Instant Anihilation when entering Jool atmosphere at high speed with heatshield

[IncredibleJeb]

The craft just heats up so fast that it gets destroyed after a few second in the atmosphere. I cant get below the first km without it happening.

In other words even when its just breazing the atmosphere it happens. I have the largest heatshild available at almost max ablator.

the ablator gets ignored almost entirely and the craft just explodes and the heatshield with it.

the same thing goes for laythe, as i tryed to aerobrake there as well.

Is there something im doing wrong?

Edited August 6, 2015 by IncredibleJeb

[selfish_meme]

Not looking at the many threads before yours.

[cantab]

The problem is partly down to very high speeds and partly to the atmospheres having "sharp edges".

[Reactordrone]

Despite the near perfect vacuum in the upper atmospheres a high entry speed will give near instant death regardless of any heat shielding with the way the heat system is currently. Try gravity braking with Tylo at Jool and be prepared to not have aero capture as an option for places other than Duna unless you drop the heating to get more realistic levels.

[OhioBob]

Below is a re-post from another thread in which this was discussed.

I did some experiments to see if it were possible to do a Jool aerocapture under v1.04. I found that it is possible under just the right conditions, but barely. The entry corridor is so narrow that if you miss the correct periapsis altitude by just a couple hundred meters you will either blow up or fail to capture. The design of the spacecraft is also very important. The spacecraft requires a large ablator and a low ballistic coefficient, i.e. low mass per cross-sectional area. A low ballistic coefficient means that the spacecraft will decelerate more quickly for every unit of dynamic pressure produced. This allows aerocapture to occur higher in the atmosphere and with less aerodynamic heating.

The vehicle I used in my tests had a mass at atmospheric entry of only 2915 kg and was equipped with a 2.5 m heat shield. This means its mass was less than 600 kg per square meter of ablator surface. I don't think that anything much higher than this would have a very good chance of survival. My velocity at atmospheric entry (200 km altitude) was 9783 m/s, which is about typical for a Kerbin-to-Jool transfer. For this vehicle and conditions, I found that the ideal periapsis was about 196,200 m ±200 m. On one attempt I came in with a periapsus a little below 196,000 m and just barely made it, having burned off all but 3 kg of my ablator (a few meters lower and I probably would have been destroyed). On another attempt I came in with a periapsis just a little below 196,500 m and failed to aerocapture (I actually got my eccentricity below 1 but was still on an escape trajcetory due to the patched conics).

Although, in this example, a periapsis altitude >196.5 km would not result in an aerocapture, the vehicle would be significantly slowed down to the point that just a small burn would be necessary to finish off the capture. This might be a good strategy to use - a combination of aerobraking and propulsive braking. Obviously the higher the periapsis altitude, the less the heating and the greater your chance of survival.

I've proven that small probes can survive a Jool aerocapture with proper design and targeting. Unfortunately, I think that large massive vehicles are a greater challenge. The need for a low ballistic coefficient means that a large craft would have to be equipped with an unwieldy number of heat shields. For Jool aerocapture you want to use a "pancake" design.

[X-SR71]

Save yourself a lot of quickloading and forget jool aerobraking.

[xtoro]

It's possible. I go between 198,500 and 199,000 and end up with just under half the shield intact. However, it's much easier to just swing by Tylo or aerobrake on Laythe.

[A_name]

Why would aerobraking on Laythe be less dangerous than on Jool?

[xtoro]

Why would aerobraking on Laythe be less dangerous than on Jool?

Because laythe's atmosphere isn't as dense and doesn't have a sharp cutoff like Jool does. Jool's atmosphere is like a brick wall. Also, if you can manage to swing ahead of Jool before hitting Laythe, its gravity can slow you down.

[Anglave]

Because laythe's atmosphere isn't as dense and doesn't have a sharp cutoff like Jool does. Jool's atmosphere is like a brick wall. Also, if you can manage to swing ahead of Jool before hitting Laythe, its gravity can slow you down.

Also, if you come in "behind" Laythe in its orbit, your velocity relative to it can be much lower, compared to your velocity relative to Jool when you've fallen all the way down its well to touch its atmosphere.

[A_name]

Also, if you come in "behind" Laythe in its orbit, your velocity relative to it can be much lower, compared to your velocity relative to Jool when you've fallen all the way down its well to touch its atmosphere.

Wouldn't coming in from behind Laythe give you an unwanted gravity boost?

[xtoro]

Wouldn't coming in from behind Laythe give you an unwanted gravity boost?

