Spaceplanes - how do they work?

[mellojoe]

I've tried so many different designs. The only ones that work, are stupid things that have ten thousand wing parts in every which way. Nothing that even resembles a plane. And those usually don't even work that well anyway.

I've tried putting center of mass in front of center of lift. I've tried putting engines on the front, engines on the back. I've tried struts. Half of them pull hard to the left before they even get off the runway, the other half blow up. I'm at my wits end.

Gallery of most recent fail attempts:

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The last one mostly flew. But it was the ugliest thing I had ever created. How do you guys do it? How do you make it look so easy?

[rflrob]

I can't quite tell how your gear is laid out, but in my (very limited) experience with the SPH, vehicles with two front wheels are more likely to pull off to one side or the other, so that might help you with the first half of your space planes. Good luck with flying!

[Snooze]

Don't attach the nose wheel to the command pod. It's something to do with how it attaches. It also helps to have twin vertical stabs. Or, you know, more.

This is the Magellan, my circumnavigation build. Note the stabs.

[sgt_flyer]

Several points for Spaceplanes :

If your wings are fixed below your craft, slightly trun them upwards (a very wide open V when you see it from the rear) - if if your wings are fixed above your craft, reverse the V shape. - this should improve stability.

You can also slightly increase the angle of attack of your wings / or only turn your control surfaces in order for your airplane to naturally place it's heading slightly above the prograde marker when inflight.

Last but not least, fuel tanks draining : as the fuel tanks drain, it will move your center of mass. Ideally, fuel flow should be set up in order to Tmake use of this CoM movement : at takeoff and climb, your center of lift should be slightly behind your center of mass, and in the reverse, when landing (after draining your fuel) the center of lift slightly ahead of your center of mass. The center of drag should be kept behind your center of mass at all time.

Also, for your turning problem, try to create an angle of attack (by having the rear gear slightly shorter than your front gear) so the plane will have a tendancy to take off on it's own. Having the rear gear closer behind the center of mass also helps at liftoff - you just need to be careful of tailstrikes

Edited July 2, 2013 by sgt_flyer

[SuperWeegee4000]

Just remember, it isn't just you. Spaceplanes are hard.

[Nibb31]

Aerodynamics are borked in KSP. There really is no point in trying to make planes or spaceplanes until a proper aerodynamics model is implemented.

[Sephta]

Couple more things:

* Jets don't really need much fuel, so use them as long as you can.

* with enough intakes you can reach 30000m and more with speeds well beyond 2000m/s - go for it.

* you can achieve an apoapsis >80km on jets alone - when you have done that you need a rocket engine only to circularize.

a stable spaceplane is hard to make, but good ones don't need much parts. For spaceplane design: less is more.

[bsalis]

It looks like you are building a small SSTO, and since I didn't have any to show, I decided to build one just for you in order to demonstrate good design.

From looking at your pics, I think your craft have too much engine mass, too much jet fuel, and are unbalanced. Even if you managed to get them to orbit, I am certain that your joy will be short lived upon re-entry.

Below is the skeleton of the Sprite SSTO Spaceplane I made.

Note how the mass of the cockpit & ASAS at the front balance the engine mass at the back. Also see how the fuel mass is also generally balanced around the CoM (remember jet tanks have less fuel) - this is so the CoM does not move much.

For an SSTO this size, one jet and two 909s are enough - any more engine mass will reduce the delta-V you can get to orbit. The jet fuel tanks are also all you need for one engine - any more will be a waste.

I did a sneaky job and fitted 4 air intakes, with 2 between the tanks and 909s. However I did do a K-prize run initially with only 2 intakes. Worked fine but was a bit of a dog.

So here it is with wings and all the trimmings, in orbit. 44 parts in total.

Craft file here (paste the text into Sprite.craft file if needed).

[iDan122]

Aerodynamics are borked in KSP. There really is no point in trying to make planes or spaceplanes until a proper aerodynamics model is implemented.

No, it is possible. You just need the skill and some Scott Manley videos.

