How to do a proper reentry with a spaceplane?

[Puddle Jumper]

I built a small and simple spaceplane. Flies like a breeze, and it functions as a proper SSTO. I parked it in a circular orbit at an altitude of 100 km. I did a de-orbit burn and then things started to go south. In short, I burned like a crisp. The question, how should I de-orbit to avoid this? My mk1 cockpit exploded due to overheating at 35 km (speed= 2070 km/h).

[Evanitis]

You are going too fast. You need to slow down before you hit the thick, lower atmosphere. First, your initial trajectory needs to be shallow. Lower your periapsis to ~40km, the friction will do the rest.

What's more important, is to not face prograde during the descent. To bleed the most speed, you must present your widest profile to the airflow as soon as you reach the air (70km). Imagine throwing an umbrella closed vs. open. A forward facing aircraft is like the closed one. Pitching up to 90° is overkill in most cases, 30-40° should be enough. You can even trim the desired landing spot by holding a specific angle of attack (AoA).

Illustration:

It's possible that you can't pitch up. The most common reason for that is the CoM shifting behind the CoL while the tanks run dry, messing up your aerodynamic properties. There's not much you can do in this case besides redesigning, unless you have well placed control surfaces. In this case, you can try landing backwards. Sounds silly, but it's totally possible.

Edited February 29, 2016 by Evanitis

[ForScience6686]

I prefer the steep trajectory with a very high aoa.  Higher drag up high and as the aero forces bring your nose down it will transfer vertical speed r to horizontal speed.  This has worked finefor my small ssto, may not be as good in heavier craft.

[Puddle Jumper]

Ah, thanks. I understand and it works. Never realised that re-entry was this realistic. I thought that a shallow approach meant that you shouldn't plummet vertically down the atmosphere, but apparently shallow means really really shallow.

[numerobis]

In the opposite direction, my best "cheap and cheerful" challenge flights using Nefrum's spacecraft (which has a Mk1 up front) achieve orbit at about 36km (2181m/s surface speed) then boost Ap to 80km (a bit faster, not really significant). It gets hot in the cockpit, but nothing melts unless I mistime the ascent and drop back down to only establish orbit around 33km.

I've dropped from 100km to the surface with a plane; just enter at a very steep angle of attack to use those giant wings to slow down fast. Another trick: roll back and forth (or all the way around) to spread the heat around more of your cockpit, not just the underside but up the sides as well.

[Evanitis]

16 hours ago, ForScience6686 said:

I prefer the steep trajectory with a very high aoa.  Higher drag up high and as the aero forces bring your nose down it will transfer vertical speed r to horizontal speed.  This has worked finefor my small ssto, may not be as good in heavier craft.

It's definitely an option, a faster and more dynamic one. In my experience, most planes are resistant enough to allow such flexibility in deorbit trajectory. And coming in nose down looks cool. Others even say that a steeper curve with negative PE is preferable, as the craft spends less time in the upper atmo gaining heat while only bleeding speed slowly. I doubt that this matters - I had no heating problems when I did many aerobraking passes on 55km - that kinda' results in the shallowest possible trajectory.

Though when someone asks how not to burn up, I'm 90% sure it's an AoA problem - but just to be sure, I mention a safe deorbit PE.

[Puddle Jumper]

I tried using a high angle of attack. Works like a charm. Does anyone know if the real-life Space Shuttle used a similar approach?

[Evanitis]

Sure it did.

[Nich]

Max Lift is at 30 degrees for wings.  With that in mind lately I have been burning 90 degrees from KSA until the orbit intersects KSA.  Hold 45 degree AOA until orbits hit Mount Whoopstooshort.  Then 30 degrees until orbit is back at KSA.  My craft had not had air brakes for a while but they would be sufficient to get you in from here but I generally have a bit more speed to bleed off (200-300m/s) So I aim short and level off for a very shallow approach.

[zolotiyeruki]

On 3/1/2016 at 0:19 PM, Nich said:

....  Hold 45 degree AOA until orbits hit Mount Whoopstooshort. ..

If that mountain range actually has a name, and that's the name, it's perfect.  Otherwise, well played sir!

[Nich]

I can not take the kredit  saw it over a year ago and absolutely loved it.

[Puddle Jumper]

Wonderful name indeed. Are there any more names for Kerbal landmarks or geographical features like this?

