Right altitude to aerobreak from Minmus to LKO?

[Brainlord Mesomorph]

Trying to get regular "taxi" service from Minmus to LKO. I've been experimenting w/ aerobreak altitudes from 50 to 35 km, but I always either have to make several passes through the atmosphere to get my Ap down to a decent level, or I just go terminal in one pass and end up landing (hopefully)

my goal would be an Ap of around 120km. hopefully in one or two aerobreak passes.

Anyone with ideas?

[Streetwind]

It's a question of precision. Because the atmosphere density grows exponentially, and slowing down more results delaying your rise back out of the atmosphere, the effects of lowering your periapsis even a little are greatly magnified the deeper you go. I'm not entirely sure if you can achieve that level of precision in KSP reliably enough without going through a lot of effort. Because even if you pull it off once through trial and error, the next time round your apoapsis may be slightly different, so the exact same periapsis gives a different result. And keep in mind that just turning your vessel with SAS can make your periapsis change by as much as 100 meters, just because the location of your vessel's center of mass in space changes, which recomputes the orbit.

[Brainlord Mesomorph]

30 minutes ago, Streetwind said:

 

It's a question of precision.

 

That would explain why trail and error isn't working.

Speed at Pe is directly relational to altitude of Ap, right? So if I use the precise same Ap and Pe every time I should get at least similar results. right?

Does mass have anything to do with it? (the ships going to be about 450t)'

[Streetwind]

Mass in relation to cross-sectional area defines the ballistic coefficient, which in turn describes how much a given amount of aerobraking will actually slow you down. So yes, if your spacecraft's mass changes, you need a different PE altitude. Or a different cross-section, but you can't change that one in KSP

Edited July 20, 2016 by Streetwind

[Brainlord Mesomorph]

5 minutes ago, Streetwind said:

Or a different cross-section, but you can't change that one in KSP

use the inflatable heat shield?

but given that its taxi service it will be the same ship and the same mass every time. 

My new 450 tonne "SuperTanker"  regular fuel service from Minmus to LKO

[FancyMouse]

20 minutes ago, Streetwind said:

Or a different cross-section, but you can't change that one in KSP

You actually can, if your ship can enter in different angles (i.e. not falling into some stable orientation, or your SAS/control surfaces are powerful enough). The way I return my spaceplane from Minmus to runway is exactly to control entry angle so that KSC appear at the right place after a few aerobrake passes.

[Streetwind]

Well, reentry-hardened spaceplanes have a little more leeway with orientation during aerobraking than a space-only assembly of rocket fuel tanks, @FancyMouse...

Also, the Brainlord is looking for a single-pass brake that reliably returns the same final apopasis height every time. You'd be hard pressed to succeed countering variations in vessel mass and orbital parameters by manually pushing the flanks into the airstream in this case.

Edited July 20, 2016 by Streetwind

[bewing]

Part of it depends on your piloting. To drop your Ap, the smartest thing you can do during your aerobrake is to be in a high drag configuration. Which may have to do with the attitude that you force the craft to hold. Which means your craft has another low-drag configuration (nose pointing prograde with airbrakes off, perhaps?).

If that's true, then what I do is to watch my Ap. Don't just passively wait until the aerobraking is complete. Wait until the Ap comes down to the "right" number (learned from experience), and then flip your ship into the low-drag configuration.

And then you just have to be able to survive the flip manuever, and that last little bit of the climb back out of the atmosphere.

 

[dire]

450 tons of mass times 3200 meters/second is a lot of newtons of force that your ship has to shed through the mechanism of heating itself and the atmosphere around it. If it was me I would put several(4?) of the inflatable heat shields on it, then drop it through 35km. That should get you into the ballpark.

You might have so much mass and so little cross-section that you are acting somewhat like an armor-piercing bullet, and you're simply not shedding a lot of velocity on each pass. In that case you need even more inflatable heat shields.

[Brainlord Mesomorph]

Sorry for the Necro, but I figured you guy’s would want a final report on this. Which I can sum up in four words:

WELL, THAT DIDN’T WORK.

First the 450 tonne tanker: I’m in a career game, I couldn’t even lift that thing to orbit with the amount of money I have. So I scale it down 75%. I build a 130 tonne “AreoTanker”  Replaceable heat shield on the front. Fins and airbreaks on the back.  I am able to get that to orbit, get it to Minmus and get fueled up.

