Aerobraking from 7Km/s to 5Km/s?

[Kulebron]

I have BTSM mod installed (which includes Deadly Reentry), and I need to fly to Gilly back and forth quickly (to be within limits of life support and also of delta-v). So make a fast transfer to Eve, in 17 days, and consequently when I enter Eve SOI, velocity relative to Eve is already 5080 m/s. As I calculated, it's about 13 km/s 7 km/s at periapsis. KSP aerocalc suggests aerobraking at 55 km.

I have the biggest heat shield, which if left alone heats up quickly and explodes, exposing the rest of the ship to heat. Then I tried another 2 sacrifice heat shields, but they exploded quickly too and left the main one to still explode.

I tried doing less deep reentry, and although it saved the ship, it still escaped SOI, flying 3 km/s faster than needed.

Question: how many heat shields you need to aerobrake this much?

Edited August 16, 2014 by Kulebron

[Dkmdlb]

I would suggest bringing more life support. Getting to Gilly quickly isn't saving you any delta-v. If you are transferring to Eve in 17 days, I would instead recommend leaving some fuel behind and bringing more life support. It will take longer to do a low-energy transfer, but it will also take less delta-v.

In addition, spamming heat shields is probably a sign that something is wrong. If you take the slower, lower delta-v transfer, your velocity won't be so high and you should be fine with one heat shield.

[1of6Billion]

I don't have Deadly re-entry, so forgive me if I talk nonsense here.

Can't you areobrake in 2 or even more sessions? So, dive in deep enough to get your apoapsis just inside Eve's SOI. Use next pass to lower it etc.

[Sirine]

I don't have Deadly re-entry, so forgive me if I talk nonsense here.

Can't you areobrake in 2 or even more sessions? So, dive in deep enough to get your apoapsis just inside Eve's SOI. Use next pass to lower it etc.

I believe it is the life support that killing his crew. He only having 1 chance. I presume.

[StuntMax]

My call with BTSM is: a fair load of life support canisters, an insanely large solar array (my first duna mission had like 40 panels - melius abundare quam deficiere ), and take all the time it needs to.

And carry no unneeded kerbals... one for the mothership and one for the lander, the third seat on the capsule is good to store one more bag of snacks. these guys eat life support like sharks.

[Kasuha]

Engine nozzles have fair resistance to reentry heat. If you build your ship shielded from the back, you can use your engine as part of the shield and you can even burn during the reentry to slow you down further.

[Streetwind]

Question: how many heat shields you need to aerobrake this much?

Depends on the heatshield in question. However, bleeding off 6 km/s in Eve's atmosphere is Not Going To Happen with expendable heatshields - capitalized for emphasis. BTSM further tweaks Deadly Reentry's settings to be more aggressive, and thus, Eve's atmosphere is more akin to a spaceship eating shark than a pillow to fall into. Example: After it finished all of its duties, I put an unshielded probe into a 91x91km orbit and then lowered the periapsis to 86km for a super gentle deorbit brake, engine first of course. It did not live to reach 60km.

Mind you, the 6.25m heatshield (yes, that's six meters) at the end of the tech tree isn't expendable (has no ablative shielding resource). It should survive almost any aerobraking load, repeatedly, without failing. Of course, by that point the integrity of your spaceship becomes irrelevant, because the g-forces during aerobraking reduce your Kerbals to a thin film of green goo on the floor of their capsule.

[Lukaszenko]

I don't have Deadly re-entry, so forgive me if I talk nonsense here.

Can't you areobrake in 2 or even more sessions? So, dive in deep enough to get your apoapsis just inside Eve's SOI. Use next pass to lower it etc.

This is the part that OP is having problems with. I had the same problem as well. When going 5 km/s, diving deep enough not to fly out of the SOI WILL destroy your heat shield.

One solution I found was using the inflatable heat shield, as it seems indestructible (although feels cheaty). But even then, it becomes a fine balance between going deep enough and not getting destroyed by the g-forces. Streetwind, is that the 6.25 m shield you were talking about?

I also don't know, but in real life do they rely only on aerocapture when going into orbit around other planets, or do they combine it with firing the engines? Seems like a highly risky and variable maneuver to pull off with only the air. I understand that when lowering an orbit using aerobraking, they make many many passes over a span of months.

