How do you land a spaceplane?

[uncle_jew]

Hello,

I built a simple SSTO spaceplane with two ramjets and a single LV-T45 engine. Getting to orbit is surprisingly easy, but how do you land the thing?

I lower the periapsis to 25km and set the pitch to about 30 degrees to induce some aerobraking before entering dense parts of the atmosphere. But after slowing down, the plane becomes nearly uncontrollable and enters a flat spin. I could regain control by pumping the remaining fuel to the forward tank (in order to bring forward the CoG) and firing the engine, but still the controls are very shaky (compared to the fully fueled plane during launch, which was a joy to fly).

I understand you can cheat by mounting huge reaction wheels (since unlike the real world, they don't need to be desaturated in KSP), but I would prefer a physically plausible spaceplane.

Is there a way to bring a spaceplane all the way down in a controllable fashion, without it spinning out of control during some of the descent? And is there a way to make the spaceplane empty both fuel tanks at the same rate, instead of draining the front tank first, shifting the CoG to the back?

Edited November 13, 2015 by uncle_jew

[SpaceplaneAddict]

You drain the BACK tanks first.

[Vanamonde]

It sounds like it's a matter of fuel usage shifting your center of mass around. Go to the SPH, empty the tanks, and see what that does. Adjust the design accordingly. Not easy, but part of the fun of making spaceplanes.

[Alshain]

Well the most obvious suggestions have been offered. Can you post a picture of the plane in the VAB with CoM/CoL on?

[GoSlash27]

uncle_jew,

I generally disable my reaction wheels on reentry in order to conserve electricity during a time that requires constant maneuvering with no electrical input.

The key to making it work is keeping track of the center of gravity during the entire flight.

If you can build a spaceplane with a center of gravity that remains constant regardless of the fuel quantity, you should do fine. It helps if you have control surfaces laid out to guide the aircraft rather than crutching it's weaknesses. Control in a spaceplane should be heavy and solid, not darty and overpowered.

The aircraft should always maintain it's static balance, it's dynamic balance, and automatically seek a pointy-end-forward and wings level attitude in all situations.

If you can achieve this, spaceplanes are easy.

Good luck,

-Slashy

[uncle_jew]

Here is a pic of my plane:

http://i.imgur.com/eCX7VFM.png

The round barrel after the cockpit is structural fuselage, not a fuel tank - I am simply loading slightly less oxidizer than fuel to compensate for extra fuel consumed by ramjets.

[edrobotguy]

Put some airbrakes near the back of the plane. During the transition from about 750 m/s to about 250 m/s your control surfaces don't do much of anything, but airbrakes will make your plane more like a badminton birdie, keeping you pointed in the right general direction and sort of in control. Once you get below about 250 m/s your control surfaces start becoming effective again (you'll see a big jump in G loads) and you can shut off your airbrakes then and just fly normally.

[zekes]

hit the ground slowly.

[Octa]

If you know how much fuel was left in the plane when you did the re-entry AND in which tank that was, check the CoM/CoL-Alignement with that fuel in SPH (you can also check if it's able to fly with that at all by launching it with the emptied tanks and flying around KSC a bit. Fuel amount in SPH can be changed with tweakables: rightclick on the tank and click&drag on the green bar to change the amount of fuel

Most likely it's that issue.

My only working SSTO has elevons on the wings, a rudder at the back (yaw only), canards and 4 airbrakes, so its a bit different from your design. i managed to did re-entry by a 100m/s retro-burn from a circular 80km orbit (just to get an idea of the re-entry speed). I pitched up at 30-45 degree and i enabled my airbreaks and intakes, to get as much drag as possible. I pitched up all the time and got some heat warnings, but nothing exploded. Worked like a charm.

[TheXRuler]

Get a nifty little plugin mod by name of RCS build aid. That will display your CoM and your dry CoM, get those markers to match ( or at least close together) and you should be fine, also, balance your fueltanks out before reentry or find a tank configuration that is relatively stable. Again a nice little plugin to make that faster and easier is TAC fuel balancer

[KerbMav]

You could also put the empty fuselage to the end of the plane to keep the COM further to the front during flight.

[Hodo]

Here is a pic of my plane:

http://i.imgur.com/eCX7VFM.png

The round barrel after the cockpit is structural fuselage, not a fuel tank - I am simply loading slightly less oxidizer than fuel to compensate for extra fuel consumed by ramjets.

The problem I am seeing is, this craft is lacking real pitch control. While the canards are there they are being asked to do to much. You asking them to control the roll, and the pitch. I would add some control surfaces to the back of the delta wings to only control pitch of the aircraft.

As for landing it. That aircraft should have a pretty decent landing speed of around 40-50m/s and if you keep your rate of decent above -10m/s you should be fine on landing.

[mabarry3]

I am confused as to how you get the wet CoM to be near equal to the dry CoM. How do you get the (a) fuel flow roughly the same on both sides of the CoM - fine for the airbreathing portion, but the rockets don't burn equally stage-by-stage - or ( the near-empty weight leaving all the fuel at the CoM (which it seems like it would be required)? I can only imagine that the solution would be carefully placed fuel lines (not really aesthetically pleasing) to keep the fuel flow stemming from around the middle to keep the CoM in line, but are there any other solutions?

[silks]

Instead of putting extra weight on the front you could move the heavy jet engines as forward as possible. I usually imagine where the center-of-fuel-mass would be and bring the COM as close to it as you can.

