RAPIERs don't suck!: A complete performance evaluation

[Captain Sierra]

Mods, if this should be in tutorials or somewhere, feel free to relocate it. I wasn't 100% sure. Back to business.

After reading the forums for a week, I've noticed an increasingly alarming number of threads confused about the RAPIER or just straight up bashing it. The usual arguments are it's lower thrust and higher mass than the usual engine choices, both jet and rocket; and it's lower efficiency than other choices like the LV-T45 Liquid Fuel Engine or Toroidal Aerospike Rocket, and TurboJet Engine.

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First off, let's just look at it's base stats.

It has an impact tolerance of 20m/s. That's >2.5 times greater than the turbojet, making it useful for VTOLSSTOs that come in for rough landings.

It masses 1.75 tons, making it the heaviest 1.25m part in the game, heavier than the LV30 and Aerospike, which both weigh in at 1.5 tons.

It is the only air breathing engine with an attach node on the bottom.

it's Isp as a jet is 800-1200 (ASL-Vac) and it's Isp as a rocket is 320-360.

Now, that's that, so let's get into some actual testing results, because numbers can't tell the full story.

I decided to fly a series of missions to identify the key performance differences in engine layouts by interchanging RAPIERs and different positions to achieve different results. These tests were done across two different test aircraft designed to represent different styles of SSTO building. While there are still more scenarios I'd like to test, I believe the results achieved by these test flights will be substantial in making the case that the RAPIER is not poorly balanced, but rather being misused.

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Disclaimer: I do not claim to be a master SSTO builder. I consider myself of average skill in building basic SSTOs. My ascents are not perfect, nor is my craft design. But in keeping with the same throughout all testing, I do claim that these results are valid and representative of results achievable in practice.

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TEST 1: Turbojet + Aerospike baseline

For the first battery of test flights, I used an aircraft which is based off the Aeris 4A. The ascent profile used is a basic 'climb to 20km, accelerate to 1200m/s+, pitch up 30 degrees, punch out of atmosphere.' This is a hybrid profile between brute forcing your way to orbit with rockets from 15km, and an air hogging '35km on jets' ascent. This profile is used across all aircraft. The target orbit is between 70 and 80km circular.

The Aerospike was used as the rocket engine of choice because it has the same thrust as the RAPIER in rocket mode.

The ascent did not climb as fast as it should have during the first 10km, likely due to lack of lift.

The aircraft was able to achieve good speed and had acceptable thrust for the duration of the jet ascent. This craft has 4 clipped ram intakes to provide air at good altitude and a intake ratio of 2:1.

This particular aircraft suffers from a notable drop-off of thrust power when the jets cut out and the aerospike alone is powering the craft.

This craft achieved orbit with 1,101m/s of delta V remaining for orbital maneuvers, rendezvous, etc.

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TEST 2: 2 RAPIER engines

For the next test, the aerospike was removed and a useless adapter was put in it's place. A nosecone was not used as they now have some control. A nosecone facing backwards on the rear of a craft will make the craft's rear want to come around and pass the front. The turbojets were replaced with two RAPIER engines.

In this configuration, and with all RAPIER tests, Auto switching was used to allow the RAPIERs to do what they do.

While overall thrust was lower during jet ascent, the lower weight made the losses less noticeable later on. During the initial climb to 10km, however, the RAPIERs produce less than 2/3rds of their thrust capacity, resulting in a slower than usual climb. Another notable fact is that the RAPIERs where able to hang on to air breathing mode for a VERY long time, way longer than the turbojets ever could.

The last key fact is that having twice the thrust during the rocket ascent was a significant difference to the last flight and the results of that were tangible.

This craft reached a slightly higher orbit with 1,348m/s of delta V remaining.

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TEST 3: RAPIER engine + Turbojets

In this test, the turbojets were put back and a single RAPIER took the place of the aerospike.

This aircraft had the best jet TWR of them all, allowing a fast climb to altitude and to cruising speed. The turbojets were shut down as they flamed out and the ascent continued on the RAPIER. Due to an action group mishap, two of the air intakes were closed when the turbojets were shut off, resulting in the RAPIER engaging rocket mode sooner than necessary. The test shall be repeated at a later time. The rocket ascent was much like the first test as the thrust was the same. The main difference was that the RAPIER was able to get higher before needing to switch mode, compensating for the lesser efficiency.

