Surefire way to calculate ablator needs

[rustysocket]

Is there a way to calculate ablator using the mass and speed of the space craft? 

[Guest]

There probably is but you'd need to know your exact reentry angle as well as orbital speed, mass, and drag of the craft.  There are probably a few other variables in the mix as well.  Best thing to do is figure out your general "reentry parking orbit" and then lower your periapsis to the desired braking altitude.  For instance, starting from 80km x 80km orbit, drop the periapsis to 25km, record the amount of ablator needed.  Considering the information that KSP gives you, or severe lack thereof, this is probably the easiest way to figure out the ablator usage.

E: But even that will cause problems because removing ablator lightens the craft.

Edited February 14, 2016 by regex

[Nich]

Also running out of albater is not the end of the world.  With a max temp of 3300 it is one of the highest rated parts in the game.  Is there still an issue with turbulent flow heating?  I remember people upset about aero breaking at jool to high nothing happens 100m lower and ship instant blows up.

 

Unfortunately last I head there were problems with heat model but it is not as simple as 1 abator absorbs 1kw of energy then just 1/2 m*v^2 because most of the heat from deorbiting comes from rapidly compressing the gas that is in your way and not from slowing you down i believe

Edited February 14, 2016 by Nich

[mikegarrison]

40 minutes ago, Nich said:

Also running out of albater is not the end of the world.  With a max temp of 3300 it is one of the highest rated parts in the game.  Is there still an issue with turbulent flow heating?  I remember people upset about aero breaking at jool to high nothing happens 100m lower and ship instant blows up.

That's a different issue. That's just because the edge of the Jool atmosphere should be calculated much farther out from Jool than it is now. So you go from absolutely nothing to very dense as soon as you cross an arbitrary altitude. It would be like if the atmosphere of Kerbin were cut off at 10K. You would get no braking at all and then suddenly hit a dense wall.

[Raptor9]

Meh, if you run out of ablator, I'm sure the Kerbals have cockpit fans mounted inside the capsule like the Russians mount in their helicopters.

[juanml82]

4 hours ago, Nich said:

Also running out of albater is not the end of the world.  With a max temp of 3300 it is one of the highest rated parts in the game.  Is there still an issue with turbulent flow heating?  I remember people upset about aero breaking at jool to high nothing happens 100m lower and ship instant blows up.

 

Unfortunately last I head there were problems with heat model but it is not as simple as 1 abator absorbs 1kw of energy then just 1/2 m*v^2 because most of the heat from deorbiting comes from rapidly compressing the gas that is in your way and not from slowing you down i believe

Well, I've dropped a rover on Eve using the MK4 mod cargo bays to protect it and I needed two 3.75 meters heat shields, one in front of the other, because that ship ran out of ablator in the first shield during descend. And when the heat shield run out of ablator, it exploded.

On Kerbin, I tried to aerocapture an MK3 stack coming from Moho - highly solar eliptical orbit, Pe at Moho's orbit, Ap between Kerbin and Duna - by setting a 25km Pe in Kerbin and it only consumed half the ablator, but the heatshield had a skin temperature above 3100 °K. In any case, it didn't burn (and I didn't aerocapture , will try a second aerobreak run)

Edited February 15, 2016 by juanml82

[wibou7]

4 hours ago, Raptor9 said:

Meh, if you run out of ablator, I'm sure the Kerbals have cockpit fans mounted inside the capsule like the Russians mount in their helicopters.

Don't mock the russian, they have space shotgun.

SPACE. SHOTGUN.

 

(ref:     https://en.m.wikipedia.org/wiki/TP-82

[Raptor9]

8 minutes ago, wibou7 said:

Don't mock the russian, they have space shotgun.

SPACE. SHOTGUN.

No mocking here.   Another interesting fact: the Soviet space station "Salyut 3" was armed with a 23mm cannon.  But I digress from the original thread topic.

[magnemoe]

7 hours ago, Raptor9 said:

Meh, if you run out of ablator, I'm sure the Kerbals have cockpit fans mounted inside the capsule like the Russians mount in their helicopters.

What? Russian helicopters has the big fan on top like everybody else.
An fan inside an capsule makes plenty of sense to circulate air. Yes you still need air recycling 

[Foxster]

My rule of thumb is to set it to 25% of max. Tends to be about right. Might be a little too much, which isn't a problem or if it's too little then it still tends to be enough to get through the hottest bit. 

If I'm really saving weight and want to get it spot-on then I hyper-edit the craft to orbit and try it. 

 

Edited February 15, 2016 by Foxster

[Dr.K Kerbal]

Probably something like...

Mass of craft divided or times by the speed of the spacecraft and divided or times by the amount of ablation in the spacecraft.

I'm the smartest in class but I'm no Einstein. I'm just a slightly above average grade fiver.

You can't blame me. And I would like to see someone try and do so.

[swjr-swis]

On 2/15/2016 at 9:48 AM, Dr.K Kerbal said:

You can't blame me. And I would like to see someone try and do so.

Well, you asked: I blame you. So there.

[Dr.K Kerbal]

Fair enough.

[Nich]

Here is a good post from one of the developers about shock heating being based off v^3

 

 

[NathanKell]

You need 100% of the ablator on a 2.5m shield to save a Mk1-2 pod and parachute hitting Eve's atmosphere at ~5.5km/sec.

You need 5% or so of the ablator on ditto to save a ditto hitting Kerbin's atmosphere at ~3000 to ~3500m/sec.

If your vessel is heavier for its drag area than that, you'll need more. If it's lighter, you'll need less.

[numerobis]

I've found that the fixed radiators are a lighter-weight replacement for ablation around Kerbin. I wonder how much ablator you could save on that Eve approach if you stuck a couple small radiators on the back of the pod?

[hellblazer]

6 hours ago, NathanKell said:

You need 100% of the ablator on a 2.5m shield to save a Mk1-2 pod and parachute hitting Eve's atmosphere at ~5.5km/sec.

You need 5% or so of the ablator on ditto to save a ditto hitting Kerbin's atmosphere at ~3000 to ~3500km/sec.

If your vessel is heavier for its drag area than that, you'll need more. If it's lighter, you'll need less.

~3000 to ~3500km/sec is pretty fast. I'm sure you meant 3km - 3.5km/sec 

Edited February 18, 2016 by hellblazer

[Spheniscine]

One thing that somewhat surprised me is that sometimes a more aggressive aerobrake is better than a less aggressive one, ablator-wise. (Too aggressive though, and you may plummet to the ground before you can slow down enough to deploy parachutes)

[JoeNapalm]

11 hours ago, hellblazer said:

~3000 to ~3500km/sec is pretty fast. I'm sure you meant 3km - 3.5km/sec 

And yet another spacecraft is lost due a floating point error.

 

-Jn-

[NathanKell]

@hellblazer thanks. :]

[Jonfliesgoats]

I'd like to get more accurate with reentry trajectories so I don't have to make maneuvering burns or fly lifting bodies around.  If we can make accurate MIRVs IRL, we should be able to do so in KSP.  Has anyone run the formulae for maximum dynamic heating and compared them to observations in KSP?