Aerobraking to Orbit

[Tibrogargan]

Short version: Is aerobraking when coming from a higher orbit in order to achieve an orbit going to save fuel (a worthwhile amount) , even though you need to accelerate out of the atmosphere?

Long version: I have a ship that I designed to take Kerbals from a station in low Kerbin orbit to the moons in order to participate in flag placing ceremonies.  The ship is designed around the 10m heat shield - it pretty much just slams itself into the atmosphere in order to dump momentum and adjust afterwards to circularize / rendezvous with the station again.  There's also a spaceplane to get the Kerbals into orbit in the first place.

It struck me that maybe I'm not really saving any fuel by doing this, since I need to accelerate back out of the atmosphere at the end - and maybe I could just re-design the spaceplane so it can make it to the moons.  How can I figure out if I'm actually saving non trivial amounts of fuel?

Edited November 21, 2018 by Tibrogargan

[Aegolius13]

To answer your first question - yes, aerobraking is absolutely a worthwhile way to lower your orbit.   It will always take a little bit of fuel to to raise your periapsis back out of the atmosphere, but this is usually much, much smaller than the amount it would take to lower your orbit without aerobraking. 

However, I'm a bit confused about the "accelerate out of the atmosphere" part.  You should NOT need to burn engines while in the atmosphere -- that kinda defeats the purpose of aerobraking, which is to bleed off speed.  You will need to do that final burn to finish into the target orbit, but that burn always happens in space.  

Perhaps the issue is that you're dipping too low in the atmosphere, which is bleeding off more speed than you want, which is making you burn to compensate. That 10m heat shield generates orders of magnitude more drag than smaller parts. I would suggest trying the aerobrake up higher in the atmosphere, and seeing where that gets you.  There's also nothing wrong with doing the brake over a couple orbits (and that's sometimes necessary to avoid burning up). 

 

[Tibrogargan]

5 minutes ago, Aegolius13 said:

However, I'm a bit confused about the "accelerate out of the atmosphere" part.  You should NOT need to burn engines while in the atmosphere -- that kinda defeats the purpose of aerobraking, which is to bleed off speed.  You will need to do that final burn to finish into the target orbit, but that burn always happens in space.  

Sorry for the confusion, I was paraphrasing.  At some point in time a burn needs to happen to get the periapsis back out of the atmosphere, hence "accelerate out of the atmosphere" - but this makes me wonder if it's worth it, since "accelerating out of the atmosphere" is often the most expensive part of any flight from Kerbin.

[OhioBob]

12 minutes ago, Tibrogargan said:

… but this makes me wonder if it's worth it, since "accelerating out of the atmosphere" is often the most expensive part of any flight from Kerbin.

You're already in orbit, albeit an elliptical one with a periapsis inside the atmosphere.  Just raising the periapsis out of the atmosphere doesn't take all that much delta-v.  For instance, let's say that after aerobraking you are in a 50 x 200 km orbit.  Burning at apoapsis to circularize into a 200 km orbit takes only 112 m/s.  Even less if your apoapsis is lower.  You're probably saving close to 700 m/s by aerobraking.
 

Edited November 21, 2018 by OhioBob

[bewing]

1 hour ago, Tibrogargan said:

and maybe I could just re-design the spaceplane so it can make it to the moons.

I think this might easily end up being your best choice overall. The nice thing about aerobraking with a spaceplane is the immense difference in drag between prograde-lock and "trying to bleed off velocity" attitude. What that ends up meaning is that you can often fine-tune the aerobraking maneuver to bring your Ap very very close to your final number. Then at your Ap, burn just enough so that you meet your space station coming around on the next orbit.

[overkill13]

Your aerobrake should never leave you with a periapsis near the ground, otherwise you are reentering and landing and while landing does require aerobraking as part of reentry, aerobraking to an orbit does not require landing.

The reason flight from the ground to orbit requires so much delta V/fuel is because you start on the ground, stationary and have to fight through air resistance to get up to speed.  When aerobraking you will never be stationary.  Any reasonable orbit is going to have an average velocity in the 2000m/s+ range which is delta V/fuel that you don't need to burn again after aerobraking.

As has been mentioned, 100 or so delta V to bring your periapsis back up to a stable orbit is far less than 700 ish to bring your apoapsis down to LKO from the Mun's orbit.

Edited November 22, 2018 by overkill13