ComradeGoat

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:

Ok, here goes. I'll write this as I do the test. 2 turbojets, 1 RAPIER (RAPIER in manual switching mode to try and get best performance out of it) At 10K in 1:33, 282 m/s, 818 fuel left (got there quicker than the aerospike variant, consumed about the same amount of fuel). Conclusion - it's fast, which you'd expect it to be with 3 jet engines running simultaneously. Started to get asymmetric flameout and had to reduce thrust lower than I normally would, as expected (25K - normally this plane can run at full power to 27K). Switched turbojets off at 30K (normally would be a bit higher) and going full thrust on the RAPIER in air breathing mode. I'm pushing apoapsis, but only just. Let's see how close this can get me to orbital speed. I'm slower than I would have hoped for at this point because I had to throttle back earlier. Presumably adding intakes would have mitigated this. I'm having to pitch up at 45º to push apoapsis - damn this thing is gutless in air breathing mode! Flameout at 36K, reduced power, it's reignited. I'm about to hit apoapsis, got to switch to rocket mode now. Reached orbit without complications. Apoapsis 108km, Periapsis 100K, 1249 m/s delta V remaining, 381 fuel, 449 oxidiser. OK, now for the three RAPIER config. Initial thoughts on pulling off the runway - this feels about as powerful as the twin jet + aerospike config. 1:50 to 10K, 816 fuel left. Second fastest, but it's using more fuel than either of the turbojet configs. Again, starting to have to throttle back at 25-26K At 32.5 K I couldn't push apoapsis any more. Throttled up, engines switched to rocket mode. Orbit, 107 * 102, 1046 m/s Delta V remaining. 371 fuel, 431 oxidiser. So in order: To 10K: 2 jets, 1 RAPIER: 1:33 3 RAPIERS: 1:50 2 jets, 1 aerospike:2:13 2 RAPIERs: 2:45 Take home message - 3 jets are faster than 2, turbojets are faster that RAPIERs (entirely expected result). Delta V remaining on 100K orbit: 2 jets, 1 aerospike: 1467 m/s 2 jets, 1 RAPIER: 1249 m/s 2 RAPIERS: 1247 m/s 3 RAPIERs: 1046 m/s Note that the more RAPIERS, the less Delta V the plane has left in orbit. 2 RAPIERs is gutless and you don't lose anything by dropping one of the RAPIERs and adding 2 jets. HOWEVER, you do need to faff about manually managing the switching mode. The two planes powered entirely by RAPIERS suck, basically, a lot. 2 jets and 1 RAPIER is fast at low altitude, but in space and high altitude jets and an aerospike beats it every time. Don't even get me started on 2 jets + 1 NERVA.

I agree that this engine has a role in helping soften the learning curve of building planes. However, I disagree with the above - the RAPIER is underpowered compared to the turbojet, which means you'll need to fly higher to attain orbital velocity while airbeathing, which means more intakes. The alternative is to start your rocket ascent from a lower speed/lower altitude, increasing your fuel requirement, which means increased complexity and increased weight. The simple fact is that the RAPIER is, basically, gutless and heavy, and to compensate for that you need more fuel, more intakes, more complexity. Turbojet designs can be lighter and faster, and if all you want is an orbital shuttle, just stick a pair of LV 24-77s on your wingtips. Job done. Regarding more complex designs, I've done a little experiment. I have a plane, the Seakrait Mk1, which is a twin turbojet+aerospike design. If the RAPIER is going to win over a turbojet design, it's here because it's one of the rare cases where a RAPIER design is lighter. I flew to a 100k orbit (ish), and saw how much delta V I had left. I also took a snapshot as the plane crossed 10,000m. I then replaced the turbojets with RAPIERs and removed the aerospike. The plane was still well balanced and flew nicely. They took off with the same level of fuel (this plane does not take off fully fuelled, but has empty tank space to allow it to reach Jool after refuelling in orbit), were both flown in air breathing mode for as long as they were able to make progress to orbit, and then switched to rockets. First the original design, with the aerospike: Passing 10K, mission time is 2:13, we're doing 220m/s and have 819 units of fuel left. Orbit. Ignore the mission time - I fast forwarded until on the light side to get a better photo. Delta V remaining is 1467 m/s, 412 units of fuel, 493 units of oxidiser. Now for the RAPIER. I wasn't able to climb as steeply to 10K, as it simply did not have the power, necessitating a shallower ascent. It passes 10K at 2:45 - 32 seconds slower, is travelling at 208 m/s, 12 m/s slower (remember, this craft is LIGHTER - most RAPIER craft will be HEAVIER than their equivalent jet powered craft) and has 816 units of fuel left, so it's burned 3 units MORE than the heavier craft. Now orbit. I really did eke out as much as I could from air breathing mode, until I had to throttle back so much that I couldn't chase my apoapsis any more. We have 1247 m/s of delta V left, 220 LESS than the HEAVIER craft, and have 396 units of fuel (less than the heavier craft) and 461 units of oxidiser (less than the heavier craft). Frankly, this is a test the RAPIER should have excelled at. As a RAPIER is heavier than a turbojet + 2 LV 48-7S engines, it's pretty much always going to suck compared to that config. It's got no hope against a NERVA powered spaceplane. If it's going to be competitive, it should be against an aerospike, and it just isn't. Not only that, the aerospike plane was much nicer to fly, given the RAPIER powered plane struggled to gain speed at low altitude. They don't lead to simpler craft, because you have to overbuild to overcome their deficiencies. As a rocket, they're OK, but heavy. As a jet, they're really quite poor.

