SSTO advice sought.

[Vanamonde]

I can routinely put landers on Mun & Minmus, put rovers on Mun & Minmus, stations in orbit around all 3 worlds, I've got some good airplanes, and I can rendezvous in orbit, but after 3 months of playing KSP almost every day, I STILL can't make a decent spaceplane that can actually get to space. Here's my best one so far:

Craft: http://www./?8ps5r8r23dp3pzi

Takes off from runway without (miraculously) diving off the side, then flies on jets to about 15,000m. When it starts to lose momentum I hit the rockets and climb at 60 degrees. It can get up to about 120,000, but only attains about 1600m/s. On descent it flies well on the jets, but before the air is thick enough for them, it can flip around and drop tail-first if you try to change attitude or heading. Now my problem is, this is as far as I ever get with any of my SSTO designs, and I get stuck here again and again. Any change I make from this point on only makes it worse. I think this one could actually make orbit if I could just increase the rocket fuel supply, but any attempt to do that causes a) the dreaded dive to the side on takeoff, followed by parts shaking/scraping off, or it can't get off the ground and shakes/scrapes apart past the end of the runway.

What the expletive and I doing wrong?

[EndlessWaves]

I've downloaded the file to have a play with but a couple of things strike me immediately.

1. Wheels mounted on wings. That'll be causing your instability problems when you add more weight due to the wings flexing. Lining them up along the bottom of the fuselage in six pairs is much more likely to get you to the end of the runway with any load that the fuselage itself doesn't bend and twist under.

2. I'm doubtful how much use you're getting out of that wing after take off. I'd try heading straight up like a rocket and see if that increases the final result.

Edited August 25, 2012 by EndlessWaves
clarification

[cardgame]

That many engines looks like a lot of wasteful weight. Can't you accomplish your purpose with less?

[Suzaku]

I can instantly tell by your pictures that your center of thrust for your space engines is way off from your center of gravity.

Remember, building an SSTO isn't a fashion show, it's about what actually works.

SSTOs are far more complicated than buiding a simple rocket, have you tried building a smaller SSTO first?

[Felix]

KISS (Keep it stupid simple). I have an ssto that can get in and out of orbit much smaller than this. perhaps try looking in the K prize thread for inspiration?

[Vanamonde]

I started putting wheels on wing mounts to keep the things upright AFTER they started diving to the side, but it does seem to have helped when I moved them to the fuselage. I don't know what you're saying might be the problem with the placement of the wing, but if it's the angle of attack issue, I fly it in a nose-up posture that keeps it generating lift.

With fewer than the 6 jet engines, this plane at least fails to generate much speed or attain much altitude. With them, I get to 15,000-16,000m and around 350m/s before igniting the rockets. That way, I figure it's achieving about what a first stage would do for a conventional rocket, while using much less fuel. I don't see how I could reduce the rockets, either, since there's no getting around the need to generate a certain amount of delta V.

Now this is the part that's going to sound ridiculous; I did start small. But they seemed to need another engine to get up to speed, and then more fuel for the additional engines, and then more wing to lift that mass, etc., and next thing you know, it's gone from sailboat to supertanker. That is mostly why this one's got a weird COG. I added to that cantilevered beam at the front to balance the weight of the engines at the back. And odd as it may look, it actually flies straight on the rockets up to and in vacuum, and only gets unruly if I try to turn too sharply during unpowered descent.

So thanks for the feedback and I'll play around with those ideas, but I'm afraid I still don't see how I'm going wrong.

[Felix]

"and next thing you know, it's gone from sailboat to supertanker." I love it.

[Suzaku]

Have you considered ditching the jets and going 100% Rocket engines?

[Vanamonde]

Wow, that's embarrassing. I was just looking at the K challenge, and most of those things are small enough to serve as hood ornaments on mine. Did give me some ideas to experiment with, though.

Have you considered ditching the jets and going 100% Rocket engines?

Well, you see, that's where my weird compusliveness comes in. The whole idea of a real spaceplane (as I understand it) is to use air-breathing engines and wing generated lift to save a lot of rocket fuel on the way to where rockets are required, so that's how I want to do it. Even if that works in the game, it feels like cheating to me. Edited August 25, 2012 by Vanamonde

[Suzaku]

Lol, you should look at my interstellar SSTO if you're looking for the heavyweight king.

[bsalis]

I had a play around with your craft. It's kinda goofy but I actually like it. I can't believe how remarkably docile it is to fly under jet power.

Anyhows, some changes later and i get it into orbit...

Solution was that it just needed MOAR!

Note that the I needed to shut down the jets after starting the rockets since the jet exhaust knocks them off now for some reason. It's also a bit of a handful part way through the burn... had to cut the engines once to get it aligned again. So... no joy to fly, but it gets there.

Edited August 26, 2012 by bsalis

[Suzaku]

I had a play around with your craft. It's kinda goofy but I actually like it. I can't believe how remarkably docile it is to fly under jet power.

