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PhylumCnidaria

Need help with SSTOs

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I'm working on a few peewee SSTOs right now. But I've reached a bit of a brick wall...

How do you re-enter the atmosphere with these things? Do you go shallow or try to decelerate as much as possible during your retrograde burn? Do you keep it pointing directly prograde through re-entry? 

All my SSTOs get toasted unless I turn down the re-entry heat in the settings, and I'm tired of doing that. Any help is appreciated.

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1 hour ago, PhylumCnidaria said:

I'm working on a few peewee SSTOs right now. But I've reached a bit of a brick wall...

How do you re-enter the atmosphere with these things? Do you go shallow or try to decelerate as much as possible during your retrograde burn? Do you keep it pointing directly prograde through re-entry? 

All my SSTOs get toasted unless I turn down the re-entry heat in the settings, and I'm tired of doing that. Any help is appreciated.

I can help a little but there are others that are a lot more educated on SSTO's. First try to make your re-entry on a slight angle if you are coming in with too much vertical your speed will increase more rather than slowing your speed. Air Brakes and a well balanced craft helps, often a craft will use lots of it fuel and when coming back on re-entry your fuel lost will have your craft with all the weight out of balance. Radiators can help with overheating, make sure they are activated when you begin to re-enter. With large craft I've noticed keeping your nose high will cause more drag to slow your speeds. I would say too much speed is one of the biggest fights I had with re-entry. 

Back to others on the forums with lots of SSTO experience, for one I would say @Rune I've learn a bunch from others on here and Rune can be a good help on this subject! 

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Begin retrobraking at 150W and burnt to get a periapsis -160kms

Edited by gilflo

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5 hours ago, Castille7 said:

I can help a little but there are others that are a lot more educated on SSTO's. First try to make your re-entry on a slight angle if you are coming in with too much vertical your speed will increase more rather than slowing your speed. Air Brakes and a well balanced craft helps, often a craft will use lots of it fuel and when coming back on re-entry your fuel lost will have your craft with all the weight out of balance. Radiators can help with overheating, make sure they are activated when you begin to re-enter. With large craft I've noticed keeping your nose high will cause more drag to slow your speeds. I would say too much speed is one of the biggest fights I had with re-entry. 

Back to others on the forums with lots of SSTO experience, for one I would say @Rune I've learn a bunch from others on here and Rune can be a good help on this subject! 

Poof! Appearing as requested! Always glad to help.

7 hours ago, PhylumCnidaria said:

I'm working on a few peewee SSTOs right now. But I've reached a bit of a brick wall...

How do you re-enter the atmosphere with these things? Do you go shallow or try to decelerate as much as possible during your retrograde burn? Do you keep it pointing directly prograde through re-entry? 

All my SSTOs get toasted unless I turn down the re-entry heat in the settings, and I'm tired of doing that. Any help is appreciated.

Great! That's a subject that I've rarely talked about, and one that deserves a lot of consideration. The main thing in order to not blow up is to shed your energy slowly. Brake too fast, and the thermal load will blow you up. So, how do you go about it, exactly? Well, the trick is to do it in the high atmosphere, where the air is thin and the drag is low. That also means, of course, that it takes a long time, so plan accordingly. I use trajectories to more or less plan things, but I put the initial impact point on the continent west of KSC, because I am going to go much farther than if I went 'ballistic' (I.E: prograde all the time).

Now, actually you don't need this at all, because if your SSTO is any good at maneuvering you have a huge amount of control about how you fall. Stay prograde, and you will fall in exactly the same way a capsule does, but if you put a bit of an attack angle, you can fall much slower, or use a negative AoA and actually fall faster. The main thing to take form this is, you can control your vertical speed, so you should keep an eye on it. That and altitude are the two only things you have to work with, and they actually sit next to each other on the top of your screen. So brace yourself, because I am going to start reciting numbers that are, of course, very specific to a kerbin reentry. Things start getting draggy below 50kms, so doing anything above that is usually a waste of time, just stay prograde. But things don't start getting seriously hot until about 35-40kms (at orbital speed), so that's the altitude you want to start your braking at. As soon as you get close to 40kms, increase AoA at least as much as necessary to arrest your vertical speed and go more or less horizontal-ish, and you will start shedding speed without increasing your temperature, and in fact your temperature will start to go down. Once you go below ~1,000m/s, it's safe to pitch down and plunge into the lower atmosphere without burning up. Too much.