See the design I posted above. No danger of spinning out, 5% lighter, faster, jets also don't use oxidiser, only needs rockets to circularise, no extra mounting points needed (you can mount a 48-7S pretty much anywhere by putting it on a cubic strut). I can't see replacing two jets with one RAPIER. The RAPIER in jet mode is slower and less powerful, so if you needed two jets to lift a heavy SSTO (e.g. one that uses a NERVA as its main engine), you'll still need two afterwards, only it'll perform worse because the RAPIER is heavier, so you'll need to switch to rocket mode earlier, burn more fuel to get to orbit, AND have less range once there because your plane weighs more. In summary, for light designs it performs significantly worse than a single turbojet and 2 LV 48-7S engines; for high endurance design it performs dramatically worse than 2 turbojets and a NERVA (good luck getting 5,000 m/s range from a single RAPIER engined design). It's heavy, slow, and it also looks like it's been beaten violently with the ugly stick. I've tried to find a use for this engine, but I can't find a single one which isn't solved better with turbojets and careful plane design. ETA: Even the twin turbojet + aerospike designs: 2 turbojets and an aerospace weighs 3.9 tonnes. 2 RAPIERS (no way you'll get a plane that heavy into orbit with just one) weighs 3.5 tonnes, slightly less, BUT: the RAPIER is less powerful than a turbojet, so you'll need to use rocket mode earlier, or airhog intakes, you still have the problem of asymmetric thrust, and the aerospike has a higher specific impulse. So for my 3 reference designs: 1 turbojet + 2 LV 48-7S vs RAPIER - turbojet wins (RAPIER design doesn't even reach orbit) 2 turbojets + NERVA vs 2 RAPIERS - not even a contest. NERVA plane will take off from the KSC, land at Laythe, plant a flag and return to Laythe orbit while the poor RAPIER plane is still looking for a filling station somewhere around Duna. 2 turbojets + Aerospike vs 2 RAPIERS (This plane has over 3000 m/s delta V in space) - Aerospike design wins, but not as dramatically as the NERVA design.

I made this: It jets! It rockets! It SSTOs! It RCSes and docks! It ion thrusts for around five Kays of delta vees! It has MASSIVE solar panels that dwarf the aircraft itself! Somewhat surprisingly, it flies quite well too.

Thanks! The Mk1 to Mk2 convertor is really handy for that extra space at the back.

Nope. This is my workhorse SSTO shuttle. It's a lovely little spaceplane that flies very fast, makes orbit very quickly, and is basically lovely. No sketchy clipping. Two cubic struts to mount the engines. If you have some sort of ethical objection to those, you can swap the LV 48-7S engines for the LV 24-77 radial mount engines. Plane still works fine. I tried swapping this engine array out for a RAPIER, btw. The plane was around 5% heavier and didn't even come close to making orbit.

A turbojet and 2 LV 48-77S engines is lighter than a RAPIER. The only engine configs I use on my current generation spaceplanes are: 1 turbojet, 2 LV 48-77S 2 turbojets, 1 aerospike 2 turbojets, 1 NERVA The only one of those where it might make any sense at all to use the RAPIER is the second: possibly a twin RAPIER design to replace the three engines, but I like to build those designs so that at a critical point, usually about 25K when asymmetric flameout starts to look like a risk, I can light all three engines, throttle down and get to orbit with a decentish ISP and a good amount of thrust without intake spamming. The RAPIER doesn't let me do that. So after a bit of initial enthusiasm and experimentation, I've found it more or less completely useless in my spaceplanes.

The issue is that other engine types lose specific impulse as the surrounding pressure increases. Aerospikes do not. The pressure is very high on Eve.

