some questions about spaceplanes

[juanml82]

I've been getting serious about building spaceplanes/SSTOs in the last couple of days, which means I now have ideas and brand new questions.

So, I have two light SSTOs, using the RLA mod to have better tiny sized parts for the 48-7s. Both ships should work using stock parts, but I think using cubic girders to attach the engines to a larger tank is silly. These are the planes

They both reach orbit, and the first one is faster to do it and more nimble. And I was able to use the first one to put a light payload in orbit. I wonder if they can be further improved - are tail fins necessary? Is it ok, better or worse to use winglets instead? Do the wings themselves count as control surfaces?

In any case, these two models work and they allow me to rescue stranded kerbals in orbit and put a few tons in orbit. So I've decided to try to build something bigger, although I'm not sure if spaceplanes can put more useful payloads in orbit - say an interplanetary transfer stage or a lander. So I've been building this but, even after I got it out of the runway, it just won't get higher than 2,500 m. Does it need more wings? And if so, is there a statistics mod like KER or MJ that would show how much?

Still stubborn, I changed the concept and built this

Which works, and gets into orbit with... 100 left over delta-v, so I'm still trying to improve it. It's also hard to maneuver, specially at high altitudes. I'm think about trying to put all turbojets behind the centerline fuselage and put the rocket engines where the outer jets are right now. Or replacing everything with Rapiers, so no engine is dead weight at any given time.

Also, the ascend profile for spaceplanes is roughly

Climb in a 45° something angle until 20,000 meters; level around 30,000 meters, get the speed close to 2,000 m/s and the periapsis as high as possible (say, around -200 km) before switching the jets for rockets, right? Or I'm missing something?

I've also read people who likes the aerospikes as engines, but I'm not sure about them: they weight as much as the LV-T30 and T43 and have less thrust. Their ISP remains the same in the atmosphere, but the rocket engines are fired at high altitudes anyway

I've seen that turbojets have to throttled down when low on air or else they'd flameout. Is there a guideline about how much, and a way to predict when to throttle them down? I've been using MJ for that, but I'd rather do it my self. Also, is there a way to restart a flamed out jet engine without editing the save file?

In the spirit of doing something different, I was thinking about landing a SSTO in the Mun to rescue a stranded Kerbal - but how are spaceplane landed in a vacuum? With a rocket, I'd lower my Pe near the point I want to land, kill my horizontal velocity and fall from the skies. I could try that with a spaceplane, and make sure I level it when below 100m (and use monopropellant or a dedicated VTOL engine to slow down I guess), but can horizontal landings be accomplished? As I think it, there is no atmosphere to slow down the spaceship, so I'd be coming down at a great horizontal speed, so the ship would have to land pointing to the surface prograde marker - ie, with the tail up front. And a gentle deorbit has to be achieved beforehand.

[Taki117]

The first issue I see (And probably where most of your problems are coming from) Is that your aircraft are all very short. This leaves little to no room for error, and as soon as your tanks empty your CoM is going to shift, and make your aircraft unstable.

Also, your flight profile is a little steep. What you should be doing is 45 degrees to 10km, then level out and gain horizontal speed until 20km or jet flameout (Whichever happens later) If your apoapsis isn't where you want it swap to rockets, if it is (plus a little extra margin) you can coast to apoapsis and circularize there.

As far as restarting Flamed out Jets, the only way to do that is to lower your altitude, or raise your speed. Another thing is that your radial intakes aren't helping you much as you increase your speed.

Edited August 9, 2014 by Taki117

[Wanderfound]

Important question: are you using stock aero, NEAR or FAR?

[Wanderfound]

Also, your flight profile is a little steep. What you should be doing is 45 degrees to 10km, then level out and gain horizontal speed until 20km or jet flameout (Whichever happens later) If your apoapsis isn't where you want it swap to rockets, if it is (plus a little extra margin) you can coast to apoapsis and circularize there.

Disagree. The air at 10,000m is still too thick for serious speed, and if your jets are flaming out below 25,000m then there's something seriously wrong with either your plane or your piloting. A well flown and built non-airhogging plane should be able to reach 30,000m before it chokes. Just below 20,000m is where I usually level off; ~35,000m is where I typically start burning oxidiser.

As far as restarting Flamed out Jets, the only way to do that is to lower your altitude, or raise your speed.

Or shut down some engines, or throttle back a bit, or reduce your angle of attack.

[Wanderfound]

In the spirit of doing something different, I was thinking about landing a SSTO in the Mun to rescue a stranded Kerbal - but how are spaceplane landed in a vacuum? With a rocket, I'd lower my Pe near the point I want to land, kill my horizontal velocity and fall from the skies. I could try that with a spaceplane, and make sure I level it when below 100m (and use monopropellant or a dedicated VTOL engine to slow down I guess), but can horizontal landings be accomplished? As I think it, there is no atmosphere to slow down the spaceship, so I'd be coming down at a great horizontal speed, so the ship would have to land pointing to the surface prograde marker - ie, with the tail up front. And a gentle deorbit has to be achieved beforehand.

