SSTO Spaceplane help - reentry FAR

[RCgothic]

Hi guys, a little help here? I've managed to get it into a 75x75km orbit with enough L/Ox to deorbit again to about 35km periapsis. Payload is currently only one passenger and about 400 units of Liquid Fuel. Powerplant is two Whiplashes and a Swivel (career limitations).

I'm struggling to de-orbit it successfully. At the moment I can't keep the periapsis up and everything ends in a lack of control and a fireball at about 22km. Any hints? I'm not clear on how airbrakes, flaps and spoilers work, so that could be a start.

http://imgur.com/a/VGB3w

[Nansuchao]

First, pump all the fuel forward before you're under 40 km. A 30 km periapsis is perfect for reentry.

Usually with not so stable planes, I use some RCS thrusters in the front to keep the nose up.

My usual reentry profile, is to keep a Radial attitude until under 50 km, then the plane begin to take a Prograde attitude. Put the SAS to Prograde and do big S turns to slow my speed. If my vertical speed is too low (-200 m/s), I keep the nose up. Usually this way, you're safely under 1600 m/s at 30 km. At that point, just keep the plane straight with SAS assist and you can land safely.

[RCgothic]

Isn't RCS really draggy?

[Goomblah]

I don't use RCS thrusters on spaceplanes anymore; they cause too much drag and often burn off during ascent or descent. You've got tons of torque anyways, given both the cockpit and the module on the tail. Definitely pump all your fuel forwards to ensure that your CoM is in front of your CoL. Keep the nose up as much as you can throughout your descent; this will generate a lot of drag and some lift, which will help slow you down more in the upper atmosphere. Nosing up will make your descent take longer, though, because all that lift is keeping you in thinner air for longer. This makes the descent much safer, but also means that you can easily overshoot KSC if you haven't prepared accordingly. Since you are using FAR, there's also a greater danger of your wings ripping off, so as you approach 30km, reduce your AoA to maybe 10 degrees or so; as you have found out, the air suddenly gets really thick and nasty at about 25km, which can easily result in unwanted explosions. Before then, angle as close to radial+ as possible (atmospheric forces will pull your nose down gradually) to maximize drag and slow your descent.

[Chaos_Klaus]

You don't want to keep your periapse up. You want to slow down. That means your periapse should fall.

However, you want to slow down before you hit the lower parts of the atmosphere (below 25km). Try to fly hard turns with large angle of attack to bleed off speed. You need large control surfaces to get control over your vehicle at supersonic velocities.

Also, beware of the CoL marker in the hangar. With FAR, the marker had problems before. I read that it was improved in the last updates, but I'd still be cautious. The Stability derivatives however seem trustworthy to me. Check these with empty tanks aswell! (Although RCS build aid shows your CoM and CoDM rather close).

[RCgothic]

Thanks for the tips. Is there anything to add regarding the use of flaps, spoilers and airbrakes? I.e. how to.

What I meant by keeping my periapsis up is generating enough lift such that even though I'm slowing down, periapsis doesn't fall too deep into the atmosphere before I'm properly slowed.

I guess I also don't understand why speed and altitude have such a negative effect on stability parameters. Why does Yaw Stability fail? It's not like the nose is getting larger and the tail smaller.

[Champ]

Thanks for the tips. Is there anything to add regarding the use of flaps, spoilers and airbrakes? I.e. how to.

For airbrakes, it's pretty simple : put them on the back of the plane (this way when deployed they put the COL in the back), then deploy them before entering, and undeploy them when you have lost enough speed.

I guess I also don't understand why speed and altitude have such a negative effect on stability parameters. Why does Yaw Stability fail? It's not like the nose is getting larger and the tail smaller.

Speed and altitude affect the strength of aerodynamic pressure.

I guess your COL shifts ahead of COM when your spaceplane is empty. When you are not low enough in the atmosphere, your control authority keeps it oriented, but when you hit lower atmosphere, aerodynamic pressure overwhelms your control authority, your ship switchs orientation and get destroyed by aerodynamic pressure.

When in SPH, always make sure the COL stays behind the COM all the time (tanks full, tanks empty, with or without cargo, etc).

[Wanderfound]

Thanks for the tips. Is there anything to add regarding the use of flaps, spoilers and airbrakes? I.e. how to.

What I meant by keeping my periapsis up is generating enough lift such that even though I'm slowing down, periapsis doesn't fall too deep into the atmosphere before I'm properly slowed.

I guess I also don't understand why speed and altitude have such a negative effect on stability parameters. Why does Yaw Stability fail? It's not like the nose is getting larger and the tail smaller.

Your tailfin keeps the plane pointed straight through Newtonian mechanics; equal and opposite reactions to the air molecules hitting it. As the atmosphere thins, those air/tailfin collisions become less frequent, and the tailfin loses power. Speed generally works in the reverse of this (i.e. more speed = more air collisions = more stable), but this is complicated by the weird things that happen to airflow as you go supersonic. Direction of travel also matters; during early reentry, you're often dropping fast enough that your tailfin is in the "shadow" of the fuselage from an airflow perspective.

As for flaps/spoilers/airbrakes: first, this is complicated by the difference in terminology between FAR and the real world. In normal use, a flap is a trailing-edge surface that deflects downwards, while a spoiler is a trailing-edge surface that deflects upwards. In FAR, however, a flap is a surface that moves in three steps (default direction downwards, but can be reversed) while a spoiler is a surface that fully deflects in a single step (default direction upwards, but again can be reversed).

Using the real world definitions for now: flaps add lift, but also increase drag. Spoilers reduce lift and increase drag. The purpose of flaps is to give you a bit more lift while flying slowly (i.e. take off and landing). The purpose of spoilers is to allow you to ditch some altitude without pitching the nose down (very useful during landing). With either of these, they work best when placed close to the CoM. Flaps or spoilers too far from CoM will introduce unwanted pitch when deployed.

The primary purpose of airbrakes is to increase drag (they can also be used as control surfaces, but that's getting a bit advanced for this context). Stick a pair of 'em behind CoM, open them as you hit the atmosphere and reentry will become trivially easy. They're rarely necessary, but often convenient.

Edited October 1, 2015 by Wanderfound

[RCgothic]

Thanks guys, all very useful! I'll make sure to double check where all the fuel is and shift it forward before my next re-entry attempt. I suspect the fuselage tasks will be basically empty with the nacelles half-full and I'll need to pump those forward.

I may try adding some airbrakes too. It sounds like flaps/spoilers won't be terribly useful for a canard/delta design though as the trailing edges are too far from the COM.

[RCgothic]

Successful landing! Returned a payload of one passenger and 270 units LF and 300 units Oxidiser. And I flew to orbit with my gear down. Airbrakes on re-entry made all the difference!

[Champ]

Congratulations !

By the way, i may be wrong (i am still learning spaceplane SSTOs), but i think you should have retracted the airbrakes as soon as your velocity was low enough to be unharmfull. That way you would have used less fuel cruising to the runway

[RCgothic]

I did, but I used them for braking on the runway once I was down.

[Van Disaster]

Wouldn't worry about airbrakes; just keep your re-entry profile flat & pitch up a couple of degrees, and if it looks like you're overheating pitch up a bit more. I can usually glide onto the runway from space; use runway 27 & turn onto final over the island airport, that way you can fiddle with your glide to get your landing speed right. Takes a while, but if you want to get home quickly use a parachute rather than wings

Here's a run up & down from one of Wanderfound's challenges; unfortunately I landed in the dark, but hey. The craft has all sorts of airbrakes, none of which got any use in the end...

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