SSTO TWR Help

[RocketTurtle]

Ok, so I usually don't build SSTO's. Mostly planes and rockets. So I thought, well, I should combine them. I did make an SSTO a few months ago, but it was pretty ugleh. So I've been designing a fairly standard SSTO, 2 jets, 1 nuke, and yeah, not very big, not very small either though, I can post pics later if needed.

But my main problem is the TWR. The Nukes TWR on my SSTO is terrible, I can't get to orbit with it. So I see 2 possible fixes, increase TWR, or get a higher apoapsis on jets. So I'm sort of just asking what you guys would recommend and how to do it. I also have a few other questions.

Intake amount, and cruising altitude for this amount?

And how much jet fuel tanks vs. LFO tanks?

Edited November 29, 2013 by RocketTurtle

[antbin]

Just look at the K-prize thread. . 107 pages (and counting) of successful SSTOs.

[RocketTurtle]

Sure, but there is nothing about TWR in there. I'm wondering how people think I should give it a higher TWR.

[capi3101]

I'm not a fan of spaceplanes my own self - that said, I've heard that the ascent profile for a spaceplane is to pitch up after you get off the runway and climb until you're just below flameout altitude (this is the part I'm not certain of; 35,000 I think), then level out and gain most of your orbital velocity (2,300 m/s or so) on jets. Only then do you kick on the rocket motor and pitch up again; a short burn should get you into space.

Ordinarily when I do a single-stage rocket take off, I shoot for a launch TWR of 1.2 (the idea being that all that thrust I have at the beginning I shouldn't need once I'm up there) and hope to be fiddling with the throttle to keep it somewhere between 2.0 to 2.3 shortly after the gravity turn. The principles of space launch are similar regardless of whether you use rocket or spaceplane, so I imagine that's where you need to aim. You might try a decoupler on the back of the nuke with an RT-10 attached, or perhaps an FL-T100 and an Aerospike. I make no guarantees as to what that's going to do to your plane's flight characteristics; really all I can offer, though. Sorry.

[RocketTurtle]

That's fine. I'm gonna test it a bit to see what I can do.

[arq]

Unless you do some serious airhogging, an LV-N alone will not get you to orbit. Add some 24-7S engines or LV-T30s to get yourself out of the atmosphere, then turn them off once you can maintain speed/climb with the LV-N alone.

[RocketTurtle]

Yeah, I'm just sticking some 24's on it. I'll see how it goes.

[Dispatcher]

I agree that LV-Ns are not the way to go; too massive thus poor TWR. I've had success with aerospikes for rocket only engined spaceplanes. For mixed jet and rocket SSTOs, I still use the spike.

A rocket engine only design: http://forum.kerbalspaceprogram.com/threads/33312-Showcase-SSTO-s%21-Post-your-pictures-here?p=735772&viewfull=1#post735772

[Kosmo-not]

It's been a while, but I believe a mk1 pod + 2 long fuel tanks + the non-vectoring rocket engine will make it to orbit. I forget what the TWR for that was.

[Tank Buddy]

You probably don't need LV-Ns if you're just going to rendezvous with a station in Kerbin orbit, especially because the LV-Ns are so heavy that you may get more delta V from just having a lighter engine.

[RocketTurtle]

Interplanetary. But I'm building bigger and everything is going ok but I can't get a fuel line to connect, it thinks it's clipping through an engine but it's not, I can put the fuel line on, then the engine, but when I reload the craft the fuel line is connected to the engine... gr...

Well I have to get off now, so I can't provide a craft or screenshots, but if anyone had ideas...

[numerobis]

If you add intakes, say 3-4 per jet, you should be able to reach the vacuum handily. At 8 intakes each, you'll get a 150-200km apoapsis. That gives you a long time to circularize; you can even do it with ions.

[bsalis]

Is this interplanetary, with or without refueling in LKO?

Also some pics would be nice.

[SofusRud]

I have had good results with a design consisting of 4 turbojets and a singel skipper. The skipper has 3/4 of a red tank and the jets are in pairs with a mk2 fuselage and the mk2 adapter fuselage. I can get it up to 1450 m/s on the jets before i switch over, got into orbit with half of the skippers fuel left. the craft transports 6 kerbals and manages to be under 100 parts. definetly recommend going big and going with the skipper engine

[el_coyoto]

Ok, so I usually don't build SSTO's. Mostly planes and rockets. So I thought, well, I should combine them. I did make an SSTO a few months ago, but it was pretty ugleh. So I've been designing a fairly standard SSTO, 2 jets, 1 nuke, and yeah, not very big, not very small either though, I can post pics later if needed.

But my main problem is the TWR. The Nukes TWR on my SSTO is terrible, I can't get to orbit with it. So I see 2 possible fixes, increase TWR, or get a higher apoapsis on jets. So I'm sort of just asking what you guys would recommend and how to do it. I also have a few other questions.

Intake amount, and cruising altitude for this amount?

And how much jet fuel tanks vs. LFO tanks?

You can do both : some more intakes to gather more speed and get a higher apoapsis, and maybe adding a few of the small engines (24-77, 48-7S) to increase your TWR during ascent without ruining your interplanetary delta V. Once in orbit, you can deactivate them, refuel and use the LV-N for cruising.

Adding more LV-N would hurt you delta V a lot because of their heavy weight, which is why I think using light (albeit with less ISP) engines to help the LV-N seems a good idea.

Personally, I tend to aim for a max speed of ~1500 m/s on jets only by flying high and adapting thrust as I climb (you can go even faster with more intakes, depends on how you like to fly), so I usually need ~700 m/s to achieve orbital velocity. This can be done with fairly weak engines, especially in a thin atmosphere (can't remember precisely, but 35 km sounds like a good altitude to disengage the jets).

And for the fuel / LFO ratio, I usually define a goal and proceed by trial and error. Some of my planes are tight on fuel after re-entry, some others can achieve LKO, re-enter THEN circumnavigate Kerbin : it mostly depends on what kind of missions the plane is designed for (cargo lifter, crew rescue, R&D, IP travel, etc...)

Interplanetary nuke-powered SSTOs are fun to design and fly, good luck with yours!

Edited November 22, 2013 by el_coyoto