Anyhows, some changes later and i get it into orbit...

Solution was that it just needed MOAR!

Note that the I needed to shut down the jets after starting the rockets since the jet exhaust knocks them off now for some reason. It's also a bit of a handful part way through the burn... had to cut the engines once to get it aligned again. So... no joy to fly, but it gets there.

Very nice! I never thought that this design could make it to orbit. Awesome job!

[Vanamonde]

Ha! All you doubters who said I'd never get this thing to orbit, you were... well, you were right. But Bsalis did! Thanks Bsalis!

I see that you added 2 rocket tanks to the engine nacelles, one rocket tank and an aerospike to the back, and a couple of struts to help steady the outrigger wheels. Anything else? Oh, and the rockets tap 2 of the jet tanks through fuel conduits. I knew it was close to the fuel needed to orbit, but couldn't figure out where to squeeze in a couple more tanks that wouldn't ruin the balance. Of course I've tried strutting the wheel mounts, but I hadn't strung the struts all the way to the fuselage. It seems to make a big difference, and it's steadier on the runway now.

Now that I have one that can finally get to orbit, though, I'm discovering other issues. It's about as easy to turn as the Empire State building, which snuck up on me because it actually handles pretty well in atmo, so as I was putting it together I didn't develop a sense of just how big the thing was becoming. Another new problem is that I have almost no practice landing the things because I couldn't get them anywhere from which to land. Empty of fuel and gliding, though, I'm pleased to see it can sustain a descent rate of about 7m/s for quite some time. In my test run just now I misjudged the de-orbit burn, had to double back, and ran out of jet fuel, having to ditch 68km from KSC. But that was after coasting for another 13 minutes after running dry at 8000m.

Did you by any chance try to land it? I have no idea if the gear will stay on under the strain of its weight.

I can't believe how remarkably docile it is to fly under jet power.

My feeling is, if you can't take your hands off of the controls for a couple of minutes at a time without something bad happening, it's not a finished aircraft.

Anyway, thanks again!

Edited August 26, 2012 by Vanamonde

[bsalis]

Did you by any chance try to land it? I have no idea if the gear will stay on under the strain of its weight.

Well, just tried landing it now. To answer your question about the landing gear...

Landing could have been better, and that was in the rough, since KSC was in the dark at the time. Note that it's not so heavy with all the tanks empty. I'm sure it's possible land in one piece.

I don't think splayed landing gear is the way to go. But meh, a few tweaks here and there and you can make it better.

[Markus Reese]

I find that large SSTO is extremely tough. I stopped doing that and changed to a jettisonable main booster stage. I also want to point out that with the 0.16 patch, SSTO that are large are much more difficult to build since you need more fuel tanks and the thrust changes, etc. On a large SSTO, you need to position the rockets on the side so you can have the fuel tanks for them come forward down the side to maintain a balance. I don't have any pictures of my 3man SSTO unfortunately, but it uses four boosters consisting of four tanks and the same engine you used just to get orbit. I had the same flip around problem on re-entry as you. Sometimes I can get a plane that doesnt, but getting frustrated, I began mounting decoupling drag chutes to the rear.

[EndlessWaves]

Ha! All you doubters who said I'd never get this thing to orbit, you were... well, you were right. But Bsalis did! Thanks Bsalis!

I'm still tinkering with it myself, I got a pretty good design with three times as much fuel as the original (4100L up from 1350L) but the jet to rocket transition proved to be too unstable. My current theory is that your design is rather sensitive to balance - I've had versions that were ridiculously front heavy but would still tilt backwards as the thrust was above the mass - but that is the latest theory of many and troubleshooting this is definitely increasing my knowledge of the subject. My own designs are generally quite different and usually have efficiency problems rather than stability problems, by the time I've added enough wings/power to get the thing up to orbit my system is grinding to a halt.

Edited August 26, 2012 by EndlessWaves
physics correction

[Suzaku]

Well, you see, that's where my weird compusliveness comes in. The whole idea of a real spaceplane (as I understand it) is to use air-breathing engines and wing generated lift to save a lot of rocket fuel on the way to where rockets are required, so that's how I want to do it. Even if that works in the game, it feels like cheating to me.

http://en.wikipedia.org/wiki/Venturestar

The Venturestar was an proposed SSTO craft that was undergoing research until a couple of years ago. It uses nothing but aerospike rocket motors to get into space.

[Vanamonde]

That will simpllify fuel storage, as I no longer need to keep the two types separate to avoid feeling like I'm cheating. On the subject of technical matters, it sounds like aerospikes are effective in vacuum? I've read about them a little, but that part isn't clear to me. If they use the air flow around the engine to form something similar to an engine bell, wouldn't they expend force in useless directions in vacuum? And in the game, does greater wing area increase the altitude the plane can attain, or just its lifting capacity?