Design Problem

[Zapo147]

First off, it's not my first rodeo in KSP. However, I found that one of my greatest challenges for interplanetary missions is designing a ship that is aerodynamically stable while performing an aerocapture maneuver. The primary reason for this is that rockets are naturally bottom-heavy, especially when launched from KSC. So while one can easily place an inflatable heat shield between the launch abort system and the command module, the center of mass will be so far back that the ship will tend to flip unless you add a ton of drag to the back end, in the form of literally tons of fins or more creative means.

That was my first instinct, and I ended up with various semi-successful designs including turbine-like fins and infernal robotics deployed drag-inducers (heat shields). This solution isn't consistent, so I started designing ships to aerobrake engine-side first. Designs, using KIS/KAS, included using winches to haul a massive heat shield in front of the engines, assembling a heat shield in situ, or robotic arms moving radial shields into a heat shield snow plow design. Then came my most successful design concept - separation. I started designing ships to separate into two equally massive halves and aerocapture separately. Then would then rendezvous and rejoin after they left the atmosphere. The two halves were always much more stable than attempting to aero break with my typical burj khalifa looking design.

My question is: what have been your successful solutions? I have a sneaking suspicion that there is a more elegant one out there.

[Foxster]

If I understand you correctly, you want to get a largish craft through aerobraking without the flipping?

The design ideas around this are:

1. Get the centre of mass close to the heatshield. This generally means the engines and larger tanks just behind the deployed heatshield. 

2. Get a bunch of drag at the top of the craft. One of the lightest and most effective ways is with a lot of (often disposable) air brakes. You might need quite a few, maybe a couple of dozen. 

3. If you still struggle then you can consider an inflatable heatshield at both ends. The one at the retrograde end acting like a big parachute.  

[A_name]

I usually go for dual inflatables, top and bottom. Sometimes integrated from the launch, sometimes attached with standard docking ports. Enter heavy side first. Consistently reliable and reasonably elegant too. 

[Aegolius13]

You could try to radially mount your engines near the front of the ship.  This also lets you easily detach spent fuel stages from the back of your skip,  without needing new engines for each stage.   I like to do this with nukes for that reason,  even when aerobraking is not an issue.  As a side benefit, having your engines mostly pull ratherthan push diminishes wobbling. 

[Vanamonde]

I can't vouch for its elegance, but with engines up front and heat shields toward the back, my interplanetary cargo ship holds attitude through atmo even with SAS turned off. 

The ship itself. 

 

and loaded with cargo. 

[Snark]

18 hours ago, A_name said:

I usually go for dual inflatables, top and bottom. Sometimes integrated from the launch, sometimes attached with standard docking ports. Enter heavy side first. Consistently reliable and reasonably elegant too. 

^ This.

Here's an example ship, which I used as a fuel hauler to bring mined fuel from Mun or Minmus down into LKO:

That's a 5m SpaceY tank that it happens to be built around, but would work just fine with stock parts.

The ship is nearly 400 tons when carrying a full load of cargo.  It has to dive down to a Pe of only 28 km in order to aerobrake to LKO in a single pass.  Gotta say that plowing 400 tons at 3000 m/s through atmosphere at 28 km gave me the longest aerodynamics-overlay drag arrows I've ever seen. 

The nice thing about an inflatable-front-and-back strategy is that it doesn't matter at all where your CoM is.  Since you don't care which end is forwards when aerobraking, you just pick whichever orientation puts your CoM towards the front, and it will be nicely stable that way.

[RetiredNavyVet]

Love to mess with that.  Can you make it available on KerbalX, please?

[18Watt]

On 1/4/2024 at 1:06 PM, RetiredNavyVet said:

Love to mess with that.  Can you make it available on KerbalX, please?

I would be surprised if @Snark still has that vessel stored, that was from several years ago, and several versions of the game ago.

But as others have mentioned, using inflatable heat shields at both ends of the entry vehicle solves the drag/COM issue nicely.

Here's one of my Eve entry vehicles, which worked: