RAPIERs don't suck!: A complete performance evaluation

[ComradeGoat]

Well, I know I can get a RAPIER engine to 28km air breathing on a 2:1 intake ratio (that shocked the crap out of me first time I did it) and my personal roleplay rule is no higher than 4:1. Hmmm. I wonder. Perhaps I can add a new test to do, max-out altitude.

The honour of Team Turbojet demands a riposte!

One turbojet, 4 radial intakes, full throttle, 35K, 2230 m/s (surface), orbital velocity, while still air breathing.

.craft file here (stock - I removed the KER part for upload).

When I replaced the engine array with a RAPIER, and let it autoswitch, this craft didn't reach orbit.

[ComradeGoat]

My Marin SSTO was originally made with the RAPIER engine in mind before I changed it to the RamJet engine, and given both were running on the 4:1 ratio (note that the intakes are not clipped and are placed on the plane which conforms to some form of aerodynamic realism) the tests for each engine is valid.

That's a very nice looking aircraft!

[ComradeGoat]

What struck me as odd was that you chose to use the Aerospike engine, which for all intents and purposes is a horribly over-nerfed engine which is rarely used in KSP except for Eve ascent vehicles. Why not use the LV-909 engines, as they are proven to be able to efficiently get SSTOs into orbit from the jet ascent profile. So arguing that the 48-7S engine is overpowered when you chose an engine which is noticeably underpowered to prove your point about RAPIERS being better, makes no sense to me.

It's also worth noting that my own tests in the other thread deliberately used the Aerospike for precisely this reason - if the RAPIER was going to beat anything, it would be an aerospike. The aerospike still won.

[Cruzan]

Hi Capt!

I appreciate you doing this test and documenting it, but I have some issues with the test overall. When I think of spaceplanes I think of craft that are designed to take advantage of the Jet engine stage of the ascent with minimal input from the less efficient rocket stage. Saying that you can't compare the RAPIER to a jet-heavy ascent profile seems...counter-intuitive. My initial impressions when the engine was released is that they gutted the jet portion which is what an efficient spaceplane relies on to perform its best.

That being said here is the test that I've been running while I'm preparing a tutorial video for SSTOs. I'd love your input. The odd thing is that I get the complete opposite performance characteristics out of the RAPIER than you did during your tests. The RAPIER will autoswitch to rockets at exactly, or close to 0.02 units of IntakeAir. The TurboJet will operate well past this point without reducing throttle, to 0.00 and beyond. Without disabling Auto switching I got anywhere between 50-75% better performance out of the TJ/48-7S setup. Disabling Auto mode helped, but the TJ still well outperforms the RAPIER.

The basic setup eliminates any question of airhogging as I only used two radial intakes.

The TurboJet/48-7S Test

Mass=8.29t

1x TurboJet

4x 48-7S

Intakes: 2 Radial Intakes

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The RAPIER Test

Mass=8.44t

1x RAPIER Engine

Intakes: 2 Radial Intakes

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I don't even have to do math to determine which result is better. The TurboJet test was done on the first run, easily. The RAPIER's best result came after four tries and that result came as a struggle. Put this engine in the hands of someone with less experience flying spaceplanes/ssto's this design won't even come close to getting orbital.

The TJ flamed out at a higher altitude, with higher velocity and with less fuel consumed and got the much quicker...

The RAPIER flamed out lower, at less speed and consumed more fuel while taking longer to get there.

Another key thing to factor in to the RAPIER's flaws is that you don't have the option to run both the Jet and rockets at the same time. There are design that benefit from turning on the rockets while the jet is on its last legs. This let's you milk the last bits of life out of the Turbo before fully switching to rocket power alone.

Going strictly on numbers rather than feel (any manually flown flight results are almost useless for making scientific conclusions here, as differences in piloting will lead to large uncertainties in the data), the Rapier is an inferior jet engine to the Turbojet - it has the same same Isp curve vs altitude and the same thrust curve vs velocity up to 2000 m/s, but with 190 kN / 1.75 tonnes instead of 225 kN / 1.2 tonnes.

