Can someone explain RAPIER engines to me?

[Jouni]

If you use the RAPIER, your plane will be bigger, heavier, take more fuel from your nuclear tug to drag to Laythe, and waste more precious fuel every time you go visit the surface in your heavy, slow, RAPIER plane.

Small and nimble still works if you're building modular. The advantages don't change. A plane that can get to LKO is lighter and smaller if it uses a jet and 2 48-7S engines than if it uses a RAPIER. It's easier to haul to Jool and gets to and from the Laythe surface much more cheaply.

But, you know, enjoy hauling that fuel to Jool for your airborne SUV. ;-)

Apart from landers, most of my designs carry far more fuel than they need. It's just easier to use standard designs that are guaranteed to carry enough fuel than to customize the ships for the task at hand. Besides, even fully fueled spaceplanes are quite light compared to things launched with rockets. My standard lifters are rated for 40 and 60 tonnes to LKO, so my tugs and pushers must also be able to haul similar things around.

My last interplanetary mission was to Duna and Ike. After the landers were back, I got bored, and didn't want to wait for a good transfer window. Instead I played with maneuver nodes a bit, until I found a 2200 m/s return trajectory. After the 15-minute burn, I warped all the way to Kerbin, completely forgetting to fine tune the approach on the way. After reaching Kerbin's SoI, I had to make an almost 1700 m/s burn to enter the atmosphere at the correct side of the planet. The mothership dove deep into atmosphere at 5500 m/s, and got rid of enough speed that I could park it into a high orbit around Minmus. That was a bit unusual, but it still tells something about how much extra fuel you might have when you rely on standard modules.

[SuperBigD60]

I think I'll run a parallel test to ComradeGoat's with FAR installed once I get home tonight.

I'm thinking FAR might be where the Rapier takes off (I make no apologies for puns)

[technion]

What about things that are not spaceplanes? I took this to Laythe but it was tested as quite capable of orbit from Kerbin. The small liquid engine above it provides a serial stage for extra travel once in orbit. Shown below, bugged out some how and not producing any exhaust. Having been to Laythe twice and unable to return to orbit in the past I'm pretty happy with the rapier.

[technion]

...Or basically any engine other than the 48-7S or LV-N.

47-7S is seriously overpowered.

You know this engine is named slightly differently nearly every time it's referred to on this forum. I'm sure I've spelt it 5-6 different ways.

Maybe it's in desperate need of a "buff" in the form of a more memorable name.

[BubbaWilkins]

I climb steeply until about 15K, when I reduce my climb to pick up speed. I'm basically horizontal at 25-27K. This is not a rocket ascent - I just don't see the point in wasting fuel to make pretty flames at 15K and 800 m/s when I can punch through 800 m/s at 20K like it's not there.

I reiterate, I am not flying a rocket ascent profile, but going horizontal too early in space planes loses efficiency to atmospheric friction. You could spend your entire fuel load at 10K and never reach 500m/s if you fly like that.

What is the point in going supersonic at 8K? It doesn't make the engines work any better, you don't need it to collect intake air, and the amount of drag is horrible

I didn't adjust the oxidiser loads at all. All planes took off with the same amount of fuel and oxisider.

ETA: Here, to prove I'm not a muppet and do actuality know how to fly a spaceplane. This is the original variant (aerospike + jets) picking up speed at 27K:

In retort, I present a craft that has more mass, less fuel, and only Rapiers. I've tried multi engine type combinations, but the mass hit kills it.

Notice the time stamp here. What you fail to realize is that travelling faster at a shallower angle than you were results in a FASTER climb rate. My craft beat yours to 10,000km by almost 10 seconds. My craft is also taking in more air with only 4 intakes than yours is resulting in only -.01 due to altitude (or half of your loss rate).

Now look at the timestamp here. I made a circular 100km orbit in 8:24. Yours took 22 assuming you took the screenshot when you got there. Obviously I have less Delta V than you do, but then my craft is heavier and carries even less fuel.

So wo points:

1) Your ascent method is actually slower, therefore less efficient.

