Flight Question Aerobraking on earth is hard

[Bonanza123d]

I am trying to use Aerobrake MFD to do some practice aerobraking from the Moon to Earth. The problem is that i don't know where to place my PeA from the earth's surface for re-entry. I was aiming for 50k from the surface and i just burned up like no tomorrow. What should i aim for when i start. I am trying to save fuel and time so i can do a high velocity return. The aerobrake will be first then the second stage would be the full re-entry.

 

[Loru]

What vessel? Some crafts can withstand direct reentry from moon/mars and some not. Details please!

 

[Bonanza123d]

I am using the XR2, XR5, and DGIV for the OFLS mission procedures for those space craft. I saw that the aerobrake can save fuel. When i was test flying a few spacecraft, i could not pull the aerobrake properly causing me to burn up.

 

[Urwumpe]

50k periapsis altitude is too low for a successful aerobraking at this speed. Try aiming higher, around 70km should be good.

NEVER fly below 80km altitude if you are faster than 8000 m/s. 50% more velocity means 35% higher peak temperatures (in Kelvin) at the same altitude. Twice the velocity means 68% more Kelvin. So, if you reenter at 16 km/s, you will be around 1700K hot at 70 km altitude.

 

[Bonanza123d]

50k periapsis altitude is too low for a successful aerobraking at this speed. Try aiming higher, around 70km should be good.

NEVER fly below 80km altitude if you are faster than 8000 m/s. 50% more velocity means 35% higher peak temperatures (in Kelvin) at the same altitude. Twice the velocity means 68% more Kelvin. So, if you reenter at 16 km/s, you will be around 1700K hot at 70 km altitude.

So when does the "thick" atmosphere start? I want an actual equation so i can make high velocity returns.

 

[Urwumpe]

So when does the "thick" atmosphere start? I want an actual equation so i can make high velocity returns.

There is no border. Thick atmosphere is everything that is too much for your mission. If you want to stay in orbit, thick starts below 140 km altitude already.

If you want to estimate how much you can take, look into Orbiters documentation, there is a density profile of Earths atmosphere.

 

[Spike Spiegel]

I actually did some trial and error with this, using a DGIV coming back from the Moon. I did a quicksave while I was on my way towards Earth, but far enough away that it was still really easy to adjust periapsis with just the RCS thrusters. Then I could just keep trying different values until I found what worked.

I found that there was a pretty narrow window to get some decent braking without burning up. I think I set my periapsis to about 68km on the way in (do your own tests to see what works for you though), which then dropped slightly as I began to decelerate in the atmosphere. I made it through, well within the limits of the DGIV, and dropped the apoapsis significantly as a result. I had done one just a few km lower, and I burned up real good. At 80km or so, I didn't get enough deceleration for my taste.

From there I just kept adjusting the periapsis on each orbit to gradually decelerate until I was roughly circular. There might be a faster, more elegant way to do it all in one go, but I'm not that skilled yet.

 

[Rtyh-12]

A sweet technique is to do what we call inverted re-entries. Basically you need to be upside-down, that is, the top of your ship should be facing Earth. You keep the normal 40 degrees angle of attack, but you'll be pointing at Earth. It's hard to explain in words, but it's not that hard to do... perhaps someone else can explain it better...

 

[Bonanza123d]

From there I just kept adjusting the periapsis on each orbit to gradually decelerate until I was roughly circular. There might be a faster, more elegant way to do it all in one go, but I'm not that skilled yet.

I am trying to do it all in one go. If you pay attention to your decent rate, i think it will work good. I will need to try it soon. Hopefully i can get everything worked out. BTW i am wondering if anyone knows the equation to calculate temperature buildup and the numbers i should put in.

 

[Wishbone]

Precession MFD (Environment page, and the source code). Please bear in mind it does not know anything about thermal properties of the spacecraft itself, and assumes it is a blunt body.

