Aerobraking and Heatshield Lift

[adinfinitum]

Okay, I've got a couple semi-related questions. I'm playing normal career with no reverts and no quicksaves, so trial and error is a rather expensive option for me.

I've got a couple ships that I'd like to be able to send to Minmus and then return to a low Kerbin orbit. The return stage is pretty much capsule shaped as far as drag is concerned, the heatshield covers up anything that's behind it. Does anyone know a good altitude to set the periapsis at so that the craft doesn't end up landing, but doesn't require more than 1 or 2 aerobraking passes? I tried it and 33.9km, and that gave me a reentry. I know that every craft creates different amounts of drag based on what's facing the airstream, but it seems like anything that's roughly capsule shaped should have similar drag characteristics.

Second, when the heatshields are facing slightly off-center of prograde, they produce lift. Are they any benefits to having the lift vector pointing a certain way? For example, if the lifting force is facing up, will the pass through the atmosphere produce less drag because you don't descend nearly as fast? And will the lift component create a significant amount of lift related drag, so that the apoapsis after passing through the atmosphere is much lower than a directly prograde alignment?

Edited May 7, 2015 by adinfinitum

[Violent Jeb]

How much dv do you have to work with? If you can add a little burn to your aerobrake you can always fine tune the Ap by burning retrograde while your in the atmosphere. I usually circularize in one pass this way, usually with less than 200 dv all in.

Typically i enter between 50-55 km. I wouldn't go any lower than 45, since even if you get the Ap to LKO you're going to have to burn prograde to bring the initial side back into orbit.

In this higher atmosphere (above 40km) your crafts direction isn't going to affect your drag or orbit significantly - there's very low air presure. Once you get lower, pointing your craft "up" (blue on the navball) is going to slow you down, and "down" (red on the navball) is going to speed you up. I find this particularly important just before you start to experience the serious re-entry heat around 20km, you can try to slow yourself as much as possible, then once your ship is on fire, you usually just focus on keeping it straight to reduce the damage (if it's a problem for your craft design). Usually I find 15t and under need no shielding if you bring them in under 3000 m/s @ 100% heat settings in a reasonable descent.

Edited May 7, 2015 by Violent Jeb

[SanderB]

While craft with similar front-shapes (ie. craft built behind a 1.25m heatshield) will have the same drag, increased mass will mean that deceleration from drag will be reduced due to the increased inertial energy from the extra mass.

induced drag from lift isn't going to reduce your AP significantly compared to regular drag. One of the benefits of reentry lift is that you can guide your capsule toward a target (such as KSC) and due a much more precise landing. You can use lift to control your trajectory, but if you don't have some way of seeing what your trajectory will look like depending on what actions you take, there isn't a whole lot you can do to do precision aerobraking especially with the lack of knowledge about the exact flight characteristics of your vehicles in stock 1.0.2.

[adinfinitum]

In this case I had about 25m/s dV. I was trying to complete a return ore to Kerbin orbit contract, so if I was able to get my periapsis up to 70km I could complete it then lower it right back down. I doubt I had enough dV to actually do that, but I don't want to add more than a couple hundred dV when I tweak my craft, to keep costs down. I'll try coming in at around 45km next time and see how that works.

I'd never noticed the orientation changing speed on reentry before, most of my other ships just entered directly prograde, I'll have to give that a try with some of my other ships.

As a side note, does anyone know the equations KSP uses to calculate drag and atmospheric pressure now? I know that without doing some serious differential equations work I won't be able to calculate the change in energy exactly, but I might be able to get some rough estimates for future ships.

[NathanKell]

Lift during reentry allows two things:

1. Lowered G and max heating flux (though same total heat load, just takes longer)

2. Controlled landing site.

Here's a video on how it works:

Orientation always matters. Half the drag when Q = 5 is still half the drag when Q = 20,000.

Pressure, up until about 68km, is an 80% Earth atmosphere (i.e. 40km on Kerbin is 50km on Earth). Temperature uses the same model, except that latitude and day/night affects temperature (though differently in different atmosphere layers). Density uses the atmospheric molar mass and pressure and temperature.

Drag is 0.5 * density * velocity^2 * area * coefficient of drag. The last two terms come from the drag cubes and the drag curves.

[adinfinitum]

Cool, thank you!

[impyre]

As SanderB said... mass is important. Drag produces force, and force = mass * acceleration. So with a given force due to drag, if you have less mass (like a tiny probe/satellite-shaped object behind a huge heatshield) then your acceleration (or technically deceleration in this case) will be greater. The ratio of mass to drag is basically the ballistic coefficient (mass/[drag coeff*cross sectional area])... for a given drag profile / AoA, higher mass results in higher ballistic coefficient. High ballistic coefficients result in craft that slice through the air with less deceleration, while something with a low ballistic coefficient behaves more like a ping-pong ball.

You'll have to experiment to figure out what's required. I use sandbox mode for such experimentation. I consider it to be analogous to combining experimental data with simulators... this allows you to figure out how things *should* behave in a given situation.