Aerocapture in 1.0.4?

[Jouni]

Pack a lot of extra fuel on your interplanetary trips so you can slow down without touching the atmosphere, both at the destination and back at Kerbin.

The amount of extra fuel isn't that high. Eve arrival is probably the worst, because you're often coming in with a heavy ship at a high speed. Aerobraking at Duna and Laythe happens at a lower speed than when returning from the Mun. Returning to Kerbin is quite safe, because the ship usually has almost empty fuel tanks, so it should slow down enough for an aerocapture without diving too deep into the atmosphere. Aerobraking at Jool is just a bad idea, when you have Laythe and Tylo at your disposal.

[Nefrums]

I just tried to arobrake at Eve. It did not work. I could barely touch the atmosphere before my ship blew up. I was coming in at about 5km/s. If i went shallow enough to survive, somewhere around 87km, my ship only lost ~30m/s.

There is a huge difference between coming in with 5km/s and the ~3km/s you get from a 100km orbit. My lander survives to the surface when braking to a 100/0 km "orbit".

[Weywot8]

Was looking around to check out aerobreaking/capture strategies for Eve under the new drag mechanics - surprised there were problems reentering Kerbin from Mun/Minimus.

Started as an engine placement/staging design quirk but found out that a wide fuel tank works to protect the craft on fast & hot re-entry. Jeb and Phoy (Jeb only rescues Kerbals with quirky names, the money was good too!) walked away unscathed.

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Note

• initial periapsis at ~30km but ended up on the surface
  
• "free-fall" return from 10,000km apoapsis orbit
  
• lost some energy before the pic (70km-34km), probably had a shallow reentry angle?
  
• parachutes on top of side tanks didn't burn up - thought those were gonners when things started glowing
  

Hope this helps anyone looking to save on heatshield weight while getting back direct-to-Kerbin in one piece.

[Signo]

I just tried to arobrake at Eve. It did not work. I could barely touch the atmosphere before my ship blew up. I was coming in at about 5km/s. If i went shallow enough to survive, somewhere around 87km, my ship only lost ~30m/s.

There is a huge difference between coming in with 5km/s and the ~3km/s you get from a 100km orbit. My lander survives to the surface when braking to a 100/0 km "orbit".

You shoud try with the best available transfer window and with a polar insertion.

The transfer window will get you there at an approaching speed of less than 4000m/s and with a polar insertion you should encounter less atmospheric drag (not so much, but everything goes there).

I never found a way to land without engines burning almost full throttle to avoid "too low too fast" turbulent convection.

[ringerc]

Weirdly I did the same with this ship

http://i.imgur.com/Pt2NWEP.png

It had this tug http://i.imgur.com/famdnBZ.png

on the back, it went almost well, did 195 km came in with nose and heatshields first. first time the atmospheric messurment instrumets blew up so I moved them inside containers with KIS and tried again,

much better except that the ship started to tumble shortly after Pe, and ended up going engines first, a couple of ladders blew up else ship was unhurt.

That'll work on 1.0.2, but be instantly crisped on 1.0.4 I suspect.

[OhioBob]

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.

Edited July 17, 2015 by OhioBob

[Rodyle]

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 drag 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.

Do you think this would've be improved in any useful way by using a reverse gravity assist using one (or more) of Jool's moons, or would it not change speed at periapsis significantly?

[SanderB]

If you dont come in with more than 1km/s encounter velocity (ie. your orbital velocity at SOI change to jool's SOI) you should be able to go around tylo and/or laythe. For that matter, you could probably use Laythe to aerobrake better than jool and at least capture into Jool's SOI.

[NathanKell]

I've done it with a Mk1-2 and 2.5m shield for the record, though I was not going much over escape at the time and didn't slow all that much. As OhioBob says, low BC is great, a 3.75m shield on a light 2.5m stack would probably work too.

[OhioBob]

Do you think this would've be improved in any useful way by using a reverse gravity assist using one (or more) of Jool's moons, or would it not change speed at periapsis significantly?

Unless you've already been captured into a closed elliptical orbit, the velocity at atmospheric entry can never be less than escape velocity, which at the top of Jool's atmosphere is 9547 m/s. That's an improvment but I seriously doubt it will improve the rapid overheating problem in any significant way.

I agree with those that say using Laythe to aerobrake would be a more survivable option. If the timing is such that the spacecraft can approach Lathye from behind (i.e. both moving the same direction) than I think the entry velocity could be as little as 3000-4000 m/s. Even a head on encounter with Laythe will likely produce an entry velocity less than an encounter with Jool.

Edited July 16, 2015 by OhioBob
grammar