This craft made orbit with 901.m/s delta V remaining, clearly showing the RAPIER's lower efficiency (as well as possibly some losses due to a less-than-ideal RAPIER ascent).

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TEST 4: Single stack SSTO

Next up, we switch to a smaller SSTO craft which uses a single stack. Before the RAPIER, clipping was required to make this craft work, as evident.

This craft is lacking in lift and thus the test will be rerun with the wing configuration used with the RAPIER powered test. The results are likely mildly exaggerated due to imperfections in the craft design.

This craft was slow to take off and suffered from mediocre TWR during all phases of the ascent. The engine layout is an aerospike clipped into a turbojet. The forward fuel tank is devoid of oxidizer to provide liquid fuel where a full mk 1 fuselage is unnecessary.

Even while maintaining the 2:1 intake ratio, the aircaft had difficulties reaching higher altitudes. The aerospike had to be called on sooner than was preferred. This was compensated by the ability to run both engines at maximum power until jet flameout due to the center of thrust being aligned with the center of mass (mostly, read on), allowing the turbojet to be milked for a good bit during rocket ascent. The loss of the turbojet was again felt as the TWR dropped off.

During the orbital insertion, it became clear I was not perfect in my clipping as the aerospike was slightly too low and the craft pitched up violently under thrust. This will have to be corrected.

This craft made orbit with 70.73m/s of delta V remaining, by the skin of my teeth.

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TEST 5: Single stack SSTO w/RAPIER

During this test, the wings were redesigned to fix the lift and controlability issues (warranting a rerun of the last test to ensure validity) and the clipped engines were traded out for a single RAPIER. In this case one engine weighs less than two, and the rocket thrust remains the same (but with less efficiency and the benefit of gimbal) and jet thrust takes a hit, or so we think.

During the initial ascent, the lower weight was immediately present as the tail section as a whole ton lighter. The RAPIER really shined at the latter stages of high altitude. The RAPIER, as I discovered, is capable of running on less than five thousandths (>0.005) of a unit of intake air, so low that the resources tab won't even register it.

Additionally, the RAPIER has just over half it's maximum thrust on takeoff, and doesn't hit more than two thirds of it until it clears 15,000 meters. The RAPIER is a super high altitude engine in every sense of the word, and while it may appear inefficient, it really is because people judge the wrong resource. In terms of intake air, it's the most efficient engine there is.

Even up to 28km and 1500m/s as I held pitch at 30 degrees, expecting them to go rocket, they held on to the very end of air breathing mode, saving loads of rocket fuel by doing so.

The following image was taken right after the RAPIERs switched, only delayed by my finger reactions.

This craft reached orbit with 1,290m/s of delta V remaining.

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In conclusion, the RAPIER is a powerful new tool for SSTO building that, when used correctly, is able to really improve the performance of spacecraft, as well as enable new and smaller constructs to take flgiht with greater efficiency than ever before.

I do intend to repeat tests three and four to validate the results as well as add some additional results to this. I plan to add two new tests with the three engine craft: 3 RAPIERs and 2 RAPIERs paired with a single turbojet. For now, I am burnt out. The second post will be reserved for posting a gallery of all my testing images, for the curious.

[Dizzle]

You're preaching to the choir, but really nice work.

[magnemoe]

Why use the aerospike as rocket engine? heavy with poor twr and probably more powerful than you need. The high isp in atmosphere is useless as you fire it at +20km attitude.

T45 would be an better choice, T30 if you don't need the gimbal.

I have an feeling that the rapier need an different approach. as its isp in atmosphere is lower you don't want an long and slow climb, you want decent twr, perhaps using an turbojet as extra engine to climb faster.

I have used in on SSTO rockets where it clearly outperform jet+rockets getting an three man pod to orbit.

[Claw]

Thanks for the comparison. I like this approach much better than the "RAPIERS suck...", "No they don't!" debate.

