Cobra reentry with a spaceplane OP

[rtxoff]

This is not rant just my observation. Everyone is praising 1.X aerodynamics to be realistic but if you reenter the atmosphere with a spaceplane with a very steep angle you will brake without any additional braking parts to 300m/s @ 25000m.

I find this to be very unrealistic.

Edit: I forgot to mention that this method also doesn't generate any heat. You will reenter without getting the reentry effects.

[Hotaru]

I've noticed this as well, I've occasionally seen smallish SSTO's reentering with aft CoG's go into a tumble during reentry around 30,000 meters and 2 km/s. Rather than disintegrating as one might expect, they almost instantly slow down to around 500 m/s at 20-25,000 meters, after which it's pretty easy to transfer fuel forward and recover.

I don't think it would be a problem if it was a little less pronounced, or if the spaceplane was more likely to break up when this happens (larger ones do, but smaller ones usually don't). Of course, real spacecraft (Shuttle, X-15) did reenter at 20-40 degrees angle-of-attack, but they didn't hit quite the brick wall that a spaceplane in KSP does when reentering underside-first.

Another questionable effect I've noticed is that airbrakes become effective before serious heating starts to build up, making it possible to avoid heating entirely on some designs by adding lots of them. I've especially noticed this on reusable rockets that need lots of airbrakes to stay stable in a tail-first reentry. They frequently make it all the way through reentry with no visible flames. (The same is true, incidentally, of reentering Kerbals.)

To be fair I don't think anybody's saying the new aero is perfect or unexploitable or anything, just that it's a huge improvement over the old souposphere. I'd be curious if the same effects happen with FAR.

[KerikBalm]

yes, its a huge improvement....

I just did a really really really bad reentry last night...

I timed my departure from the Mun a bit off, and the KSC wasn't far enough east to do a realtively shallow reentry and landing, and I didn't have a lot of spare fuel to fly a long way back.... it was too late to adjust perapsis for a mere aerobraking pass.

So... I pushed the nose down, and came in really steep.... my air intakes nearly overheated, and I relented and pitched up breifly.... but it was pretty much as aggressive of a reentry as possible... and nothing overheated...

I think its going to be pretty hard to balance OHing in a way that SSTOs don't burn up on ascent, but you still need to worry about descent.

Lower drag may help get into lower atmosphere with more speed to cook you... it won't help SSTOs much because top speed is limited by the engine curve anyway...

I somewhat agree with some who thought 1.0 aero simply needed a higher drag co-efficient for rentering pods, and not a universal drag increase.

[zarakon]

Yeah, I've found that the best way to re-enter without blowing anything up is to create as much drag as possible and spread it over as many parts as possible while high in the atmosphere. For a simple capsule+fueltank+engine ship, you can easily re-enter by just going into a tumble or orienting sideways while high up

[KerikBalm]

Which is somewhat similar ot real life... the space shuttle doesn't enter nose first, it enters belly first.... almost like a capsule.

The leading edges, and protrusions heat the most.

In reality, you want to spread the drag out as much as possible too.

[Archgeek]

I'm pretty sure this effect is a least partly legit. The night of the 1.0 release I took an Aeris...4? The one with the the LV-T45 on it upstairs, ran out of gas and electric charge, which is beside the point, and got it back down using just its RCS, which I also used to take a nearly 90 degree angle of attack very very high up, easing up a bit once the atmosphere showed up noticabably to try and deflect upward some to stay higher longer, since I was downright terrified of trying to get down an unshielded space plane. This resulted in enough gradual slow down that I barely saw any re-entry flare, though it did take a while. Note that this was in the 1.0 un-soup.

[cicatrix]

Ehm, right now I have a craft approaching Laythe at 8 km/s... I wonder will this method work there?

[rtxoff]

Ehm, right now I have a craft approaching Laythe at 8 km/s... I wonder will this method work there?

i'm pretty sure it will, just hold the nose of your plane as high as possible.

[cicatrix]

i'm pretty sure it will, just hold the nose of your plane as high as possible.