Now, notice that I said 'increase AoA at least as much as...". That 'at least' is very important, because in it lies the trick to not only fall safely, but with a lot of precision, so you can nail KSC every time and really milk those SSTOs with 100% recovery. See, with AoA you not only control the rate of descent, you also control your drag. Go horizontal with the least AoA required, and you will glide more than half the planet before running out of energy and falling to the lower atmosphere, where drag won't let you go very far. But if you increase AoA above ~30º, you will not actually go up, but instead stall the whole wing, reduce lift, and greatly increase drag. Think of it as turning you whole wing into one giant, heat-resistant airbrake (which is why I don't use airbrakes). You need enough control authority to do high AoA maneuvers, but with them you can almost stop on a dime (or at least a continent), and control where you want to fall exactly.

So three distinct phases: first, fall prograde until you are approaching 40kms and stuff starts to get hot. Then you pull up, not-quite-leveling the flight (I like to keep vertical speed at about 100m/s, but that's me) and start shedding energy slowly, while you cruise around kerbin as stupendous speeds. If you feel that you are going to overshoot, you pull up sharply to 45º or more, and start increasing drag, and if you are actually going up and worried about it, you can pull down hard, and negate your vertical speed very quickly. Two or three of those pullup maneuvers should get you to the east mountains of KSC with about ~900m/s and ~20kms of altitude, and from there it's basically a straight line to the runway, the lower atmosphere will brake you to subsonic speed by the time you get close to the ground.

 

Rune. And if you still overshoot KSC, use the energy you have to perform a horizontal 180º turn, then glide yourself back. Happens to all of us, and it's way better than blowing up.

Edited by Rune

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2 hours ago, Rune said:

Poof! Appearing as requested! Always glad to help.

Great! That's a subject that I've rarely talked about, and one that deserves a lot of consideration. The main thing in order to not blow up is to shed your energy slowly. Brake too fast, and the thermal load will blow you up. So, how do you go about it, exactly? Well, the trick is to do it in the high atmosphere, where the air is thin and the drag is low. That also means, of course, that it takes a long time, so plan accordingly. I use trajectories to more or less plan things, but I put the initial impact point on the continent west of KSC, because I am going to go much farther than if I went 'ballistic' (I.E: prograde all the time).

Now, actually you don't need this at all, because if your SSTO is any good at maneuvering you have a huge amount of control about how you fall. Stay prograde, and you will fall in exactly the same way than a capsule goes, but if you put a bit of an attack angle, you can fall much slower, or use a negative AoA and actually fall faster. The main thing to take form this is, you can control your vertical speed, so you should keep an eye on it. That and altitude are the two only things you have to work with. So brace yourself, because I am going to start reciting numbers that are, of course, very specific to a kerbin reentry. Things start getting draggy below 50kms, so doing anything above that is usually a waste of time, just stay prograde. But things don't start getting seriously hot until about 35-40kms (at orbital speed), so that's the altitude you want to start your braking at. As soon as you get close to 40kms, increase AoA at least as much as necessary to arrest your vertical speed and go more or less horizontal-ish, and you will start shedding speed without increasing your temperature, and in fact your temperature will start to go down. Once you go below ~1,000m/s, it's safe to pitch down and plunge into the lower atmosphere without burning up. Too much.