I've been mostly playing with TurboJets and small rockets, trying to learn and tweak. Maybe I'll spend a little more time with the RAPIER on my smaller space planes again.

I've been trying on my larger space plane (80+ tons), and I like the TurboJet/RAPIER combo better than TurboJet/Aerospike by far. Maybe I'll try all RAPIERS, but that will make it heavier than I was aiming for.

[BubbaWilkins]

You're preaching to the choir, but really nice work.

There are other threads which would indicate otherwise.

Given that the results were drawn using identical methodology, I have no complaints. Well done.

I would say as point from my recent SSTO experiences with Rapiers, that the early ascent to 10k-15k is as much a factor in performance as anything else. I think too many people just go nose up at 45deg's a call it good. The problem with this is that the gravity and drag losses are much greater resulting in slower ascent times to these first keystone altitudes. It seems counter intuitive, but an angle of 20-30 deg's on the initial ascent allows you to build up more velocity resulting in a slightly faster rate of ascent. But the key difference is that you will be travelling 2x-3x and thus be much deeper in the "powerband" of most of the normally aspirated engines. This results in much harder acceleration through the upper altitudes and an overall shorter time to orbit. End result is less DeltaV wasted in the atmosphere.

For reference: Here is one of my crafts which made 100k orbit with Rapiers in 8:24.

I have an feeling that the rapier need an different approach. as its isp in atmosphere is lower you don't want an long and slow climb, you want decent twr, perhaps using an turbojet as extra engine to climb faster.

I have used in on SSTO rockets where it clearly outperform jet+rockets getting an three man pod to orbit.

It's ISP and Thrust are both effected by altitude and velocity.

On the runaway, just starting out in air mode.

standing still, ISP:830, Thrust:19.9kn

at 200m/s, 400m Alt, ISP:1100, Thrust:105kn

at 330m/s, 5000m Alt, ISP:2500, Thrust: 119kn

at 540m/s, 10km Alt, ISP: 1800, Thrust: 132kn

at 745m/s, 15km Alt, ISP: 1400, Thrust: 175kn

at 991m/s, 20km Alt, ISP: 1280, Thrust: 183kn

I leveled out there, reached 1800 m/s, 20km, ISP: 1303, Thrust: 105

So it appears that the ISP sweet spot is around 330m/s, but thrust doesn't really hit until 15km (as noted above by Captain).

Pretty decent numbers considering the dual mode functionality.

Edited February 1, 2014 by BubbaWilkins

[Captain Sierra]

Why use the aerospike as rocket engine? heavy with poor twr and probably more powerful than you need. The high isp in atmosphere is useless as you fire it at +20km attitude.

T45 would be an better choice, T30 if you don't need the gimbal.

As clearly stated in the main post, it's because it has the same thrust as the RAPIER in rocket mode, so replacing the aerospike with a RAPIER would not change the power of the rocket ascent profile. It was a choice to further 'sterilize' the testing and produce more uniform results by removing power from the equation in certain tests. Also, the LV-T30 got heavier in .23 and is now the same weight as the aerospike (even though I disagree about that).

I have an feeling that the rapier need an different approach. as its isp in atmosphere is lower you don't want an long and slow climb, you want decent twr, perhaps using an turbojet as extra engine to climb faster.

It seems to do fine as soon as you get it above 15km. I climb at 45 degrees to get above 10km, then drop the climb down to 30 to start building some speed and finally 20 degrees at 15km. By that point, the RAPIERs are doing their job quite well.

I have used in on SSTO rockets where it clearly outperform jet+rockets getting an three man pod to orbit.

I wouldn't say that's the RAPIER's main job. It works amazingly on aircraft and it also works great in tandem with turbojets. I suspect some of my best results will come with the 2 RAPIER, 1 turbojet test which I intend to fly tomorrow.

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.

Edited February 1, 2014 by Captain Sierra

[WhiteWeasel]

Also An LTV is much longer than an aerospike, making it more awkward to incorporate into a design that won't smash it on takeoff.