[jason0735]

Ask Tim Barrett he can help he's the master of spaceplanes... no LITERALLY.

[Nibb31]

No, it is possible. You just need the skill and some Scott Manley videos.

I know you can build them and fly them, but they don't respond to proper rules of flight mechanics, therefore there really isn't much point.

[Tex_NL]

I know you can build them and fly them, but they don't respond to proper rules of flight mechanics, therefore there really isn't much point.

Then why are you playing this game in the first place? There are many features of this game that aren't true to real physics, the game is full of 'shortcuts' and 'quick fixes'.

[nhnifong]

I believe you are being misled by the word "plane". KSP is a rocket simulator with a hastily added loose approximation of at least one limited aspect of aerodynamics.

If you want to build "planes" you should install FAR

[mellojoe]

Wow.

Slightly angling things works! I'm an engineer by background. I'm very left-brained by nature. I love straight lines and square blocks and mirror symmetry. In planes, and spaceplanes, having wings slightly angled downward (or upward) seems to help. And having the landing gear slightly angled outward also helps. Plus having two tailfins, also slightly angled. Having everything slightly angled, plus using lots of intakes, I have managed to successfully launch a SSTO Spaceplane. I got into a 400k by 400k orbit. I deorbited and landed.

WOOT!

Thank you all. Use angled everything. Thats the trick.

[Snooze]

Wow.

Slightly angling things works! I'm an engineer by background. I'm very left-brained by nature. I love straight lines and square blocks and mirror symmetry. In planes, and spaceplanes, having wings slightly angled downward (or upward) seems to help. And having the landing gear slightly angled outward also helps. Plus having two tailfins, also slightly angled. Having everything slightly angled, plus using lots of intakes, I have managed to successfully launch a SSTO Spaceplane. I got into a 400k by 400k orbit. I deorbited and landed.

WOOT!

Thank you all. Use angled everything. Thats the trick.

/highfive

Congratulations!

[Autochton]

nhnifong has the right of it if you want to really get into planes and atmospheric flight. Stock KSP's flight model is... creative. I can't use it at all, myself, feels like the planes don't fly so much as magically slide through this non-Euclidean goop, powered mainly by sorcery. So FAR is a must-have mod for me - not though, it does not make things easier, in fact, quite the opposite. But it does make aerodynamics a lot more realistic. Expect your working planes to look more like planes, and less like the wing-and-intake-studded monsters you often see when looking at stock spaceplanes.

Disclaimer: I come from a past of really crunchy flight sims where realistic flight profiles are paramount. So my problems with stock KSP probably stem from that.

[Castun]

Also you'll want to keep in mind that your Center of Thrust is just as important. Having your CoT out of alignment with your CoM may not cause problems upon takeoff, but once you get up above 20km, the thinner air means your wings are a lot less effective at keeping your craft straight. This is mainly due to just how...goofy KSP's aerodynamics engine is. I find it annoying too, but not sure I'm ready to give FAR a shot either.

Edited July 3, 2013 by Castun

[Geschosskopf]

There's another issue with spaceplanes to keep in mind that I never see mentioned: wingloading.

Let's say you build a spaceplane very carefully so the CoM won't move significantly as fuel burns, the CoM, CoL, and CoT are all lined up correctly, and the thing takes off and orbits like a champ. Yay! But then you try to bring it home and the thing tumbles uncontrollably right after the re-entry flames die out. What the...? I mean, it flew perfectly going up, so what's gone wrong?

I submit that this is due to the change in wing loading caused by the loss of fuel mass. When a spaceplane starts out, it's as heavy as it will ever be and it's moving slowly. Thus, you have high mass and low lift, so a fairly high wing loading. As the plane goes up, mass decreases while lift increases with speed but also decreases as the air gets thinner. As things typically work out, the air peters out long before the lift vs. mass balance gets very far out of whack. Thus, a spaceplane can be stable from runway to orbit.