Now, notice that I said 'increase AoA at least as much as...". That 'at least' is very important, because in it lies the trick to not only fall safely, but with a lot of precision, so you can nail KSC every time and really milk those SSTOs with 100% recovery. See, with AoA you not only control the rate of descent, you also control your drag. Go horizontal with the least AoA required, and you will glide more than half the planet before running out of energy and falling to the lower atmosphere, where drag won't let you go very far. But if you increase AoA above ~30º, you will not actually go up, but instead stall the whole wing, reduce lift, and greatly increase drag. Think of it as turning you whole wing into one giant, heat-resistant airbrake (which is why I don't use airbrakes). You need enough control authority to do high AoA maneuvers, but with them you can almost stop on a dime (ar at least a continent), and control where you want to fall exactly.

So three distinct phases: first, fall prograde until you are approaching 40kms and stuff starts to get hot. Then you pull up, not-quite-leveling the flight (I like to keep vertical speed at about 100m/s, but that's me) and start shedding energy slowly, while you cruise around kerbin as stupendous speeds. If you feel that you are going to overshoot, you pull up sharply to 45º or more, and start increasing drag, and if you are actually going up and worried about it, you can pull down hard, and negate your vertical speed very quickly. Two or three of those maneuvers pullup maneuvers should get you to the east mountains of KSC with about ~900m/s and ~20kms of altitude, and from there it's basically a straight line to the runway, the lower atmosphere will brake you to subsonic speed by the time you get close to the ground.

 

Rune. And if you still overshoot KSC, use the energy you have to perform a horizontal 180º turn, then glide yourself back. Happens to all of us, and it's way better than blowing up.

Thanks! Im a little mad at you now because you gave me a reason to finish my SSTOs, when I have to do homework. Nice work :wink:

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5 hours ago, PhylumCnidaria said:

I'm working on a few peewee SSTOs right now. But I've reached a bit of a brick wall...

How do you re-enter the atmosphere with these things? Do you go shallow or try to decelerate as much as possible during your retrograde burn? Do you keep it pointing directly prograde through re-entry? 

All my SSTOs get toasted unless I turn down the re-entry heat in the settings, and I'm tired of doing that. Any help is appreciated.

I have always used the following technique for re-entry. it's very simple but you have to equipped your SSTO with some vernor engines on top of cockpit and below cockpit. They must be powerful enough to help maintaining Cobra attitude on re-entry, which is just 10° under the Radial+ position on your Navball. You keep nose up position around 80° from 60kms altitude to 30 kms or less. To keep your heading you have to help with RCS thrusters on your wings so that they can thrust left or right to keep your nose center at 12 o'clock .

Before taking Cobra position you must burn Retrograde on your back - cockpit facing Kerbin, so that you have to flip 90 to 100° to take Cobra position when you reached 60 kms altitude

Start burning between longitude 150W and 140W to aim at Kerbin base. For a 100kms orbit I burn retrograde to reach an apoapsis of -150 to -160 kms, then I keep retrograde, cockpit facing Kerbin down to 65kms, Then I began flipping to cobra position with the help of my cockpit vernor and I maintain this position as long as possible using vernors and RCS thrusters.

You can also try from retrograde to flip to Radial + position with the help of ASS and RCS.with this technique you will never overheat and you will get out of cobra position in line for flying....

Just look at this vid from KSP 0.90 and you will understand

 

Edited by gilflo

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I have an SSTO that can do a high AOA reentry that doesn't require vernier or RCS. It's all about balance. Just wanted to add that for the record in case people begin to believe they are required.

 

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18 minutes ago, Space Cowboy said:

I have an SSTO that can do a high AOA reentry that doesn't require vernier or RCS. It's all about balance. Just wanted to add that for the record in case people begin to believe they are required.

 

Yup, it's all a matter of enough control authority to overcome your natural stability. So, if you are just barely stable, it's actually pretty easy. Also, you don't need to go the full 90º to the airstream, wings turn into airbrakes much sooner, around 45º AoA. I actually think I have a couple of pics to illustrate exactly the point....

Here we have lots of lift, and are pushing out impact point eastwards:

vBLOsWE.png

And here, just a few degrees more, and drag starts to dominate, lift goes down the drain, and the impact point moves westwards:

gIzUuv0.png

 

Rune. I know, I should have kept the UI up, but you can see I was only about 30-45º over the horizon.

