thats a bug or intentional). The result is a good 500 m/s difference, which saves a ton of fuel/oxi.

[Levelord]

The rapier just cuts out in air breathing mode at a lower altitude and speed, even with manual switching set and throttling down, while the TJ keeps going (at very low thrust and also reduced throttle) even when the air intake bar shows 0.00. (not sure if thats a bug or intentional). The result is a good 500 m/s difference, which saves a ton of fuel/oxi.

From how I understand it, the intake air values shows how much intake air is in 'storage' to be used. When it shows 0.00 intake air, it simply means that the engine is consuming air faster than the intakes can 'store', but other than that, the intakes are still taking in air.

[Scarecrow88]

My single engine SSTO can get to 36,000m and 2000+ m/s in air breathing mode, so only needs a relatively short burn in rocket mode to achieve orbit, so it's a thumbs up for the RAPIER from me. By docking and refuelling, it's possible to get to Mun and back.

[SkyRender]

The RAPIER engines are very useful indeed, as long as you have a well-designed craft. One of my favorites is my Airbus Kerbin.

Using FAR, RAPIER engines become even more overpowered, since they're effectively a free semi-efficient rocket engine attached to a turbojet. I managed to get a similar payload to orbit using FAR with literally half the fuel and engines.

[Wanderfound]

I found that straigt-up replacing a rapier with a TJ and two 24-77's (or 48-7s's) halved the consumed fuel to orbit, to the point where the same craft that could orbit and land once with a few % fuel to spare, (after engine-swap) could take off, orbit, land, take off, orbit and land again before running out of fuel.

That also means I could take off half the fuel tanks, and replace them with usefull cargo/passenger space, improving my SSTO capability from single pilot to orbit, to tons of cargo and dozens of passengers.

The rapier just cuts out in air breathing mode at a lower altitude and speed, even with manual switching set and throttling down, while the TJ keeps going (at very low thrust and also reduced throttle) even when the air intake bar shows 0.00. (not sure if thats a bug or intentional). The result is a good 500 m/s difference, which saves a ton of fuel/oxi.

It depends heavily on whether you're in stock aero or FAR.

By the time the RAPIERs swap over to oxidising mode, I'm usually on the edge of hypersonic (Mach 5+). Due to the way FAR has altered the speed/thrust curves on air-breathing engines, turbojets (and air-breathing RAPIERs) are virtually useless at those speeds (realistically; there are limits to how fast you can effectively spin a turbine, which is why scramjets exist). Even at full throttle and with a generous air supply, they're only going to put out about 15kN each. I still have turbos on a fair few designs, but that's mostly because I like the look and sound of them. RAPIERs sound awful.

Pretty much all of mine can reach orbit with tanks half full. The Longreach can do KSC to Minmus and back, unrefuelled, with a VTOL landing on Minmus in the middle. The Wedgetail can hit orbit with over 4,000 units of oxidiser left in the tanks. These are both normal-sized and very fast planes, not lumbering Whackjob-style monsters.

There just isn't any need to sacrifice performance for the sake of maximum fuel efficiency.

Edited August 31, 2014 by Wanderfound

[Hodo]

There are a lot of assumptions and incorrect usage of RAPIER/SABRE engines out there, most of the time these faults in fuel use are pilot error related. People try and fly them like a rocket when that isn't the best way to do anything.

I use RAPIERs quite a bit, and SABREs which are just bigger RAPIERs mainly because I am trying to reduce part count and keep life simple when building a craft. Not because I want something that can Single Stage To Eeloo. I follow the same flight profile as the planned Skylon craft. Which means I gain as much speed as possible between 15-25km altitude on jet engines before I switch over to closed cycle, then I change my climb rate to climb as fast as I can while using closed cycle.

This is one of my designs that I am currently using. It uses 4 RAPIERs and has no problems hauling up to 20 tons to orbit. I have made larger FAR larger craft that can haul over 120 tons into orbit using SABRE-M 2.5m engines, which are almost 5 times more powerful then the RAPIER, and use almost 9 times the fuel.

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If you fly it wrong YES you are going to have horrible fuel consumption. If you fly it right you can get the simplest things into orbit.

[SkyRender]

And now, proof that RAPIER engines plus FAR equals absurdity.

Here's a very sleek and aerodynamic SSTO spaceplane that has the same passenger capacity as Airbus Kerbin. In stock, it would just barely be able to reach orbit. (The parts are from Spaceplane Plus, shortly to be implemented in stock. Fun times.)

With FAR, it reaches orbit with over a full third of its fuel remaining. FAR is the best thing.

[Wanderfound]

And now, proof that RAPIER engines plus FAR equals absurdity.

http://www.skyrender.net/lp2/ksp/spp_ssto.jpg

Here's a very sleek and aerodynamic SSTO spaceplane that has the same passenger capacity as Airbus Kerbin. In stock, it would just barely be able to reach orbit. (The parts are from Spaceplane Plus, shortly to be implemented in stock. Fun times.)

http://www.skyrender.net/lp2/ksp/spp_ssto2.jpg

With FAR, it reaches orbit with over a full third of its fuel remaining. FAR is the best thing.

Parallel evolution

That's essentially the same plane as the one I posted upthread, cracking Mach 11 and escape velocity while still well inside Kerbin's atmosphere. Streamlining works.

[Wanderfound]

I'm new to ksp. especially planes. I've managed to get the core of a space station in orbit and some other stuff. I want to fly some spaceplanes. So I went to full throttle then pressed space. All I see is sparks come out and then nothing. What do I do?

Put some air intakes on it. Turbines need to breathe.

[DocMoriarty]

RAPIER's are good when part count really gets an issue. I create all my transport space planes with them. It makes it also easier to get the alignment of engines right if you can't make a symmetric design. Here an example delivering a Kerbodyne S3-14400 based tug to LKO. On the image the tug is currently undocking. The total assembly (plane & tug) is around 470 parts. Using a different setup with turbojets and rocket engines would most likely take more parts and be much harder to align both engine sets right.

[SkyRender]

Turns out FAR and RAPIERs get along better than I'd realized. Yes, that is 10 crew cabins.

[Wanderfound]

http://www.skyrender.net/lp2/ksp/spp_ssto3.jpg

Turns out FAR and RAPIERs get along better than I'd realized. Yes, that is 10 crew cabins.

...which is good. Finally we can build serious passenger liners that don't look like Whackjob monstrosities [1]. I'm hoping to do better than a 100 Kerbal audience for the Minmus ice races.

Given the interplanetary fuel range of the Migration​, you could easily add another four passenger cabins on the nacelles and still make it to orbit, BTW.

[1] No offence intended to Whackjob; your creations are cool. But it's nice not to have to do that.