I may be going out on a limb here, not having built an Eve ascender yet, but aren't you supposed to use aerospikes for that?

Test flights of my latest single pilot orbital shuttle, the Labrys Mk III (named after the double headed battle axe its wings vaguely represent), after delivering a pair to Laythe. Remote piloted one down to the surface to test it in an unmanned scenario, then flew down with an older generation plane with a pilot, landed nearby, taxied over, swapped planes and flew the Labrys back up to my orbital station. On the way up, something quite photogenic happened. May use it for my new flag. ETA: Speaking of Laythe, things are getting kinda busy there: So I've started work on a second station - docking at this one is getting to be a bit of a 3D jigsaw puzzle!

This is utter win, right here.

Not sure you will. Get a low Jool orbit, just outside the atmosphere. Thrust retrograde until your periapsis is negative, thorn prograde, wait to enter the atmosphere. Enter the atmosphere, take the sample, full throttle and get the hell out of there. Should be able to do the whole manoeuvre for a few hundred delta V. Climbing back up the gravity well from low Jool orbit is left as an exercise for the reader.

Hmm, I suspect your solution could involve being at about 120km, a periapsis just disappearing, haste, and a ZOMG AEROSPIKE! Possibly more than one.

Get torn off the ladder, I expect.

Not true. You just need to exit Kerbin such that your perihelion is lower than Kerbin's orbit by a suitable amount so you can catch Duna up, and your aphelion gets the intercept. It's still faster than waiting for the window.

If you're nearer the last launch window than the next one, it's easy: just burn past it until you get an intercept, keeping your perihelion at Kerbin. If you're nearer the next launch window than the last one, it's harder, because you'll have to lower your perihelion to speed your orbit up and catch it up. You might even have to resort to a manoeuvre node to get an intercept, or something. Anyway, you can always get there, more or less. As someone said above, you just have to arrange to be in the same bit of space at the same time it is. Treat it like orbital rendezvous, only bigger, because that's what it is.

Why can't the main ship aerocapture?

Just been experimenting by throwing stuff at Duna well outside the launch window (after it by making aphelion most of the way to Dres, letting Duna catch up, and intercepting on the downswing). I expected it to take a lot more fuel. It really doesn't; couple of hundred delta V extra, if that. Think I might give up on waiting for Duna launch windows and just send stuff there when I feel like it. The transfer time gets quite a bit longer though.

Designing a good Duna lander is hard. My current one uses 2 LV 48-S engines and a drogue chute, and has proven to be something of a deathtrap. Basically, if it works on the Mün, it'll work on Duna if you add a parachute, isn. The problem comes with parachutes being kinda unpredictable on Duna because the air pressure varies greatly when they open, depending on terrain height. After losing too many Kerbals, I have given up on the "Stick a chute on a Mün lander and hope for the best" approach. My new Duna lander, of which I have a couple in transit to my Duna station, uses 3 radial parachutes, 3 of the little ones from the small capsule, programmed to open after the radials, is reinforced with struts to hold the thing together when the chutes "snatch", and uses a ZOMG! AEROSPIKE! on the bottom. Marginal TWR be damned. Check out this bad boy:

Duna and Kerbin have quite similar orbital periods, so it takes a long time between transfer windows, 2-3 kerbin orbits. Contrast with Jool, which has just under 1 transfer window per orbit of Kerbin, and Moho, which has one every 30 days or so.

Others have commented on a slow, nose-up approach to scrub off your horizontal velocity. Here's what it looks like if you get it right: my Labrys Mk-III spaceplane gliding into a landing. It landed at about 20 m/s, SAS held the nose wheel off for a bit longer, and it drifted to an uncomplicated stop. Obviously it lands a bit faster with fuel on board, but a good design should still be able to glide to a landing at about 30 m/s horizontal. Kerbal Engineer Redux really helps with this (as seen here on the left):

The tipping force often comes from pivoting forward onto the nose wheel. An easier way to avoid this is just disable the nose wheel brake in the SPH, using tweakbles. Instant stability!

Yes, important to read the key; note the little arrow, which indicates aerobraking as a possibility. That means that arc is almost free in that direction. BTW, I'd want at least 1,600 on a Mün lander, just to be sure. I like to have 2,500-3,000 in them, so I can do precision landings and not worry too much about my fuel budget. The figures posted there assume a perfect suicide burn, which is more-or-less how you land on Tylo with a ship of any sensible size, but a Mün lander with 2,500m/s delta-V really isn't very big. Go on, spoil yourself!

Agree with Skorpychan - trying to make jet ascent rage rockets is annoying. Leave 'em for planes and just stick another Mainsail stage on your rocket. You'll pull less hair out that way, IME.