Then to achieve orbit, you have to do a fairly large burn at apopsis, which takes a lot of fuel. Then you probably burn more to synch up with a space station, and then de-orbit. This brings the mass way down at a point where there's no lift at all and the thing's flying like a rocket, so you have no idea of the aerodynamic changes you're making.

But now you re-enter. Typically, nothing very aerodynamic happens (even with FAR) until after the re-entry flames stop. Then suddenly you're in pretty thick air (about 20km) going WAY fast, much faster than you passed through this same altitude on the way up. So suddenly this HUGE amount of lift kicks in, from the original wing area moving very fast through thick air. But at this point, your spaceplane is as light as it will every be, minus all the fuel it used going up and whatever payload it delivered to orbit. Thus, the lift force totally overpowers the weight force and the spaceplane goes into a death tumble.

If the wing area isn't too big, then eventually you'll regain control before you run out of altitude or the spaceplane breaks up during the tumbles. But the bigger the spaceplane, the more wing it needs to get off the ground, so the worse the problem becomes.

If anybody has a solution to this problem (especially if it's FAR-friendly), I'd love to know it.

[Castun]

Using the TAC fuel balancer can certainly help, as it can automatically balance fuel between tanks on the fly. Your CoM will still likely shift somewhat unless your plane is somehow perfectly balanced.

[Autochton]

Geschosskopf:

Reenter belly-down. Basically, line your ship up so that your nose is 30-35 degrees, or potentially even more, over your prograde, and keep it there while you lose speed - your drag should be utterly massive, and as a side effect this flight profile enables you to come in with a mod like Deadly Reentry on too. Preferably you should be down to a stable flight regime (whatever that is for your design - for me it's ~1500 m/s, around Mach=5 @ 25km) by the time you start hitting deep atmosphere. That way, you're not hurtling along at speeds far above your ship's rated maximum (Orbital velocity comes to Mach=8 or 9 at 25km IIRC), but instead you'll be able to fly normally.

At the least, this approach worked well for me with my shuttle, although I did have stability issues until I rebalanced my fuel load.

[Geschosskopf]

@ Autochton:

Yeah, that's what I try to do. But I can never get it to work. Occasionally, my spaceplanes weathervane in the thin upper air to go nose-first despite my attempts to keep them bellying in. Then it's infinite lift vs. minimal weight. More commonly, however, I can keep the nose up until the flames die out but then I immediately tumble. I believe this is a very accelerated stall due to the extreme angle of attack caused by the belly-first attitude.

It's probably that I just suck at both designing and flying spaceplanes. Or it could be the fact that my SPH crews are all disgruntled employees whom I've transferred there as punishment after they've botched a more conventional mission .

[KerbMav]

As much as id like to try out spaceplanes - I took a stock plane (the smallest) out on a test flight and I utterly failed as soon as I wanted to do any maneuver.

Turning worked kinda, although it twisted in my desired direction and rubberbanded halfway back - slowing down to land ... I wont even mention this.

I had my experience with flight sims, but this was totally discouraging. Maybe I should try some user made models?

[Aragosnat]

As much as id like to try out spaceplanes - I took a stock plane (the smallest) out on a test flight and I utterly failed as soon as I wanted to do any maneuver.

Turning worked kinda, although it twisted in my desired direction and rubberbanded halfway back - slowing down to land ... I wont even mention this.

I had my experience with flight sims, but this was totally discouraging. Maybe I should try some user made models?

That sounds like a good idea. As right now landing one for me requires parachutes to land.

Edit: Another thing is you also have to worry about how balanced the top and bottom of the plane is. As any imbalances off the CoM will make it spin if using thrust greater then the torq can handle.

Edited July 4, 2013 by Aragosnat

[mellojoe]

Don't forget the CAPS LOCK button. This makes your inputs less sensitive so that your corrections don't become over corrections. You'll notice the pitch/yaw/roll controls turning from orange to blue.

[Novamarine]

No, it is possible. You just need the skill and some Scott Manley videos.

Scott Manley is awesome!

He practically has a Phd in KSP!