 

Edited by Rune

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Yes 45° AOA should be the best in fact, whatever you do to set it !

My tankers got  COG very aft of fuselage to have dry COG and fueled COG at the same place, that's why i need some vernors to get full authority to aim at 45° AOA

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I've split the above posts from SSTO picture thread, as they were off topic.

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My own approach -- which I've found works pretty well across a broad range of spaceplane designs, once they're balanced to be controllable in it -- is to lock in an AoA of 40-50 degrees, just enough to stall my main wings, and set my periapsis to around 40-45km. That brakes you quickly and early, thereby dramatically reducing peak heat loads. Once you're down to 1km/s, you can nose-forward into a hypersonic glide and cruise to touchdown. The trickiest part is in the balance, since a plane that can perform high-angle reentries purely on aerodynamic control is generally very close to being, or outright is, aerodynamically unstable. RCS units can help cheat this somewhat and buy you a more stable layout in other flight regimes.

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I typically use an unpowered reentry, at orbital speed.

Heat goes with the cube of the velocity. So, unlike Rune, I like to scrub my first 200 m/s in the very high atmosphere, above 55km -- I just lock my plane to radial out hold (Surface mode) with SAS. That gets rid of the really scary part of the reentry velocity. Then I pick my first target -- I want to be going 2100 m/s (orbital) over the coast at the east end  of the Great Desert (so use AoA control to manage speed, drag, and altitude). I don't use huge amounts of reaction wheels and no RCS at all in my spaceplanes, so it gets hard to hold a very high AoA after the Great Desert. But I hold it as high as I can going across the ocean. My next target is 1900 m/s (orbital) directly over the west coast of the KSC continent -- at around 35km altitude. Then 1500 m/s (surface) over the mountains west of KSC. Like Rune, I find it pretty easy to scrub the remainder quickly as I get within 30km of the runway.

But part of it depends on which parts you use. If you have a SciJr or any MK1 crew cabins -- then your spaceplane is in much greater danger from the heat. In that case, I turn retrograde above the east coast of the Great Desert, and burn retrograde until my orbital velocity is 1500 m/s (so 800 m/s of dV). Yes, it's moderately expensive in fuel, but there isn't even the slightest heating problem from even the most delicate parts in the plane as it glides down to the runway after that.

 

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So many opposing approaches lol !  Mine is different again.

If concerned about heat, i just hold the nose about 20 degrees above prograde for maximum lift.    The idea is to stay as high as possible for the speed you are doing.    

Craft design makes a  huge difference.

The main 2 things are

1. use inline cockpits ie.   heat resistant part at front, fragile stuff as far back as possible

2. wing area.   The more lift it can make, the slower it will be before falling into the peak heating zone.  I can usually maintain over 35km till below 1200 m/s.  Most capsules burn to a PE of 20km and start chewing through their ablator at 35km and 2400 m/s,  on a high lift spaceplane you'll barely be doing half that at this stage.

In this video i'm re-entering a mk1 cockpit ssto with a PE that's set below ground level (hardcore).  If you were playing safe, you'd obviously aim for 40km plus,  but the purpose of this video was to intentionally undershoot then demonstrate how you can use your crossrange glide capability to recover.  Since this vessel is in little danger from re-entry heat even with a re-entry with the PE at ground level, i'm not pitched up to max lift 20 degrees, i'm going for max glide range.  But if you read the annotations you'll get the idea.

  

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The thing about flying high-lift reentries is that you're stuck in the upper atmosphere for ages, and what you can wind up doing is thermally soaking the craft and having parts explode from core temperatures instead of skin. You want to stall; it gives you maximum drag, which brakes you safely through peak heat with no problems at all, and it gets you quickly down into the lower atmosphere to allow you to brake even faster, which eliminates any possibly of soak and means your reentry doesn't take an hour. Only reason to fall off the high-angle before you transition to the cruise is if the g-loading becomes excessive, but that's not likely.

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