2) This was just a quick run and I didn't really maximize my performance.

EDIT: Much better run.

So I reached orbit with 42% of my oxidizer left. I could probably improve that even further. But I think the results clearly show that Rapiers are viable. I don't know if they beat the other configurations shown in the thread, but I think this clearly shows that the perceived ascent profile is not ideal. I also see now a slight mechjeb failure with a low peri. oh well.

Edited February 1, 2014 by BubbaWilkins

[Jouni]

What about things that are not spaceplanes? I took this to Laythe but it was tested as quite capable of orbit from Kerbin. The small liquid engine above it provides a serial stage for extra travel once in orbit. Shown below, bugged out some how and not producing any exhaust. Having been to Laythe twice and unable to return to orbit in the past I'm pretty happy with the rapier.

My biggest issue with RAPIERs is the attachment. Even when I use them in spaceplanes, engines sometimes fall off in hard landings. In vertical configuration, the situation is much worse.

Yesterday I was designing a three-kerbal Laythe lander. I wanted something lighter than my Duna lander, because a heavy lander at the nose of my mothership significantly reduces its maneuverability. First I tried using 2 or 4 RAPIERs, but the prototypes always failed flight tests. Sometimes one of the engines fell off already at launchpad, while other prototypes dropped their engines when parachutes opened. I could fix the problems only with a huge mess of struts, so I switched to turbojets, and made this accidental SSTO:

There is a nice thing in using turbojets in vertical SSTOs. Because the ship has no wings, you don't have to watch out for engine flameout. The ship may start spinning when the first turbojet dies, but SAS gets the situation quickly under control, and then you just reorient the craft and fire the rockets. If things were as easy with spaceplanes, I would use turbojets in them more often.

[Claw]

I would say that it's hard to compare climb profiles with two completely different airplanes, mods, weights, lift ratings, etc. I would also argue that specific climb profiles are going to be airplane dependent, rather than a perfect one size fits all ascent.

[Captain Sierra]

I would say that it's hard to compare climb profiles with two completely different airplanes, mods, weights, lift ratings, etc. I would also argue that specific climb profiles are going to be airplane dependent, rather than a perfect one size fits all ascent.

Answer to this entire thread: http://forum.kerbalspaceprogram.com/threads/68030-RAPIERs-don-t-suck%21-A-complete-performance-evaluation

[Claw]

Answer to this entire thread: http://forum.kerbalspaceprogram.com/threads/68030-RAPIERs-don-t-suck%21-A-complete-performance-evaluation

I'm sorry, I think you aimed that thread at the wrong person. I never said RAPIERS suck. I just said when you take a completely different airplane (all stock vs. mods, different weights, different engines) and compare the results of a climb schedule, you taint the comparison of the way you climb. I didn't say you invalidate the comparison of the airplane. Just to say that "all space planes must be at 600 m/s at 10km to be at their best performance" is a reach when you compare two completely different craft.

I more than welcome the use of RAPIERS. I've tried on small craft and they just don't do what I want. That being said, I don't use aerospikes on my small planes either. I'm far from an expert space plane maker. In fact, I think of myself as pretty ameturish for anything beyond small, but I like the debate.

I like that your thread uses the same climb method, and compares engine performance. This I can buy into. But if you changed everything and threw in B9 and FAR in between, I would be skeptical. That was my point.

[Captain Sierra]

It wasn't specifically aimed at you or anyone in general. It is, in fact, an attempt to answer this entire thread. The goal being to prove where and how the RAPIER should be used to great effect, and where it is lacking.

[Claw]

It wasn't specifically aimed at you or anyone in general. It is, in fact, an attempt to answer this entire thread. The goal being to prove where and how the RAPIER should be used to great effect, and where it is lacking.

My apologies. I thought it was aimed at my comment specifically. And thanks again for your RAPIER comparison.

Since this thread wandered into the realm of climb performance profiles, here's a test I did tonight. It's all with the same craft, but flown with different profiles. Single TurboJet, one ram intake, and roughly 7.9t on the runway.