It's not, strictly speaking, a plane. Just an ordinary spacecraft. I tried aerobraking already. It burns at 35 km in a second, at 45 - in two seconds.

[Acemcbean]

cicatrix, you know what to do then. Do it Apollo, with multiple passes at high altitudes. Multiple passes means you should have a rather high Periapsis over Laythe, and since I don't know EXACTLY where the atmosphere ends there, just go with like 55km maybe? Let it pass two or 3 times, or, if you get lucky, 55 COULD be a sweet spot for whatever you are doing

[Jouni]

I've noticed this as well, I've occasionally seen smallish SSTO's reentering with aft CoG's go into a tumble during reentry around 30,000 meters and 2 km/s. Rather than disintegrating as one might expect, they almost instantly slow down to around 500 m/s at 20-25,000 meters, after which it's pretty easy to transfer fuel forward and recover.

As a rough guideline, spaceplanes lose at least half of their mass before reentry. If it's reasonable to expect the plane to survive 10g maneuvers on the way up, then they can probably withstand 20g maneuvers on the way down.

[cicatrix]

cicatrix, you know what to do then. Do it Apollo, with multiple passes at high altitudes. Multiple passes means you should have a rather high Periapsis over Laythe, and since I don't know EXACTLY where the atmosphere ends there, just go with like 55km maybe? Let it pass two or 3 times, or, if you get lucky, 55 COULD be a sweet spot for whatever you are doing

LOL atmosphere over Laythe is 50 km and... I can't do it multiple times, I'm on Joolian escape trajectory. I'm trying to find some trajectory around Joolian moons so that a good gravity assist from Laythe or Tylo could keep me within the system. Before 1.0 I simply plunged into Jool to kill speed, now the task is more delicate, aerobraking is not an option.

[Red Iron Crown]

This is not rant just my observation. Everyone is praising 1.X aerodynamics to be realistic but if you reenter the atmosphere with a spaceplane with a very steep angle you will brake without any additional braking parts to 300m/s @ 25000m.

I find this to be very unrealistic.

Edit: I forgot to mention that this method also doesn't generate any heat. You will reenter without getting the reentry effects.

Do you mean that the parts didn't heat at all? Or just that the reentry effects never appeared?

[moogoob]

LOL atmosphere over Laythe is 50 km and... I can't do it multiple times, I'm on Joolian escape trajectory. I'm trying to find some trajectory around Joolian moons so that a good gravity assist from Laythe or Tylo could keep me within the system. Before 1.0 I simply plunged into Jool to kill speed, now the task is more delicate, aerobraking is not an option.

You can try a Jool aerobrake, certainly. You're being more efficient that I usually am - I usually just go the route of "bring way too much fuel and circularize normally".

[rtxoff]

Do you mean that the parts didn't heat at all? Or just that the reentry effects never appeared?

Just the effect never appeared.

However the heat produced this way is almost negligible compared to a reentry with flat angle.

[martinborgen]

Just the effect never appeared.

However the heat produced this way is almost negligible compared to a reentry with flat angle.

I might be wrong, but I think the problem of re-entry too flat is the G-forces destroying the craft. No Idea about the heat levels involved though.

[problemecium]

This sounds legit to me too o_O

In fact I've wondered why NASA hasn't tried using a blimp for space travel - it would float up to the upper atmosphere, then use comparatively smaller and more efficient engines to get the rest of the way to space, and on the way down the balloon would create so much drag that reentry would be slow and peaceful.

[Armchair Rocket Scientist]

This sounds legit to me too o_O

In fact I've wondered why NASA hasn't tried using a blimp for space travel - it would float up to the upper atmosphere, then use comparatively smaller and more efficient engines to get the rest of the way to space, and on the way down the balloon would create so much drag that reentry would be slow and peaceful.