[I_Killed_Jeb]

From my experience so far the RAPIER really shines when used in high air-breathing TWR configurations, taking the craft to high velocities quickly and efficiently and punching out to orbit, minimizing gravity losses and preserving lots of dV even in the face of increased mass

[Captain Sierra]

Also An LTV is much longer than an aerospike, making it more awkward to incorporate into a design that won't smash it on takeoff.

This is true. Given the landing gear was positioned to allow easy take-off under 100m/s, I almost tail struck the RAPIER on the smaller SSTO twice, and it's about the size of a turbojet, lead alone an LV45!

From my experience so far the RAPIER really shines when used in high air-breathing TWR configurations, taking the craft to high velocities quickly and efficiently and punching out to orbit, minimizing gravity losses and preserving lots of dV even in the face of increased mass

Given that it can cling on to air breathing longer than I thought possible by the game scripting, I have to agree with you. It's ability to maintain air breathing efficiency in the face of common sense is uncanny. I have no doubt that makes for great results when you're packing high TWR. I expect to see this with the triple RAPIER configuration test tomorrow.

[Dispatcher]

Very nice to read, Capt'n. I'll not hog your thread by repeating my graphs, but will refer to them. Some points. While the RAPIER (air mode) only dead lifts about a third of the mass that an aerospike can dead lift, this is certainly offset by the use of lifting surfaces and other aerodynamics. Also, while the aerospike excels by lifting and burning a maxed mass of mostly fuel to an altitude of about 22,500 km, the RAPIER Air excels by lifting a maxed mass (50% non fuel, this time) to an altitude of about 630 km or so. Actually, it does well with about 7 or 8% starting fuel mass. It also excels in an efficiency test (only the smallest fuel tank used). In fact, in followup graphs, I excluded results from either the aerospike or the RAPIER Air (depending on the graph) in order to make the results of other engines clearer.

On a more practical note, my best pre-RAPIER SSTOs combine the use of turbojet and aerospike engines. These were very challenging for me to design and fly to orbit. Now, RAPIER only SSTOs make it easy for me to achieve orbit, and my designing and testing time has been greatly reduced. The RAPIER can be a very useful engine.

My test result graphs are found in this thread: http://forum.kerbalspaceprogram.com/threads/67403-Graphs-of-Engine-Comparison-Flights

[SuperBigD60]

I don't know how well represented in the math it is, but for the stock game, usually the most efficient ascents for SSTO spaceplanes involves using enough intakes such that you can be in airbreathing mode as high as 32-35 or so km and reach orbital velocity up there. I know there was a thread on it at some point, and I vaguely remember testing out designs like that myself.

I wonder if this principle is still true, if it can help your point, and also if the Rapier would make this ridiculous sort of altitude easier to achieve.

You know, food for thought

[Captain Sierra]

I don't know how well represented in the math it is, but for the stock game, usually the most efficient ascents for SSTO spaceplanes involves using enough intakes such that you can be in airbreathing mode as high as 32-35 or so km and reach orbital velocity up there. I know there was a thread on it at some point, and I vaguely remember testing out designs like that myself.

I wonder if this principle is still true, if it can help your point, and also if the Rapier would make this ridiculous sort of altitude easier to achieve.

You know, food for thought

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.

[Brotoro]

I wouldn't say that RAPIER engines 'suck' at all, but I've gotten better results on my spaceplanes using the old turbojet/rocket engine combos.

In my Longterm Laythe series, I have tested two spaceplanes in recent episodes (parts 23, 24, and 26).

They are almost identical spaceplanes: One is the Ladyhawk, powered by a turbojet and four Rockomax 48-7S engines:

The other (below) is the Raptor, powered by a single RAPIER engine:

I am certainly not an expert at flying SSTO spaceplanes, but what I found was, first, that you can get a big difference in how well the spaceplane does depending on how you fly it (as is well known by those who fly spaceplanes). In my first test flight of the turbojet Ladyhawk, I reached Laythe orbit with 599 m/s of delta-V remaining onboard. In my second flight, I was cruising along at a higher altitude with the turbojet sipping fuel, and found the magical moment where my apoapsis was rising rapidly before I switched to rocket power…that flight made it to Laythe orbit with 2567 m/s of delta-V remaining.