The numbers in parenthesis in the key is the amount of fuel used to reach 30km for that ascent profile. Basically all except the last run used the same type of mechanics. Climb out at X degrees, reset to X degrees at 10km and let it ride till 30km. I didn't bother changing the profile during the climb to obtain orbit, as I wanted to compare acceleration performance for each profile to see if they converge. And (per the chart) they do when flying 30 deg climb above 10km.

The worst fuel performance was climbing 30 deg nose up the whole time. Took 38 units of fuel and 3:21 to reach 30km. The rest of the runs, climbing out at something greater than 30 deg, then leveling to 30 deg at 10km, used about the same fuel and took between 2:30 to 2:48 to get there. The steeper the initial climb, the faster to get to 30km. They all ended up reaching about 1200 m/s at 30km.

So I ran another profile. Climb at 45 degrees till 8km, then go to 15 deg and let it ride (the brown line at the bottom). That took 4:55 to reach 30k, although it was going about 1400 m/s at the time. So a lot better than the 30 deg climb, but it used up 47 units of fuel.

The last profile I ran (70/15 Parabolic) is the typical profile I try to run. Climb out 70 deg nose high till about 8km and slowly chase the prograde marker down to (in this case) 15 deg. This ended up at 30km in 2:53. So a little slower than the other 30 deg profiles, but faster than holding 15 deg or 30 deg the whole time. It took 34 units of fuel, so again slightly more than the 30 deg profiles, but less than holding 15 or 30 deg the whole time. But the big benefit was that it was going 1480 m/s at 30km.

I suppose you could argue that the last profile isn't necessarily a fair comparison since I didn't "set it and leave it." I did it for comparison to show how the steep climb can converge with the 15 deg ascent and have better fuel/time economy.

I might duplicate this test tomorrow for RAPIERS, but I suspect the results will be similar once higher weight and lower thrust are taken into account. I'll be interested to see the fuel/time at the end though.

[ComradeGoat]

Now look at the timestamp here. I made a circular 100km orbit in 8:24. Yours took 22 assuming you took the screenshot when you got there.

I clearly stated that I didn't - go reread my original post. I arrived in orbit at night and time warped until it was daylight in order to take the pictures.

My test was a comparison of RAIPERs and other engine types ON THE SAME CRAFT, and the other engines won every test.

A race to orbit is an entirely different test. I'm interested in getting there with a plane that weighs not much for what it does (hence is cheaper to haul to Laythe), doesn't use much fuel (hence I don't have to ship as much fuel to Laythe) and, for some designs, can carry 3000-5000 m/s delta V in space.

1) Your ascent method is actually slower, therefore less efficient.

That's not actually true in the general case either. There's a tradeoff between running the jets for as long as possible, maximising the Oberth Effect, and minimising atmospheric friction. If you want to get to space as quickly as possible, build a device that has a TWR of 2, stand it on its tail, and follow the ascent profile of a rocket. Then burn to circularise at apoapsis. That'll get you there very quickly indeed, but it won't get you there effficiently.

If you want to get there efficiently, build a craft that can still breathe air at 3-35K, get up there as quickly as your wings will let you, and accelerate until you are actually in orbit while still in the atmosphere. Point your nose down to remain at periapsis and burn until your apoapsis is about 200km or more. Coast to space using slight bursts of the engine to recover speed lost due to atmospheric friction (which won't be much once you pass 40K). At apoapsis, circularise, which will require so little effort you can do it with RCS if you want.

That's the efficient way to reach orbit with a spaceplane, but it is also slower.

Most designs we build pick a compromise between being a rocket with wings and an airhoggy super cruising stratoglider and fall somewhere in the middle. RAPIERs suck at either extreme for two very simple reasons: the turbojet has a better TWR; the turbojet is lighter.

[BubbaWilkins]

I'll concede that my ascent methods might not work on smaller/lighter craft. In my mind they should, but clearly what I have experienced is not universal as demonstrated by others in this thread. I am using B9, FAR, Firespitter after all. So it is far from the same experience if I was playing full stock. For most of my designs, the initial TWR is too low to manage anything greater than 45degs at low altitude and still accelerate. Anything steeper slowly bleeds velocity until it stabilizes or stalls.