Now I’m able to start my aerobreaking experiments. Two words:

NO WAY.

As dire predicted, the thing punches through the atmosphere like a bullet.  Even as low as 43km I’m only getting about 100 m/s of dV per pass. And creating literally Hell’s own heat. I was guessing that the replaceable heatshield would last for 5 or 6+ flights, its GONE after the fifth pass!

And then I made a fatal error.  I forgot to redeploy the solar panels after one pass, and by the time I notice,  batteries are gone. So now I have a 130 ton dead bird up there on a decaying orbit!

I scramble to find ANYTHING I have in orbit that can intercept and dock with it and wake it up but nothing has the fuel for it – this has all the fuel I was planning for those ships!

So … the thing comes down in a HUGE fireball, parts of it exploding from 30km up all the way to ground. A big chunk of it is still intact to hit the ground, and even after that a shower of debris continues for several seconds .

So the take away here is that its going to be easier to send ships up with just enough fuel to get to Minmus, than to try to carry fuel from Minmus to LKO.

 

Edited August 5, 2016 by Brainlord Mesomorph

[StrandedonEarth]

Well, you could have simply launched a jumpstart package (batteries, solar panels, probe core, RCS, monoprop, and docking port) to rendezvous with and revive your tanker. While you were doing that, you may have noticed that since you're not focused on your tanker, the orbit is now "on rails" and not decaying. The only time you need to worry about an 'on-rails' vessel is if the periapsis is under ~23km, at which point the game considers it crashed.

Or just use the Alt-F12 cheat meu to enable infinite electricity. Saves a lot of frustration that way. I assume the tanker was uncrewed? A crewman on EVA could pop open a solar panel, although I'm not sure if that needs an engineer these days or not.

If you're playing for max realism, then a crewer or launching a jumpstart pack (unless you have KCT) would be your only hopes

Edited August 6, 2016 by StrandedonEarth

[Brainlord Mesomorph]

Well not MAX realism, but I do like to own my mistakes (esp, if they don't kill Jeb).  I only had a few hours, not logically time to build and launch anything (didn't even think of it)

And thing was that when I discovered the problem the orbit was far too eccentric for any of the dozens of things I have in LKO to intercept it, i know I could leave it up there on rails, (a)  that's cheating  but (B) I thought after another aerobreak pass (or another)  THEN I can intercept.  but no  and then things started getting all exploedy very quickly 

 it was unmanned. 

Edited August 6, 2016 by Brainlord Mesomorph

[Geschosskopf]

On 7/20/2016 at 5:38 AM, Brainlord Mesomorph said:

Anyone with ideas?

Well, when you areobrake or aerocapture, the Pe altitude you need to achieve a certain desired Ap afterwards depends on a number of things:

• The design of the ship.  This includes a number of factors itself:
  • The ship's mass---heavier ships don't slow down as quickly as light ones, due to momentum.  If you're several trips on a tank of gas, this will be different each time.
  • The ship's drag---sleek ships don't slow down as quickly because drag is what slows you down.
  • How much heat the ship can stand before some of it explodes.  To do 1-pass aerobraking, you will need to keep pretty much everything behind a heatshield or you'll lose something.  And this puts a limit on the service life of your taxi.
• The ship's arrival speed, which will vary every time you come back from Minmus, due to Minmus being in a different position in its misshapen orbit.

So at the bottom line, you'll have to do trial-and-error every time.  And if you want to do all the braking in 1 pass, you'll have to dive deep, get VERY hot, and burn ablator very quickly.  You can't "refuel" ablator so once it's gone, it's gone, and you're ship is out of service.

For all these reasons, you might be better served to have your taxi brake on thrust alone.  Sure, that costs fuel, but presumably you need your taxi service because you've got a permanent MInmus base, which probably produces fuel, right?  So just give the ship enough fuel to make a round trip from Minmus to Kerbin and back, and refuel it between trips.

[GoSlash27]

The way I do it:

Ablator in the nose and about 100 m/sec reserve fuel. I shoot for 42km Pe (your mileage may vary). I add throttle to maintain apoapsis as required. If another pass is necessary, I raise Pe accordingly, rinse and repeat.

 I plan on trying replaceable ablators using clamp-o-trons later, but I haven't tried that yet.

Best,
-Slashy