For the RAPIER-powered Raptor test flights, I first tried to cruise along at high altitude…but I just wasn't getting good results like that second Ladyhawk flight, even after 30 minutes of trying. The Raptor made it to Laythe orbit with 952 m/s of delta-V remaining. For the Raptor's second flight, I tried using the RAPIER at a lower altitude and higher throttle, and got better results, making it to Laythe orbit with 1467 m/s of delta-V remaining.

So I think spaceplanes with the two types of engines need to be flown differently for best results (and I expect spaceplane experts could get better results than I did).

As to HOW I flew the planes, the graphs below show the altitude, speed, apoapsis altitude, periapsis altitude, and scaled throttle and intake air measurements for the two best flights.

First is the Ladyhawk (turbojet) second flight (2567 m/s delta-V remaining):

And below is the Raptor's (RAPIER) second flight (1467 m/s delta-V remaining):

Remember, these are flights done at Laythe, so the scale height of the atmosphere is 80% that of Kerbin's atmosphere (so I expect similar effects can be seen flying at correspondingly higher altitudes over Kerbin).

[Dispatcher]

Brotoro, what did you use to capture your results? MechJeb or something else? What graphing app? Calc or something else?

Nice work and nice planes BTW.

[Brotoro]

Brotoro, what did you use to capture your results? MechJeb or something else? What graphing app? Calc or something else?

Nice work and nice planes BTW.

I was taking screen shots every 30 seconds (or more often if interesting things were going on), and I had MechJeb's orbit data and surface data windows displayed. I used those to enter the data in Excel for plotting. Yes, I expect there is some nice way to automatically capture and plot the data, but I have not looked into that.

[Captain Sierra]

I think the 48-7S being as overpowered as it is definitely has something to do with those results, but I cannot say anything factual until I run some of my own tests on it.

In the meantime:

SuperBig asked how far you could push the RAPIER on air breathing. Let's find out!

[UNSC]

Poor aerospike; it used to be so OP

[Levelord]

I've had mixed results, but the RAPIER is more forgiving to introduce newer players to the concept of SSTOs while not being particularly efficient, while the traditional Jet/Rocket combos are for more experienced players who simply want to min/max their crafts with the drawback of being more difficult to control.

The RAPIER seems to be less useful for my light aircraft, so I often use a jet/rockomax combo for mine.

Sourced from the wiki:

RAPIER ENGINE:

Atmosphere:

Weight: 1.75t

Thrust: 190kN

ISP: 800s

Fuel Consumption: 0.30 l/s

Space:

Weight: 1.75t

Thrust: 175kN

ISP: 360s

Fuel Consumption: 9.90 l/s

Now compared to a TurboJet + 4X rockomax combo you get:

TurboJet Engine

Weight: 1.2t (Turbojet wins out on the weight)

Thrust: 225kN (Turbojet wins out on thrust. Spends less time in the thick part of the atmosphere lifting the plane)

ISP: 2500s (Turbojet wins out on the ISP, it is more fuel efficient allowing a light craft to carry less fuel)

Fuel Consumption: 0.11 l/s (Consumes only a third of the liquid fuel compared to the Rapier in air breathing mode, allowing less even less fuel to be carried)

Rockomax 48-7S

Weight: 0.1t (x4 = 0.4t) (Wins out on weight, being 1.35t lighter)

Thrust: 30kN (x4 = 120kN) (Has less thrust, but has a higher T/W ratio)

ISP: 350s (has 10s less ISP)

Fuel Consumption: 1.75 l/s (x4 = 7 l/s) (Consumes 2.9 l/s less fuel)

Now combining the TurboJet and the 4x Rockomax, we get a combined:

Combined Weight: 1.2 + 4(0.1) = 1.6t↑ (Lighter than the RAPIER)

Thrust atmo: 225kN↑ (more thrust than the RAPIER)

Thrust vac: 120kN↓ (Less thrust than the RAPIER, but like ion engines, you probably only need a little nudge to get out of the atmosphere)

ISP atmo: 2500s↑ (higher ISP than RAPIER)

ISP vac: 350s↓ (only 10s less than RAPIER)

Fuel Consumption atmo: 0.11 l/s↑ (1/3 less fuel consumption than RAPIER)

Fuel Consumption vac: 1.75 l/s↑ (2.9l/s less fuel consumption than RAPIER)

The RAPIER probably has its uses for heavier craft, but for lighter craft, I find the jet engine/rocket combo eaier to handle given the stats. I hardly build very large SSTOs anymore, but when I do I'd probably use the RAPIER.