All in all, a great indicator of just how deep the gameplay really is.

[Claw]

I'll concede that my ascent methods might not work on smaller/lighter craft. In my mind they should, but clearly what I have experienced is not universal as demonstrated by others in this thread. I am using B9, FAR, Firespitter after all. So it is far from the same experience if I was playing full stock. For most of my designs, the initial TWR is too low to manage anything greater than 45degs at low altitude and still accelerate. Anything steeper slowly bleeds velocity until it stabilizes or stalls.

All in all, a great indicator of just how deep the gameplay really is.

Yep. I fully agree, the climb profile is highly aircraft dependent. I can't make claims that my climb profile is good for anything other than what I tested it on. It has a TWR a little over 3.5. Clearly if you have anything at or less than 1 it wouldn't work at all. Most of mine are usually TWR around 2, which means TWR on takeoff around 1. I climb those out differently as well.

I do like the way B9 aircraft look though.

This thread sure wandered a lot, and I like to see all the different angles.

Edited February 1, 2014 by Claw
Typo

[WafflesToo]

(...just because I think the mood could use a little lightening up in.)

"Can someone explain RAPIER engines to me?"

Well, you see Clockwork, when a rocket motor and a turbojet REALLY love each other, the rocket motor gives the turbojet a very special hug...

[Gauss H2K]

I use the Rapiers with a LV-N engine to create an SSTO that has engines that aren't just dead weight on non oxygen atmo worlds. I also use FAR and I think I've gotten much better performance from using the RAPIERs due to it.

[Wahgineer]

ComradeGoat:

My test was a comparison of RAIPERs and other engine types ON THE SAME CRAFT, and the other engines won every test.

Because you used a plane designed to use the other engines. Which skews the results, because the other set-ups will always win since they're being used on a plane designed to use them.

[MarvinKitFox]

The RAPIER is a somewhat crappy air-breathing jet, and a distinctly crappy fuel+oxidizer rocket.

In either role there is a definitely better solution.

Its strength is that it is both types, at the same time. And with rare exception, it beats any combo of jet+rocket hands-down, **when you are not allowed to drop stuff off your plane**

Meaning: It is tailor-made for true SSTO.

[Dispatcher]

According to their stats, rapier engines didn't look that good to me. ...

You know the saying, "lies, damned lies and statistics!" Regardless of the published stats, check out some engine performance test results:

http://forum.kerbalspaceprogram.com/threads/67403-Graphs-of-Engine-Comparison-Flights?p=946903&viewfull=1#post946903

Consider that the RAPIER (air mode) performance is in the same group of engines as are the jet engines (as well as the LV909 and the LV-N and some others) in some of my testing (see the first graph). Granted; the LV-N is lifting its own mass as a higher percentage of total mass. Also note that the RAPIER (vacuum mode) performance in the same testing is in the same group of engines as are the LVT45 and LVT30 (also the Aerospike and some others). The test in this case was of maximizing the fuel mass. But, this doesn't show us the altitudes achieved.

Edit: see http://forum.kerbalspaceprogram.com/threads/67403-Graphs-of-Engine-Comparison-Flights?p=949688&viewfull=1#post949688 for newer graphs comparing mass & altitudes (maxed, with 50% non fuel).

When I substituted half the fuel mass with non fuel mass, the RAPIER (air) well outperformed any other engine, including the jets. In this test, its the only one which could achieve orbit without flying it as part of a plane. The best performing rocket (closed cycle only) engine was the Aerospike in one test.

Of course, most people who use the RAPIER (and jets) are making planes in general and spaceplanes in particular. So the dynamics change when we add lifting surfaces and other aerodynamic parts.

As to why the "obsession"? For me, its because I can design/ build and test an SSTO plane in about 10% of the time it takes me to make a traditional multiengine (turbojet/ your rocket engine here) plane. And I can get it to orbit easier.

Edited February 5, 2014 by Dispatcher