[Geschosskopf]

17 hours ago, Puddle Jumper said:

Wonderful name indeed. Are there any more names for Kerbal landmarks or geographical features like this?

I believe @Kuzzter came up with Mt. Whoopstooshort.

Other rocketry-related landmarks:

• Bay of Boosters / Boosterbottom Gulf:   The ocean east of KSC
• Overshoot Island:  variously applied to either Airbase Island or the large island NE of KSC about halfway across Boosterbottom Gulf
• MECO Point/Peninsula:  The south-pointing land that reaches the equator on the far side of Boosterbottom Gulf from KSC

[AeroGav]

Re-entry is the process by which friction converts a spacecraft's kinetic and potential energy to heat energy.

Firstly, if you're worried about burning up,  you want to pitch up to an angle of attack that gives maximum lift.  This keeps you out of the thicker air and causes the re-entry heat pulse to be spread over a longer period, which gives hot components more time to dissipate their heat, leading to lower peak temperatures. 

 Angle of attack is the difference between the yellow prograde circle and the orange -w- symbol on your navball, with the navball in surface mode.    Lift increases rapidly up to about 15 degrees angle of attack,  after that it continues to go up slowly but you get an increase in drag.  Peak lift is at 30 degrees AoA, after that the plane stalls and lift falls off very rapidly.

Now, because we're human and holding a perfect pitch attitude isn't easy,  I seldom aim for more than 20 degrees, because lift goes up so slowly beyond this and falls off a cliff if you overdo it. 

The drawback to "flying in like an airplane"  is that all of the re-entry heat tends to get concentrated on the nose & leading edges of your vehicle.   However, by deploying landing gear, opening cargo bays etc. to increase drag, these items can do some of the thermal -> kinetic energy conversion too.  This has the practical effect of shortening the re-entry duration, which can help peak temperatures.

Now, as a practical matter,  even my Mark 1 cockpit spaceplanes have no issues re-entering on Kerbin, because my designs have lots of wing area and don't drop into the soup till going pretty slow.   I like to make patchwork quilts of Big-S delta wings and keep all my fuel there - fuselages are for cargo, systems and kerbals only.     I also prefer inline cockpits with a few things ahead of them in the vehicle stack, that way they're not the first thing to blow up if you do get it wrong and not the first to smack the ground if you prang the landing.    

For the belt-and-braces approach,  put 1.25m heat shield with a little ablator right behind the nose cone of your aircraft.  The nose cone will be first to pop, exposing the heat shield, which will then blanket the cockpit and stop it heating any further.

In terms of placing the landing site accurately, i'll admit it's tricky.  I normally aim for a 35km periapsis in the middle of the desert in the continent west of the space centre.

If I'm undershooting, bear in mind that  the best lift-drag ratio at hypersonic speeds is at 5 degrees angle of attack.  You will actually glide furthest and travel furthest from your initial re-entry point at this angle.      Above 5 deg, drag rises faster than lift so the distance downrange will be less the more you pitch up.   

So, if you're undershooting and not in danger of blowing up, try lowering the nose bit closer to the "best glide" angle.  If overshooting, pull back to a near stalled condition and make more drag.     I know it sounds kind of backwards, and for the first couple of seconds pitching down will make altitude drop faster as height is converted to speed, but pretty soon the trajectory line will show your landing point moving further away.

As for pulling back on the stick to correct an overshoot,  I actually think that's easier on the plane than making S turns.  You're using lift to Up rather than sideways, so will stay higher and cooler longer,  and you might welcome the buffer if you start having thermal issues.      

So, assuming your craft is like mine and doesn't come particularly close to melting on a normal re-entry,  i'll start o

[mrclucks]

While I'm not an SSTO master, I've built a few of them and landed(or at least reentered successfully).  I do an AoA of 20 degrees at an PE of 40-35 km from the start and usually start skipping up around 50-45km where I'll either lower my AoA to -20 or 5 till I start coming back down.  I'm nearing 20km around the ocean or the desert and will usually have to turn on my engines at the minimum to make it over the mountains. On the flip side if you're initial descent seems as if you will overshoot, you should combine pointing 15+ degrees down with 15+ degrees up to kill vertical speed faster so you can be low enough to land on or near the runway