Now the case in point is how I use the combo on my latest Marin light SSTO

Keep in mind the engines are clipped with octagonal struts to make it look neater, but in reality, I could just place the struts and engine outside and the plane would still work exactly the same.

Here's a variant using the Rockomax 24-77 for you non-clipping purists out there. It will works the same for light craft and is more useful than the RAPIER in terms of stats.

I believe the biggest problem I saw with the RAPIER given it's roughly similar stats was that it has a MASSIVE fuel consumption rate. So you often need to carry relatively more fuel in order to get the things into SSTO. I'd argue that the reason why it's easier to get into SSTO with the RAPIER is that by the time you reach the atmospheric border, you've consumed so much fuel in the first place they your ship is light enough to make the transition jump into orbit anyway. Which is thy the RAPIER probably isn't suited for lighter SSTO craft because it eats through so much fuel.

[Captain Sierra]

You cannot compare my results to an SSTO using the 48-7S, and there are a couple of reasons.

Such SSTOs generate less rocket thrust and require a more jet intensive ascent profile, which I did not use. Jet intensive profiles do not favor the RAPIER engine over turbojets due to it's lower thrust and efficiency. The RAPIER is favored by a slightly more aggressive ascent profile while still not being brute force. 48-7S powered SSTOs do not have the thrust to do this. Jet intensive ascent profiles are something reserved for the more veteran SSTO building community, whereas my results are intended to represent a wider range of playstyles.

The 48-7S is also horridly overpowered considering it's high efficiency, really strong thrust for it's size, and low weight.

[Levelord]

You cannot compare my results to an SSTO using the 48-7S, and there are a couple of reasons.

Such SSTOs generate less rocket thrust and require a more jet intensive ascent profile, which I did not use. Jet intensive profiles do not favor the RAPIER engine over turbojets due to it's lower thrust and efficiency. The RAPIER is favored by a slightly more aggressive ascent profile while still not being brute force. 48-7S powered SSTOs do not have the thrust to do this. Jet intensive ascent profiles are something reserved for the more veteran SSTO building community, whereas my results are intended to represent a wider range of playstyles.

The 48-7S is also horridly overpowered considering it's high efficiency, really strong thrust for it's size, and low weight.

So you're basically saying "RAPIERs don't suck because I fly my ascent profile differently", which has nothing to do with the actual raw stats of the engines in the first place.

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.

My conclusion still stands with the RAPIER being a good introductory engine to fresh pilots on SSTO crafts but with the drawback of less overall stats, and the traditional rocket/jet combo being better for people who really want to push everything they want out of their craft. Both have their uses, but for different reasons.

Edited February 1, 2014 by Levelord
additional notes about the aerospike choice comparison

[Captain Sierra]

So you're basically saying "RAPIERs don't suck because I fly my ascent profile differently", which has nothing to do with the actual raw stats of the engines in the first place.

My conclusion still stands with the RAPIER being a good introductory engine to fresh pilots on SSTO crafts but with the drawback of less overall stats, and the traditional rocket/jet combo being better for people who really want to push everything they want out of their craft. Both have their uses, but for different reasons.

No, I'm saying that my conclusions are based around basic designs representative of what I see quite frequently. The demonstrator craft I used was based of the Aeris 4A.

Now, what I am saying is that 48-7S powered SSTOs are designed to so differently, that it's comparing apples to oranges.

While the 48-7S SSTO design may be better in a lot of different ways, it serves a very different role in the world of SSTOs, and that role seems to be up with more veteran SSTO builders, who probably already know the results of my findings and were not the intended target of this guide.

On another note, I may have been wrong to say that RAPIERs were outclassed in the high altitude, high speed jet intensive ascent. I just circularized . . . in the atmosphere.