I've been looking for something like this, thanks!! Did you collect the data? It pretty much reflects what my initial impressions were. Squad just turned the dial down on the Jet portion of the RAPIER. I think this was a mistake on their part, at least turning it down as much as they did.

There really isn't any other conclusion that I can see. The RAPIER's only appeal is for someone's first couple designs, but honestly if they don't run in manual mode and learn how to control throttle through the flameout region of the engine's ascent, they will still find getting a SSTO to orbit more difficult than it needs to be.

The staple of spaceplane design is to take advantage of the highly efficient jet stage of the ascent. Gutting that portion of the engine in the RAPIER's design was bound to make it less effective when compared to more traditional engine compositions. This even factors out air-hogging. My tests were done on a terrible set-up using two radials. Improving the intake assembly only favors the TurboJet in the end.

EDIT:

Raw stat wise, a good TurboJet/Rockomax combo blows the RAPIER out of the water, but for ease of use, the RAPIER wins out. The choice of which engine you want to use depends on the type of craft you are making, what you want it to do, and what ascent profile you want to fly. I use the RAPIER engines for my heavier planes that spend a lot of time in space, and I use the traditional jets for very small planes that have less fuel to spare.

While I agree that user-friendliness is a something to consider when choosing engines, I honestly can't see any application beyond beginner designs that would opt for a RAPIER engine. For spending longer periods of time in space there are equivalent TurboJet/Rocket compositions in the TWR/Isp range of RAPIER. If I have to disable the one feature that is supposed to attract me to the RAPIER in order to get any decent performance out of it, then that defeats its purpose...

Edited February 1, 2014 by Cruzan

[Comrade Jenkens]

I personally find rapier designs much easier to achieve orbit with than Turbojet/rocket designs. Though they struggle with power for the first part of the climb.

[Brotoro]

One thing I have noted in these kinds of discussions is that no one mentions air intake to engine ratio...

In my post I was clear that almost exactly the same plane design was used (with just the engines changed), so the intake to engine ratio is the same and has no effect on the difference in performance. Both planes have two ram air intakes to feed the single air-breathing engine.

I found that the turbojet could operate at a higher altitude than the RAPIER with the same intakes.

[baberbot]

my perfered use of the rapier is the rapier, turbo, nuke combo (cliped)

turbo for low atmo, when TJ burn out occurs turn on rapier jet, somehow stops TJ burn out, turn off rapier (i managed to get to 35km with this once), when butn out prevention no longer works use rapier to get a decent appoaps then use nuke to circularise, head to minmus etc.

[Disclamer] some one else on the forums invented this strat so i can't take credit for inventing. But do use it frequentily.

I run tests most other clipped comboes (dual TJ + various rockets, dual TJ + rapier, 3 rapiers etc.) gave me most DV at end.

[ComradeGoat]

I found that the turbojet could operate at a higher altitude than the RAPIER with the same intakes.

I'm finding exactly this too. It's probably because the intakes generate more speed the faster you go, and turbojets have a higher TWR.

In every test I've done, the turbojets go higher and faster before I need rocket power, and that's even with the RAPIER's signature user-friendly autoswitching feature disabled.

[tavert]

The RAPIER will autoswitch to rockets at exactly, or close to 0.02 units of IntakeAir. The TurboJet will operate well past this point without reducing throttle, to 0.00 and beyond.

Be sure to right-click on the engine and watch the thrust carefully. Starting in 0.23, jets will now auto-throttle themselves down when running low on IntakeAir, down to a threshold of 10% for basic and turbojets, or 33% for Rapiers (I think only when auto-switching is disabled).

I've been looking for something like this, thanks!! Did you collect the data? It pretty much reflects what my initial impressions were. Squad just turned the dial down on the Jet portion of the RAPIER. I think this was a mistake on their part, at least turning it down as much as they did.

Not really, the data itself is contained in the part.cfg files. K^2 finally figured out the proper way to interpret it here http://forum.kerbalspaceprogram.com/threads/67606-Fuel-consumption-as-a-function-of-atmospheric-pressure and we verified against a large set of points that numerobis had collected. I just plotted it all up.

I think the Rapier's balance is about right, for what it is. It's a slightly worse jet than the turbojet, a slightly worse rocket than the aerospike, put together into a package that weighs more than either individually but less than the two combined. It's popular for its simplicity, but it's objectively less efficient because that's the role it's supposed to have.