 

[Moach]

A sweet technique is to do what we call inverted re-entries. Basically you need to be upside-down, that is, the top of your ship should be facing Earth. You keep the normal 40 degrees angle of attack, but you'll be pointing at Earth. It's hard to explain in words, but it's not that hard to do... perhaps someone else can explain it better...

you mean, flip upside down, then pitch "up" to 40 degs below flightpath? :blink:

never thought of that - sounds like one hell of a ride :headbang:- now i HAVE to try it! :focus:


i'd imagine it to work well for DG-like vessels, where you're otherwise fighting off lift thoughout most of the descent - gotta try that on the G42 sometime

which reminds me... i should probably sit down and finish up that ship one of these days... it's been a while since i last had a decent chance to make some progress on it....



i wonder how hot an XR1 would become flying like this..... :hmm:

 

[Rtyh-12]

Well, actually, it shouldn't get too hot... the idea is that you slow down much faster, but you don't heat up as much. So you can perform crazier flights without blowing up.

And yes, pitch "up"... so down, relative to Earth... so actually down and up at the same time...

Oh, and if you try this, try making S-turns. It really helps. :thumbup:

 

[Bonanza123d]

last time i tried that, it went horribly wrong. I only had 30 seconds of re-entry before a hull breach in the XR2. Like I said, i will try it. Hopefully i can get it to where i am able to do a aerobrake so i am able to bounce back up into orbit again but my ApA is at 160k.

 

[Rtyh-12]

Well, try to make it just a liiiiiiitle higher and you'll be fine

 

[Linguofreak]

you mean, flip upside down, then pitch "up" to 40 degs below flightpath? :blink:

Well, you've got an AOA of 40 degs, but generally not a *full* 180 degrees of bank. Usually more like 90 + a few.

 

[Hlynkacg]

Are you aero-braking (as in entering from a interplanetary trajectory and using the atmosphere to change planes/velocity) or are you trying to re-enter and land.

If you are trying to land, there are plenty tutorials you can follow. In the xr2 simply adjust your pitch to maintain approx 80ms decent rate and you should be fine. (use S-turns to adjust point of impact)

If you are simply braking or turning you want to keep your decent rate much lower. The exact amount depends on what maneuver you are attempting. Once you're close to your target Inc or speed pitch your aircraft so that you begin to ascend again. you may need to do a correction burn but the Dv required will be much lower than if you had relied solely on your thrusters.

A sweet technique is to do what we call inverted re-entries. Basically you need to be upside-down, that is, the top of your ship should be facing Earth. You keep the normal 40 degrees angle of attack, but you'll be pointing at Earth. It's hard to explain in words, but it's not that hard to do... perhaps someone else can explain it better...

Sounds like fun in game, but I suspect that in real world G-Loading and Aerodynamic Instability would quickly rear their ugly heads.

 

[C3PO]

That method is a lot of fun on Mars. The scenery really flies by.

 

[Cairan]

Sounds like fun in game, but I suspect that in real world G-Loading and Aerodynamic Instability would quickly rear their ugly heads.

Actually no, an inverted (roll angle > 90 degrees left or right) is the same as far as loads are concerned, the difference is the lift vector now works WITH the gravity vector, which helps stay in the atmosphere for aerobraking/aerocapture when the trajectory is hyperbolic (eccentricity > 1)

However, you want to maximize the LIFT you get in the thin atmosphere while you are zooming in at hyperbolic orbital velocities, with time I've found that the best attitude is actually with a 20-ish degree angle of attack, and doing S-turns... The twist is that you have to be rather quick in doing the roll reversals (going from left bank to right bank) because it can cause you to dip too far down in the atmosphere if you are not careful... Once you are captured and your periapsis is well within the atmosphere, you can roll back to a normal attitude and increase your angle of attack to maximize drag...

 

[C3PO]

Sounds like fun in game, but I suspect that in real world G-Loading and Aerodynamic Instability would quickly rear their ugly heads.

The Shuttles can actually use this technique in case there's a problem with the reentry burn, and the angle is too shallow. I can't remember where, but I've read the procedures for it in some NASA document.

 

[Bonanza123d]

this will be fun to pull this on orbiter. Is it possible to do this at a super hyperbolic speed, like a high DV sling like a Neptune>Jupiter>Saturn>Mars>Venus>Mercury>Venus>Earth. I would like to know how to capture the atmosphere there because i will not even have enough fuel for the de-orbit burn because i will be at such a high velocity. i would probably aim for 120k