EDIT: According to Wikipedia, an aerocapture has never been performed in real life. So if it seems difficult in KSP, now you know why

Edited August 13, 2014 by Lukaszenko

[Sirrobert]

Try beeing less deep in the atmosphere, and also burning retrograde?

[Streetwind]

Yes, that is the one I meant. Mind you, I've never tried slamming myself into Eve at 13 km/s to test how sturdy it is, but if I have even the slightest clue about how Deadly Reentry works, then that should be your best bet.

As far as real life goes - aerobraking is used very carefully and very sparingly, generally to lower an existing orbit. In the first place, RL spacecraft don't travel fast enough to really need more than a small burn to capture into orbit, due to the huge amount of dV required to do an accelerated transfer, which drives up the cost of the spacecraft. As such, a Mars probe for example can propulsively capture into a highly elliptical orbit and then use aerobraking over many days to slowly lower its apoapsis to where it needs to be. However, even then it sometimes is not sensible to aerobrake, because the craft may be too fragile, or because the maneuver requires constant human attention - and that means buying a lot of expensive signal time on the DSN. Not to mention having your ground crew at its stations for a pass every couple hours for days on end.

[Godot]

If you use a crewed vehicle it sounds to me you have a problem no matter what you do ... aerobraking at a too high speed will kill your crew nevertheless, as they are exposed to too high G-Forces

[cantab]

I don't have Deadly re-entry, so forgive me if I talk nonsense here.

Can't you areobrake in 2 or even more sessions? So, dive in deep enough to get your apoapsis just inside Eve's SOI. Use next pass to lower it etc.

Capturing into highly elliptical Eve orbit will require around 1300 m/s less than capturing into low orbit, but when you're coming in from an "express" transfer you need to shed a lot of speed either way.

For the OP, if you're throwing enough delta-V at the problem to make a 17 day transfer, you may as well throw more to do a power-assisted capture, burning hard retrograde just before entering the atmosphere.

[Kulebron]

To answer why I tried to do a fast transfer, there's a relatively small return transfer window, so I planned an early departure, and about 3-5 days for stay (mostly maneuvering to get to Gilly). It was also a convenient travel with _just_ 3 orange tanks. Total DV required was about 1700 + 1700, and about 1000 to get to Gilly orbit. If someone shows me a better transfer combo, will be very grateful.

I did think BTSM use stock DR settings. What a mistake!

this is the first version, with the heatshield in question. I added another 2 smaller heatshields after this. I guess I'll need that inflatable one.

[MarvinKitFox]

You are better off spending mass on more life support + recyclers, and less on heatshields.

After a lot of fiddling around, i realised that an engine, with **lots** of small parts behind it, bleeds off heat better than any but the inflatable heatshield.

If you are very close to the burning point, rotate! you spread the heat better.

Also: Consider *not* jettisoning that empty propulsive stage. Between its engines, and several tanks, couplers & whatnot, an expired lower stage make for *great* ablative armor!

[AlexisBV]

According to Wikipedia, an aerocapture has never been performed in real life. So if it seems difficult in KSP, now you know why

LIES! It has been done, and is documented here:

[Matokage]

LIES! It has been done, and is documented here:

Well, it didn't happen, it's from a BBC program called Space Odyssey: Voyage to the Planets, here is the making of: https://www.youtube.com/watch?v=cl9nxG9C2dg

[Mr Shifty]

EDIT: According to Wikipedia, an aerocapture has never been performed in real life. So if it seems difficult in KSP, now you know why

The Galileo probe did a successful aerocapture, not to orbit, but to a parachute descent into the Jovian atmosphere. It went from 47 km/s Jupiter-relative to subsonic in about 2 minutes, experiencing about 230g and losing more than half of its heat-shield mass during the braking.

[Sirrobert]

LIES! It has been done, and is documented here:

Aaa yes, I remember where I was when humans walked on Jupiter

[MarvinKitFox]

an aerocapture has never been performed in real life. So if it seems difficult in KSP, now you know why

No aerocapture has ever been done, and none is planned that I know of.