Edit: This would all be a different story if jets were rebalanced and given unique roles. Things would be more interesting if "conventional" jets had a more realistic upper speed limit, and there were alternate types of air-breathing propulsion like ramjets, scramjets, and precooled engines like the SABRE. These all have different characteristics in terms of weight, thrust, efficiency, speed, cost, and complexity. But this likely gets into too complex of a setup for the purposes of a game. The more complicated engineering details you put into the game past a certain point, the more difficult it is to make sure it's still entertaining to play for a wide audience.

Edited February 1, 2014 by tavert

[Cruzan]

Be sure to right-click on the engine and watch the thrust carefully. Starting in 0.23, jets will now auto-throttle themselves down when running low on IntakeAir, down to a threshold of 10% for basic and turbojets, or 33% for Rapiers (I think only when auto-switching is disabled).

Yup! I didn't state that very clearly, but I used the concept in the high-alt video I made. It just seems to be yet another little victory to add to the TJ's list of advantages.

Edit: This would all be a different story if jets were rebalanced and given unique roles. Things would be more interesting if "conventional" jets had a more realistic upper speed limit, and there were alternate types of air-breathing propulsion like ramjets, scramjets, and precooled engines like the SABRE. These all have different characteristics in terms of weight, thrust, efficiency, speed, cost, and complexity. But this likely gets into too complex of a setup for the purposes of a game. The more complicated engineering details you put into the game past a certain point, the more difficult it is to make sure it's still entertaining to play for a wide audience

I wouldn't doubt that might be considered down the road, but I would assume additions to science and general rocketry will get more prioritization. I get a feeling SPH junkies are a minority in KSP . No data to back up that statement though. It would be very cool to get more complex stock air-breathers and I don't think people would view it as too difficult to grasp. If they can choose between a Mainsail and a LV-N properly they could probably figure out proper applications for scramjets and whatnot.

[tavert]

I wouldn't doubt that might be considered down the road, but I would assume additions to science and general rocketry will get more prioritization. I get a feeling SPH junkies are a minority in KSP . No data to back up that statement though. It would be very cool to get more complex stock air-breathers and I don't think people would view it as too difficult to grasp. If they can choose between a Mainsail and a LV-N properly they could probably figure out proper applications for scramjets and whatnot.

You're absolutely right on the prioritization. None of this makes much sense within the stock aerodynamic model either, and as far as I've seen the devs haven't shown much interest in improving it, or simply adapting some or all of FAR as stock (not saying this is their best option, but it's a possible option). KSP's not a flight simulator and Squad doesn't seem to want to make it one, but the fact that it's much easier to build your own aircraft in KSP than in real flight sims means people will continue to play around with aircraft in KSP. I don't think your minority feeling is justified or accurate, not sure why you would get that impression.

Rocketry design tradeoffs at the simplified level KSP presents come down to a handful of numbers, and once you're in space those numbers are static. Design for atmospheric flight depends much more on operating condition, and covers a pretty wide range. You could fiddle with the curves within KSP's current setup and get something that may feel decent - modders are already working in that direction.

[ComradeGoat]

I get a feeling SPH junkies are a minority in KSP . No data to back up that statement though.

We probably are, but some of us got here by necessity, after avoiding the place because the start of the learning curve was daunting. I had spent ages (and many Kerbal lives) trying to design a working air breathing Laythe lander in the VAB, and in the end concluded the solution to all my problems was learning how to make spaceplanes.

It takes quite a bit of perseverance to get past the "why won't it take off" and through the "why does it spin out of control" and the "how do I land this without it exploding" stages though.

[bsalis]

I guess i'm alone in that I loves all the engines for SSTO Spaceplanes.

They all have a use, depending upon the nature of the craft and what you want for it. You can also mix engines together to get the sweet spot you are after.

There is also this perception that the ultimate worth of an SSTO Spaceplane is the dV you have left when you get to orbit (which I find odd, since IMO that only matters for interplanetary flights without refueling). What about time-to-orbit? Or how much dV you have when you refuel in orbit?