Stick a few Vernors on the bottom, balanced around the CoM; they should be enough for a VTOL Mun landing. A few of the smallest radials would also do the trick.

You can do a rolling landing on the Mun, but it's tricky and you'll have trouble finding a sufficiently flat landing strip.

[juanml82]

The first issue I see (And probably where most of your problems are coming from) Is that your aircraft are all very short. This leaves little to no room for error, and as soon as your tanks empty your CoM is going to shift, and make your aircraft unstable.

Also, your flight profile is a little steep. What you should be doing is 45 degrees to 10km, then level out and gain horizontal speed until 20km or jet flameout (Whichever happens later) If your apoapsis isn't where you want it swap to rockets, if it is (plus a little extra margin) you can coast to apoapsis and circularize there.

As far as restarting Flamed out Jets, the only way to do that is to lower your altitude, or raise your speed. Another thing is that your radial intakes aren't helping you much as you increase your speed.

Yes, they are difficult to fly on reentry, when they are almost dry - I'll have to get back to the drawing board on that one

Important question: are you using stock aero, NEAR or FAR?

Forgot about that. So far I'm using stock. I was thinking about FAR, but I wonder whether it would affect rockets flying in the atmosphere. IE, my existing designs, even if it's only during take off.

Stick a few Vernors on the bottom, balanced around the CoM; they should be enough for a VTOL Mun landing. A few of the smallest radials would also do the trick.

You can do a rolling landing on the Mun, but it's tricky and you'll have trouble finding a sufficiently flat landing strip.

I was thinking in those lines. I should try on Minmus first, and with these designs, I will need to refuel quite a bit along the way. I'm thinking about trying a VTOL landing and a STOL take off - it's not like Minmus requires a long runway.

[Wanderfound]

Forgot about that. So far I'm using stock. I was thinking about FAR, but I wonder whether it would affect rockets flying in the atmosphere. IE, my existing designs, even if it's only during take off.

I was thinking in those lines. I should try on Minmus first, and with these designs, I will need to refuel quite a bit along the way. I'm thinking about trying a VTOL landing and a STOL take off - it's not like Minmus requires a long runway.

Minmus has perfect runways: the greater and lesser flats. Better than KSC.

FAR does affect rockets, but in a good way: it makes aerodynamics matter. With FAR, nosecones and fairings are actually functional, instead of being the counterproductive cosmetic pieces that they are in stock.

You do need to adapt your builds and flying style to FAR's more realistic aero, though. No more pancake rockets and no more "straight up to 10,000m then immediately slam it to 45°". Try that under FAR and you'll flip, break your rocket in half, or both. As with aircraft, you need to keep your nose fairly close to the velocity vector and turn gradually; start gently nosing over immediately after liftoff and gradually increase your lean over the first 15,000m of the ascent.

I'd strongly recommend trying FAR/NEAR if you're getting into spaceplanes; if you don't like it, it's easy to switch back. Spaceplanes are even more fun when they actually behave like real planes.

[Taki117]

Disagree. The air at 10,000m is still too thick for serious speed, and if your jets are flaming out below 25,000m then there's something seriously wrong with either your plane or your piloting. A well flown and built non-airhogging plane should be able to reach 30,000m before it chokes. Just below 20,000m is where I usually level off; ~35,000m is where I typically start burning oxidiser.

I play with FAR, so I start loosing thrust at about Mach 3, at Mach 4 I start flaming out or overheating (Depending on what my payload is)

Here's my General Purpose SSTO, Can get up to 350km circular orbit and back again, It also has a passenger variant that is a tad shorter.

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[Wanderfound]

I play with FAR, so I start loosing thrust at about Mach 3, at Mach 4 I start flaming out or overheating (Depending on what my payload is)

I also play with FAR; with the right plane and piloting, you can get beyond 2,000m/s and 30,000m on turbojets alone, without the need for more than a couple of intakes (I normally use one nacelle/ramscoop combo per engine). Once you get your altitude high enough, it only takes a tiny amount of thrust to continue slowly accelerating.

Climb slow, minimise your angle of attack, throttle back and shut down as many engines as possible (I generally build with an odd number of engines to avoid having to worry about asymmetric flameouts; at extreme altitude, the central turbojet is the only one left running).

Give it a try; build yourself a basic plane with a single turbojet, a couple of ramscoops and a pair of delta wings. See what's the absolute maximum speed and altitude you can get out of it while you circumnavigate Kerbin.

It will take some time to get there, but you'll be surprised at how much piloting influences flameout altitudes.

Edited August 10, 2014 by Wanderfound