[Brotoro]

I don't think the success of my turbojet SSTO spaceplane was the result of the Rockomax 48-7S engines. The rocket engines really did very little in getting the plane into orbit compared to what the turbojet accomplished. Using 24-77 engines would have given very similar results.

[Clockwork_werewolf]

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

I suspect that those who do not like Rapiers are using 8-15 intakes per engine (air hogging). These people would find the jets power them to a very high alt with little fuel use, then all they need to do is add a little rocket thrust when out of the atmo. They would pick a good rocket engine not an atmo rocket engine.

People only using 1 or two intakes per engine (without clipping it is hard but not impossible to have more) will have a different feel. For these people the rapier cuts out at a much high alt and then given them the force to punch though the rest of the atmo without needing to bring lots of rockets lowering T/W. New players that do not know about clipping tricks or players that see it as not in the spirit of the game will therefore find the rapier better.

Personally I find adding 12 intake per engine feels like cheating as I can throw a ship out of the atmo(but not a stable orbit) without needing any rocket engine. If I can do this then all I need is one nuke and I have no need of the rapier but this feels wrong to me. I think the ram air intake should weight 0.5 tons but produce 8 times the air intake.

[tavert]

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.

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

That's a decent point.

The lower jet TWR of the Rapier is only worth it if you get more rocket performance out of the difference in mass than whatever bipropellant engine you choose to pair with the turbojet. If you're already nearly at orbital velocity when you switch from jets to rockets, you rarely need much TWR. At lower speeds and altitudes, your TWR requirement will be higher.

However any fans of the Rapier due to its simplicity should look carefully at your ascent trajectory. Even with small numbers of intakes, substantial efficiency gains are possible by building up as much horizontal speed as you can at the highest altitude your intakes allow you to fly on jet power. The mode switch on the Rapiers should also be toggled to manual, since I believe the automatic switch will occur at the first sign of flameout at full throttle whereas in manual mode you can continue flying higher in air-breathing mode by throttling down.

Assuming a proper trajectory, the Rapier is never going to be more efficient than pairing a turbojet with an LV-909 (or a few 48-7S engines which do outclass every other rocket in most situations - a pair of radial 24-77s is also a good choice if you're staying within the Kerbin system, simple to construct and not overpowered), no matter how many intakes you have. But it will be more forgiving of suboptimal trajectories. The thesis of the thread title of the Rapier not sucking depends on your skill level more than anything else. If you know how to get an efficient jet ascent, on few intakes or many, then the Rapier does suck. If you're new and still figuring it out, then it doesn't.

Edited February 1, 2014 by tavert

[Levelord]

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

I suspect that those who do not like Rapiers are using 8-15 intakes per engine (air hogging). These people would find the jets power them to a very high alt with little fuel use, then all they need to do is add a little rocket thrust when out of the atmo. They would pick a good rocket engine not an atmo rocket engine.

People only using 1 or two intakes per engine (without clipping it is hard but not impossible to have more) will have a different feel. For these people the rapier cuts out at a much high alt and then given them the force to punch though the rest of the atmo without needing to bring lots of rockets lowering T/W. New players that do not know about clipping tricks or players that see it as not in the spirit of the game will therefore find the rapier better.

Personally I find adding 12 intake per engine feels like cheating as I can throw a ship out of the atmo(but not a stable orbit) without needing any rocket engine. If I can do this then all I need is one nuke and I have no need of the rapier but this feels wrong to me. I think the ram air intake should weight 0.5 tons but produce 8 times the air intake.

The discussion is about the engine power given identical intake ratios for each. You are changing the variables to argue that one engine is better than the other when the tests have already established that each engine is running on identical intake ratios.

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.

The only benefit I see to using the RAPIER is allowing you to have a different ascent profile, like what Captain Sierra has done with a more rocket-like ascent. While the TurboJet engine takes advantage of the lifting surfaces of the plane so that you spend more time horizontally gaining speed.

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.

Picking one engine over the other as a clear superior engine and the other to be thrown away just shows that some people can't see that the two engines are used in very different situations and roles, so they end up limiting themselves design-wise in their crafts, and then complain that their crafts have a difficult time getting into orbit etc etc.