[ComradeGoat]

What about time-to-orbit?

Make it light, use a turbojet for its TWR and a pair of 48-7S engines because they weigh nothing.

Or how much dV you have when you refuel in orbit?

Two turbojets and a NERVA!

[Jouni]

There is also this perception that the ultimate worth of an SSTO Spaceplane is the dV you have left when you get to orbit (which I find odd, since IMO that only matters for interplanetary flights without refueling). What about time-to-orbit? Or how much dV you have when you refuel in orbit?

I find the real time from the first idea to the end of the first succesful mission to be the most important efficiency measure. Fuel doesn't cost anything in 0.23, so its value is mostly based on the time needed for refueling operations and maintaining the refueling infrastructure. On the other hand, most of my spaceplanes aren't used in the same configuration that many times, so the time needed for designing, testing, and fine tuning them is an important part of their cost.

[Themohawkninja]

To go off on that point about how RAPIERs can hold onto their intake air longer, I'd like to also point out that since RAPIERs are both hybrid engines, and they don't have any delay between modes, there is pretty much zero chance of your craft going into a flatspin due to the unpredictable nature of engine flameouts.

[Voculus]

Why use the aerospike as rocket engine?

Easy. It's not a mile long, and makes avoiding ground strikes a breeze, particularly if the plane is intended to land off-runway.

[ComradeGoat]

To go off on that point about how RAPIERs can hold onto their intake air longer, I'd like to also point out that since RAPIERs are both hybrid engines, and they don't have any delay between modes, there is pretty much zero chance of your craft going into a flatspin due to the unpredictable nature of engine flameouts.

Not so. That danger from flameout is pretty much eliminated in 0.23 because of the throttle reducing behaviour. However, twin engined designs still reduce asymmetrically, so you get an induced yaw, just not as much as with a flameout.

Now if your design is well built, it'll hold its course on the rudder and you can see SAS applying yaw and know to throttle back. If it isn't, it'll go into a flatspin. This will happen regardless of whether you're using RAPIERs or turbojets.

[Dispatcher]

With the topic of air intakes being addressed (rightly so), I'll point out that my testing (mentioned in an earlier reply) was done for each engine with ONE circular air intake (whether the engine is an air breather or not BTW; thus mass and drag were the same, as far as that component is concerned).

While Talvert's graph shows that in his testing, the RAPIER (Air) shadows the Turbojet in its performance (and outdoes the Basic Jet), its also true that the RAPIER obviously can switch to closed-system-only while the Jets cannot. So far this discussion has shown to me that each engine has its uses, flight methods and styles differ, and the RAPIER does have its uses.

I'm of the opinion that almost any combination of engines, with the right design, can allow an SSTO to achieve at least LKO. What's cool about KSP is that we can favor certain engines and get results using them. I do have my own favorites of course.

Twin spikes (so rocket only). I'm not the best pilot or SSTO designer, so in this flight I used up all fuel for orbit, but monopropellant can nudge me back down. One of my half round fuel tanks caps the back end of the central stack. The nosecone sits atop of a docking port:

Hmmm. No pics of mixed engine (spike/turbo) or RAPIER SSTOs. Perhaps I'll get to that another time. I'm interested in trying out other combinations, as some of you have done.

[ComradeGoat]

Easy. It's not a mile long, and makes avoiding ground strikes a breeze, particularly if the plane is intended to land off-runway.

I use it on my heavy Duna lander. It's heavier than an LV909, but that's largely mitigated by the extra TWR and lower profile, so it's a good engine for that scenario.

[Captain Sierra]

I see a lot about turbojet vs RAPIER. Now, I did some testing of that late last night. The results are intriguing. This was the craft.

The test was done with both RAPIERs and turbojets.

With RAPIERs, they switched over at over 30km and 2100m/s. That's the first plane I've ever circularized in atmosphere. Those engines can hold on to a non-existent ammount of intake air.

Now, I repeated the test with just turbojets. They flamed out at about 27.5km and I was unable to achieve the same height, though their higher power meant I was going faster, ~2250m/s.

With some 48-7S engine pods on the wings, that would have got to orbit too, probably more efficiently than the RAPIERs, though I think that can be at least partially attributed to how overpowered those engines are.