As for reentry, here's what works well for me: no SAS, disable reaction wheels, pitch up with ALT+S when there's enough atmosphere and control the descent rate by doing S-turns with roll.

How do you get the (a) fuel flow roughly the same on both sides of the CoM - fine for the airbreathing portion, but the rockets don't burn equally stage-by-stage - or ( the near-empty weight leaving all the fuel at the CoM (which it seems like it would be required)?

Balancing rocket fuel flow usually doesn't matter since once you cut the jets you're out of thicker atmosphere. Keep oxidizer tanks near COM and you'll be fine (or transfer the fuel by hand while flying).

Edited November 11, 2015 by silks

[Snark]

Also, try adjusting your vertical stabilizer to be yaw-only. When you have an "unpaired" stabilizer like that (i.e. fin on top doesn't have corresponding fin on bottom), it can cause problems because it uses its actuator to try to help you with roll, and ends up inducing yaw. I can imagine that this might be exacerbated in your case because your craft has very little aerodynamic roll authority (just those two little canards, and they're close to the ship axis so they don't have much lever arm to work with), which might cause your vertical stabilizer to grab a large share of the roll authority with tragic results.

Also, consider what happens when your vertical stabilizer tries to control your yaw. Let's say the plane has deviated so that its nose is pointing slightly left of center, so SAS tries to use the stabilizer to turn you to the right. This will make the plane roll to the left (since the stabilizer is unpaired). So SAS will try to correct to roll right. And if you haven't disabled roll authority on the stabilizer, it'll try to use the stabilizer to roll the plane to the right, which is directly fighting against the need to yaw right, so you've just neutered your yaw authority.

I would also advise adding some ailerons out near the wingtips on your big delta, to give you some aerodynamic roll authority. Even after you've turned off roll authority on your vertical stabilizer, it's still going to induce some roll every time it corrects for yaw, so you need some aero roll authority that doesn't involve the stabilizer in order to compensate for that.

Plus, in general it's always better to have control surfaces at the back of the plane rather than the front. A control surface that's in front of the CoD becomes less effective as angle of attack increases, and if the angle of attack goes past the surface's max deflection angle, it actually becomes part of the problem rather than part of the solution. Whereas a control surface that's behind the CoD becomes more effective with increasing angle of attack. It's okay to use canards as a way of giving your plane some lift near the front to move your CoL forward, but don't rely on them as your primary pitch control-- put something at the back of the plane for that.

So, try that and see what happens? And in general, for all aero control surfaces, it's generally a good idea to disable all control axes that you're not adding them for; that is, for example, if you're adding a surface to help you with pitch and roll, then disable yaw, and so forth. Helps to avoid unexpected control behavior.

Edited November 11, 2015 by Snark

[Alshain]

Here is a pic of my plane:

http://i.imgur.com/eCX7VFM.png

The round barrel after the cockpit is structural fuselage, not a fuel tank - I am simply loading slightly less oxidizer than fuel to compensate for extra fuel consumed by ramjets.

Move your front wheel further forward.

You have no control. The canards on the front are not going to be enough to roll and possibly even pitch this plane. I'm surprised you even got it off the runway let alone, space.

[TheXRuler]

I am confused as to how you get the wet CoM to be near equal to the dry CoM. How do you get the (a) fuel flow roughly the same on both sides of the CoM - fine for the airbreathing portion, but the rockets don't burn equally stage-by-stage - or ( the near-empty weight leaving all the fuel at the CoM (which it seems like it would be required)? I can only imagine that the solution would be carefully placed fuel lines (not really aesthetically pleasing) to keep the fuel flow stemming from around the middle to keep the CoM in line, but are there any other solutions?

You shift around heavy parts (and light parts) that don't contain any fuel to change your dry CoM. Also structural fuselages in front of your tanks can help by using the relatively heavy cockpit to shift the CoM. In addition to all that you can add smal fuel tanks to the front of your plane, lock these and then tweak fuel levels (dunno if RCS build aid actually detects locked tanks though) or experiment in the editor a little. Remember where your CoM is when full (maybe mark it with a massless part) and then empty most of the fuel, simulating how much you have left upon reentry. Then shift the fuel around to find the fuel configuration puts the CoM back where it should be. Then after your reentry burn but before reentering the atmosphere use TAC-Fuel Balancer to dump any excess fuel and pump the fuel to where it should be. I used to have an enormous SSTO (~380 tons on the runway) that could get 60 tons to orbit. To make it fly with any payload weight I stuck the wet CoM dead centre of the 24m long cargo bay. When leaving the atmosphere I pumped fuel around to shift the CoM, thereby pitching the plane (similar to the SR-71 Blackbird) because I had nowhere near enough pitch authority. Upon reentry I had a few disasters until I figured out that I had to pump ~7000 units of fuel right to the front to make it stable when dry. This had the added benefit that I could send it into a controlled crazy and uncontrolled spin if I overshot KSC by too much, dumping about 1000m/s in a few seconds, and then pump everything as far forward as possible to make it superstable and let it recover itself. Pump it around a bit again and I could fly it down to the runway safely. Good 'ol times hehe

[uncle_jew]

Thanks for the suggestions!

[Lohan2008]

Get your speed as close to stalling as you can & pray.

[Hodo]

Get your speed as close to stalling as you can & pray.

I heard it best from a F-15C Eagle pilot when I was a kid. He said, "Landing an Eagle is just the act of making a controlled crash that the plane can take off again later from."