Kerbin Aerobraking

[Das_Sheep]

So I've got a plan for an absurdly sized freight shuttle to ferry either rocket parts, ore, or whatever in-between that gives me the best deal for shipped items from a mining base on Minmus to an orbital assembly facility to Kerbin orbit. Let's just say this thing shall end up less than aerodynamic. What's a good rough altitude to aim at for aerobraking without risking detonating a good few thousand units of expensive stuff? Or is it even worth bothering and just accepting I use a bit more fuel?

[Spricigo]

I think this may be useful https://alterbaron.github.io/ksp_aerocalc/

 

Also,  since you are going to rely on aerobrake that much, it's a good idea to design it to be at least barely aerodynamic stable.  

[bewing]

Aerobraking depends heavily on the mass and average density of your vessel at that moment. A vessel full of ore is going to be very massive and very dense. It will not slow down the tiniest bit unless you go extremely deep in the atmosphere, and then it will blow up. You can only aerobrake light vessels. So it will all depend on the exact cargo you are carrying, and you cannot plan it out in advance. Which means it's basically not worth the effort, I'd say. You can set your Pe to something in the range of 55 to 62km to take advantage of maximum oberth when you do your retroburn at Kerbin, but it is unlikely that you will get more than a few m/s of aerobraking in that case.

 

[Spricigo]

57 minutes ago, bewing said:

Aerobraking depends heavily on the mass and average density of your vessel at that moment.  

Mass and mass/volume... 

Almost correct,  it's dependent in  mass and exposed area.  A vessel full of ore make things difficult indeed but by no means impossible. 

On the other hand, the added area to make aerobrake practical comes with a weight penalty. 

[Aegolius13]

Going from Minmus to Kerbin, it's often fairly convenient to grab a Mun gravity assist to bring your apoapsis down a bit.  It's not a lot of delta-v saved, but unlike aerobraking, there should be no risk of explodey failure. 

[Snark]

On 5/20/2017 at 1:10 PM, Das_Sheep said:

What's a good rough altitude to aim at for aerobraking without risking detonating a good few thousand units of expensive stuff? Or is it even worth bothering and just accepting I use a bit more fuel?

A really heavy, dense ship (i.e. an ore carrier) hitting Kerbin's atmosphere at Minmus-reentry speeds (e.g. 3000 m/s or thereabouts) is going to need heat shields or it's gonna go kaboom.  Nice explanation from @bewing above.

That said, you can totally make this work, as long as you have heat shields.  Note that you don't actually have to have ablator, so you can still have a re-usable ship.  The inflatable heat shields work great for this purpose:  they're huge, so they do a good job of slowing down really massive ship.

Here's an example Minmus-fuel-hauler ship I had a few careers ago:

It's built around the biggest-size 5m liquid fuel tank from SpaceY.  I'm transferring refined fuel rather than ore, but the same principle applies.  This ship can aerobrake just fine, and repeat the cycle ad infinitum.  Totally re-usable.  Note the engines' slight outward angling, so that their exhaust misses the heat shield in the rear.

The ship is nearly 400 tons when carrying a full load of cargo.  It has to dive down to a Pe of only 28 km in order to aerobrake to LKO in a single pass.  Gotta say that plowing 400 tons at 3000 m/s through atmosphere at 28 km gave me the longest aerodynamics-overlay drag arrows I've ever seen. 

Anyway, stick an inflatable heat shield or two onto your ship, and you'll be fine.  The right altitude will totally depend on the mass and shape of your ship, though, so you'll almost certainly need some trial and error to find the right altitude for any given ship.  F5 is your friend.

[Loren Pechtel]

On 5/20/2017 at 5:36 PM, bewing said:

Aerobraking depends heavily on the mass and average density of your vessel at that moment. A vessel full of ore is going to be very massive and very dense. It will not slow down the tiniest bit unless you go extremely deep in the atmosphere, and then it will blow up. You can only aerobrake light vessels. So it will all depend on the exact cargo you are carrying, and you cannot plan it out in advance. Which means it's basically not worth the effort, I'd say. You can set your Pe to something in the range of 55 to 62km to take advantage of maximum oberth when you do your retroburn at Kerbin, but it is unlikely that you will get more than a few m/s of aerobraking in that case.

 

So long as your periapsis is within the atmosphere you will slow down no matter how heavy you are.  Even something without a heat shield can be handled this way, you just have to keep the periapsis high and accept that it will take many passes.

I have spent many an orbit in the upper few hundred meters of the atmosphere collecting flying high science, you keep slowly bleeding energy even that high up.  I'm seriously considering bringing something like an Ant along next time I do this, the tiny burns needed to stay up there are a pain to do.  I've done it with my solar panels deployed, if they can take it anything else you bring along can also.

Edited May 25, 2017 by Loren Pechtel