(As in aerobrake from interplanetary transfer speeds to orbit, not simply slamming in and falling in the atmosphere)

We have done plenty of aerobrake maneuvers though, almost all of the MARS-something probes employed aerobraking.

p.s.

Airbreaking by 6Km-s

air BREAKING? That is easy.

it's the aero-braking-but-non-breaking bit that is hard.

Edited August 14, 2014 by MarvinKitFox

[Lukaszenko]

No aerocapture has ever been done, and none is planned that I know of.

(As in aerobrake from interplanetary transfer speeds to orbit, not simply slamming in and falling in the atmosphere)

Yeah, that's what I gathered. I'm still confused on the term though. Is "aerocapture" an aerobrake and only an aerobrake to orbit from interplanetary speeds? As in, if you aerobrake while firing an engine, it is no longer an aerocapture?

I'm asking because of that Mars probe that got destroyed due to the wrong units...I understood it went to deep into the atmosphere while attempting to do some sort of aerocapture maneuver, no? Or was it supposed to avoid the atmosphere altogether?

[MarvinKitFox]

I'm still confused on the term though. Is "aerocapture" an aerobrake and only an aerobrake to orbit from interplanetary speeds? As in, if you aerobrake while firing an engine, it is no longer an aerocapture?

Aerocapture:

Using an aerobrake event to transition from hyperbolic orbit to "captured" orbit around the target body.

Any amount of aerobraking would qualify, but it is usually understood as "the vastly greater part" of braking is done by aerobrake.

Some thrust is always required, of course, to raise periapse out of the atmosphere again.

And given the ridiculous degree of trajectory accuracy required, it is expected that the vessel will be under constant positive control all the way through the maneuver.

In KSP we have a nice cheaty universe, with the atmosphere standing still(no wind), and dissipating in an exactly regular way, AND a clean clear cut-off altitude.

This makes aerobraking and aerocapture maneuvers predictable.

In real world, Mars's nominal high altitude atmosphere changes in density by !!!two whole orders of magnitude!!! between seasons, solar flares, and comets.

(this year's comet C/2013-A1 is expected to increase the orbital drag on the orbiting Mars Reconaisance Orbiter by a factor of 25-30 times normal, simply by very lightly brushing the atmosphere with its cometary coma.....)

Predicting the drag for an aerocapture under such circumstances is..... problematic.

[von Ziegendorf]

... when I enter Eve SOI, velocity relative to Eve is already 5080 m/s. As I calculated, it's about 13 km/s at periapsis.

How did you get such result? I calculated it would be only 6950m/s

[Kulebron]

How did you get such result? I calculated it would be only 6950m/s

I used this formula:

μ/r₁ + v₁²/2 = μ/r₂ + v₂²/2

And it transfrms into

v_p² = μ(2/r_p - 1/a) = μ(2/r_p - (2μ - rv²)/μr) = v² + μ(2/r_p - 2/r)

The numbers were:

μ = 8.1717302e12

r = 8e7 (80Mm, current altitute)

v = 5080 (current orbital velocity)

r_p = 1e5

In [3]: (5080**2 + 8.1717e12 * (2 / 1e5 - 2 / 8e7)) ** .5
Out[3]: 13749.040239231246

Verifying these numbers, I got where the mistake was: I forgot planet's own radius. I add 7e5 and everything is correct:

In [8]: (5080**2 + 8.1717e12 * (2. / 8e5 - 2. / 8.5e7)) ** .5
Out[8]: 6785.526855438888

Edited August 15, 2014 by Kulebron

[Kulebron]

In real world, Mars's nominal high altitude atmosphere changes in density by !!!two whole orders of magnitude!!! between seasons, solar flares, and comets.

(this year's comet C/2013-A1 is expected to increase the orbital drag on the orbiting Mars Reconaisance Orbiter by a factor of 25-30 times normal, simply by very lightly brushing the atmosphere with its cometary coma.....)

Predicting the drag for an aerocapture under such circumstances is..... problematic.

But this has to deal with upper atmosphere, little amount of air and tiny drag, like orbital decay (some metres per second enough to dive very deep), but does this affect interplanetary capture that much?

[Kulebron]

air BREAKING? That is easy.

it's the aero-braking-but-non-breaking bit that is hard.

Lol. When do we start writing Euroenglish to avoid such confusions?