I think his intent is the comparison to Jool. However, I can also understand that while aerobraking behind Laythe, its gravity is also helping by pulling you radially in the direction that the planet is travelling, which is the path you want to take to achieve orbit around both Laythe and Jool.

[A_name]

I think his intent is the comparison to Jool. However, I can also understand that while aerobraking behind Laythe, its gravity is also helping by pulling you radially in the direction that the planet is travelling, which is the path you want to take to achieve orbit around both Laythe and Jool.

Thanks for your replies xtoro. A follow up question: Would it be more efficient, then, to come in behind or in front Laythe?

[xtoro]

Thanks for your replies xtoro. A follow up question: Would it be more efficient, then, to come in behind or in front Laythe?

I think that depends on your intended destination and if you're going to aerobrake on Laythe or not. If aerobraking and headed for an orbit in the Jool system, come in behind.

If not aerobraking and looking for gravity braking to hit up Jool or another one of its moons other than Laythe, then come in ahead of Laythe.

If trying to slingshot using a gravity assist to reach another planet further out and not aerobraking, come in behind.

All sounds legit to me, but it would be nice if a guru could chime in to confirm because I've only started using these tricks a few months ago.

[OhioBob]

For typical entry into Jool's atmosphere, a spacecraft is probably traveling about 9800 m/s. For that same spacecraft swinging by Jool out at Laythe's orbit, it will be traveling only 5068 m/s. Laythe orbits Jool at a velocity of 3224 m/s. Therefore, if the spacecraft approaches Laythe from behind, the relative velocity is 5068 - 3224 = 1844 m/s. If it approaches Laythe head on, the relative velocity is 5068 + 3224 = 8292 m/s. We then have to account for the fact that Laythe's gravity will pull the spacecraft in and speed it up before it hits the atmosphere. In the minimum case (approach from behind), the spacecraft will enter the atmosphere at 3246 m/s. In the worst case (a head on approach), the spacecraft will enter the atmosphere at 8712 m/s. So even in the worse case scenario, the entry velocity is still nearly 1100 m/s slower than an entry into Jool's atmosphere. In the best case the velocity is about the same as a spacecraft returning to Kerbin from Mun.

[Edax]

For typical entry into Jool's atmosphere, a spacecraft is probably traveling about 9800 m/s. For that same spacecraft swinging by Jool out at Laythe's orbit, it will be traveling only 5068 m/s. Laythe orbits Jool at a velocity of 3224 m/s. Therefore, if the spacecraft approaches Laythe from behind, the relative velocity is 5068 - 3224 = 1844 m/s. If it approaches Laythe head on, the relative velocity is 5068 + 3224 = 8292 m/s. We then have to account for the fact that Laythe's gravity will pull the spacecraft in and speed it up before it hits the atmosphere. In the minimum case (approach from behind), the spacecraft will enter the atmosphere at 3246 m/s. In the worst case (a head on approach), the spacecraft will enter the atmosphere at 8712 m/s. So even in the worse case scenario, the entry velocity is still nearly 1100 m/s slower than an entry into Jool's atmosphere. In the best case the velocity is about the same as a spacecraft returning to Kerbin from Mun.

If the destination is Bop, is it still worth it to aerobrake at Laythe? Or is it better to just fly towards Bop before reaching Jool's sphere of influence?

[OhioBob]

If the destination is Bop, is it still worth it to aerobrake at Laythe? Or is it better to just fly towards Bop before reaching Jool's sphere of influence?

That's hard to answer because Bop's orbit is so much different than Laythe's. Using Laythe to aerobrake will get you into a Jool orbit, but you'll then have to perform a series of burns to get the Bop. Bop's orbit is also 15 degrees out of plane with Laythe, so that doesn't help. By the time you do everything necessary to get a good intercept and orbit insertion at Bop, I'm not sure you've saved all that much by doing the aerocapture.

I've only been to Bop a couple times. The method I used was chosen because I thought it was easiest, not necessarily the most efficient. I just used engines to brake into a circular orbit out near Bop's apoapsis, incorporating any necessary plane change. This would take a ÃŽâ€v of about 1500 m/s. From there I would drop my periapsis at the appropriate time to approximately match Bop's orbit. I would then just wait for an opportunity to create an intercept, much as I would if I were rendezvousing with another ship. All of that would take maybe another 300-400 m/s.

[IncredibleJeb]

Okay, thanks guys. Ill try to use Laythe instead and try to minimize entry speed.