There have been some ideas that are a little similar to that. Rockoons are suborbital rockets with a balloon as the first stage, which gives them a pretty big performance boost because when you're just going suborbital the extra height actually translates to significant dV reductions, and sounding rockets are small vehicles with high TWRs, so they suffer major drag losses. Ballutes are balloons used for drag instead of buoyancy, and are sometimes used when supersonic deployment is necessary. The wiki says ballutes have been considered for extremely small (Cubesat-size) reentry vehicles, but something larger would probably need a real heat shield.

Someone has actually proposed a vehicle like you're thinking of, but honestly it seems completely unworkable. First of all, ANY air-launch-to-orbit proposal has the problem that no matter how high your starting altitude is, you can only save about 1 km/s out of 9 km/s. To get a reasonable payload capacity, you also need an ENORMOUS carrier aircraft. I doubt any practical air-launch system could handle more than 20 tons of payload.

This proposal is even worse. They're accelerating the entire balloon to orbit, which means their system is basically an SSTO with a high-altitude launch pad. To make matters worse, balloon envelopes are extremely heavy compared to their contents. Even if you filled the balloon with hydrogen and used that as propellant, good luck getting a mass ratio anywhere near what you'd need for orbit. And remember all that drag the balloon is supposed to generate on reentry? It's generating that same amount of drag on the way up, but is supposed to spend several DAYS at mesopheric altitude. 50-100 km is where relatively compact metal capsules do a lot of their deceleration. Columbia broke up in that altitude range, and expendable spacecraft probably break up even higher. Even if the balloon survived, the drag losses would be astronomical. Finally, assuming the balloon makes it into orbit, it's envelope is extremely thin, on the order of a few microns. It is also an extremely large (tens of thousands of square meters) target for micrometeoroids and orbital debris. The thing will be turned into swiss cheese.

[Red Iron Crown]

I tried this "cobra braking" maneuver with my small cargo spaceplane tonight, and I have to say I'm impressed. It bled off speed quickly like an airbrake-spammed craft, without using the airbrakes on the plane at all. The plane still gained heat, but the high heat gain period was much shorter due to the greater deceleration.

Whether realistic or not, this is a handy technique for plane reentry in stock aero. Good find, gpisic, rep++.

[ajburges]

I didn't realize this wasn't already well known.

As a tip. Do this with areo forces displayed. At a critical angle you maximize body lift (cyan vectors). It will significantly alter your vertical velocity and keep you higher for longer as you decelerate.

High AoA braking isn't as powerful as air brakes, but if you rely on it first, your air brakes give you greater landing site control.

[arq]

FAR prevents this by adding structural failures due to aerodynamic forces. The stock game would need something akin to this to remove this exploit or "exploit." G-limits for kerbals (akin to what DRE does) would resolve this for manned vessels, but still would leave the technique viable for drones.

[SkyRender]

Really what's needed to nip that one in the bud is an overall pressure tolerance threshold for parts (which combines G-forces with atmospheric effects). As noted, FAR does have this factored in the form of aerodynamic failure, and in fact does exactly that: it checks the combination of atmospheric density and G-force being applied to parts and breaks things when that threshold is exceeded. This is why a spaceplane taking off from Eve at sea level in FAR is generally a suicide mission. I think the main reason why high dynamic pressure simulation isn't in the game at this point is, well, it's generally not much fun to have your craft literally disintegrate around you without much warning or means of preventing it by extension.

[Jouni]

FAR prevents this by adding structural failures due to aerodynamic forces. The stock game would need something akin to this to remove this exploit or "exploit." G-limits for kerbals (akin to what DRE does) would resolve this for manned vessels, but still would leave the technique viable for drones.

If you brake at 1g during reentry, the aerodynamic forces are weaker than for a fully fueled plane at level flight.

[Red Iron Crown]

Er, you folks realize that aero failures are a thing in stock aero now, right? It's more forgiving than FAR but it can still happen.

[Wanderfound]

Er, you folks realize that aero failures are a thing in stock aero now, right? It's more forgiving than FAR but it can still happen.

Extremely forgiving. Extended 15G turns, 45° reentry AoAs.

Which is fine. If you want more realistic aero, go FAR.