[Brotoro]

There is also this perception that the ultimate worth of an SSTO Spaceplane is the dV you have left when you get to orbit (which I find odd, since IMO that only matters for interplanetary flights without refueling).

In my case, I quote that figure as a measure of how efficient my ascent profile was in multiple ascents. The more delta-V remaining, the less was used getting to space. This is not just the amount of delta-V available for orbital maneuvers, but also a measure of how over-designed the spaceplane is for simply reaching orbit with its current payload mass.

[BubbaWilkins]

BubbaWilkins,

I'm not so sure on the gradual climb thing. In my experience with this testing, there are a few things conspiring against that. First is massive drag losses. You want to expend as little fuel down there as you can because that's less dV claimed by drag.

The other thing is that the RAPIER and turbojet both don't like the lower atmosphere. They have to get up high. The RAPIER has it worst, delivering only ~60% of max jet power at sea level. Neither of those two engines really come into their own until you clear 15km. I always rocket up through the 1-10km zone as fast as possible to get my engines to max power and to get clear of the drag. The 10-15km climb cannot be overdone or else you could flame out prematurely and with the RAPIERs, that will cost you precious rocket fuel. Yeah, the ascent is a tricky thing to do. The ease of use of the RAPIER probably tricks people into not flying proper ascent profiles, so they get confused and angry and blame the engine.

It may be the difference between FAR and stock, or perhaps due to the craft mass and TWR. My smallest craft still tend to be much larger than those I see other people using. I've never really had much luck with minimalist designs. I thought it was interesting that the ISP for a Rapier spiked as early as it did. I'd be curious to see how it responded to other atmospheres other than Kerbin. Sadly, I have no easy way to test this.

[theend3r]

It never even crossed my mind to make a spaceplane without LV-N and for that the RAPIERs are obviously useless.

[Levelord]

I see a lot about turbojet vs RAPIER. Now, I did some testing of that late last night. The results are intriguing. This was the craft.

http://i.imgur.com/uRy1ueH.png

The test was done with both RAPIERs and turbojets.

With RAPIERs, they switched over at over 30km and 2100m/s. That's the first plane I've ever circularized in atmosphere. Those engines can hold on to a non-existent ammount of intake air.

http://i.imgur.com/KtFh0sP.png

Now, I repeated the test with just turbojets. They flamed out at about 27.5km and I was unable to achieve the same height, though their higher power meant I was going faster, ~2250m/s.

With some 48-7S engine pods on the wings, that would have got to orbit too, probably more efficiently than the RAPIERs, though I think that can be at least partially attributed to how overpowered those engines are.

I took the liberty of measuring the fuel payload fraction of the craft, which means that your craft saved

Liquid Fuel: 290/540 x 100 = 53.7%

Oxidizer: 440/660 x 100 = 66.7%

When it reached orbit.

A ship I made in comparison (I took a single LV-909 engines since the main argument people keep throwing up is that the 48-7s engines were too overpowered, intake ratio is also standardized to your 4:1 ratio) has this fuel payload fraction:

94-95km stable circular orbit:

Liquid Fuel: 203/360 x 100 = 56.3%

Oxidizer: 267/440 x 100 = 60.6%

And keep in mind this was also carrying an extra monopropellant tank and a SAS module as per my standard build requirements for a spaceplane and it still achieved orbit, with the same relative payload fraction as yours and carrying 180l less fuel and 220l less oxidizer at launch. That's an entire FL-T400 tank of fuel I decided to not add to the plane. With an ISP of 390s (compared to the RAPIER's 360s) I can do significantly a lot more orbital maneuvers as well, since TWR is less important compared to ISP in space.

In all honesty the height at which the engines flame out is not important because the thing that determines if you will reach orbit is your horizontal velocity to push out your parabolic trajectory. At 2250m/s the measly ion engines are actually enough to get you into an orbit because 2250m/s is already orbital velocity. I'm surprised you didn't take the advantage to push out of the atmosphere with a bit of rocket power instead of waiting for the jet engines to flame out, it's such a waste considering how fast you were going.

Edited February 1, 2014 by Levelord
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