Goozeman

As I'm sure you're aware, reentry heating is complex, so this is the best that I can come up with for a simple heating/ablation model. The shock temperature you have already, and is found simply from the normal or oblique shock relations. There are two primary modes of heat flux into the shield, convection and radiation. Convection dominates at lower re-entry speeds, and radiation dominates at higher speeds. I'll just deal with convection for the time being. For convective transfer: q'' = h*(T-T_inf) where q'' is the heat flux per unit area, h is heat transfer coefficient, T is the temperature of the object, and T_inf is the applied temperature. Relations for h can be found from the Nusselt number correlations. For laminar flow (valid at high altitudes): Nu = C*Re^.5*Pr^(1/3) Nu = h*L/k Making some simplifications, one can arrive at h = C'*sqrt(rho) where C' is a tweakable constant, and rho is atmo density. For the ablation of the heat shield, the Arrhenius equation should suffice. It is: dm/dt = B*m*exp(-T_0/T) where B and T_0 are tweakable constants. One caveat is that the rate of ablation never really hits zero (it gets close) with this model, so a lower cut-off would be beneficial. Or don't and call it vacuum ablation :-P. Finally then, the rate of heating/heat rejection is m*c*dT/dt = -dm/dt*L -epsilon*A*sigma*T^4-h*A*(T-T_shock) where m is the mass of the shield, c is the heat capacity of the shield, L is the heat of pyrolyis of the shield, epsilon is the emissivity of the shield, sigma is the Stefan-Boltzman constant, and A is the area of the shield. d()/dt is the time rate of change of that variable. Do note that T appears without a finite difference in the rate of ablation and radiation terms, so it must be given in absolute temperature, i.e. either Kelvin or degrees Rankine. With regard to the tweakables: C' is simply a function of the flow. Shield properties do not come into play. B and T_0 are properties of the shield, and must be sorted by trial and error with possibly some help from literature. c, L and epsilon are properties of the shield that could be estimated or obtained from literature about the material. Finally, radiative or thermal soak type shields can also use this model simply by dropping the ablation term.

Interesting. By my reasoning, the temperature of the heat shield is directly related to the incident heat flux (convection from the ram-heated air). The rate of heat loss by pyrolysis should be close to equal to the rate of heat input. If the shield is warming, the heat loss is slightly less than the incident flux, and visa-versa. The rate of pyrolysis is directly proportional to the heat rejected by pyrolysis. The rate of pyrolysis (ablation) should then slightly lag behind the temperature of the shield. Obviously, there is a lower limit on the temperature at which pyrolysis occurs, but so long as you are above that, the reasoning should hold.

Curious question. Is the rate at which material ablates related to velocity or temperature? I noticed on reentry with a standard Mk1-2 capsule that the heat shield was ablating rather quickly at ~350C then after a certain point it stopped ablating even though it was at ~550C.

I guess that's where I'm confused. Pressure doesn't really mean anything to aerodynamic calculations. It's all about density and relative changes in pressure due to motion, not absolute pressure. As far as temperature modelling goes, talk to Farram4. Just watching the Mach number readout in the FAR mod, it seems that he has done some good work with temperature.

To get you started, my notes say that Mars' atmo is 0.015 kg/m^3 at the surface and the scale height is ~11km. Don't know how that would translate to KSP, though.

I checked his orbital elements, and they are correct, or at least the LAN, argument of periapsis, and MA at epoch match the J2000 elements, which would be what would severely affect the positions relative to one another. Also, it's impossible to see Earth in the Celestia screen cap. Rotate your KSP screen 180 degrees and check again, cause Mars/Duna, Jool/Jupiter, and Dres/Saturn all seem to be in relatively the right place. Kerbin's elements don't match JPL's Solar System Dynamics elements, but only because the LAN isn't zero. The offset in ARG and LAN is the equal and opposite though, so the orbits should pretty much match. If Kerbin's inclination is zero, then the LAN, by definition, should also be zero. "Kerbin" Earth Ecc. 0.01671 0.01671 Inc. 0 ~0 Arg. 114.21 102.93 LAN 348.74 0 MA 357.51 357.51

I see your point, but even the atmosphere of Venus (Eve?) is 92% CO2. Whether or not that 8% of "other" makes a significant difference or not, I don't know. Given that it's sulfuric acid rain/vapor, it very well could. Also, if anything, Duna's atmo is more uniform, being largely CO2 with traces of other things, so the exponential atmo should work there. Jool is a whole other can of worms. There's no way you could get that close to Jupiter without frying electronics anyways. Anyways, so long as the scale height is adjusted for each planet (and I'm pretty sure it is), the model should be relatively accurate.

I think that's more an issue of people opening the parachutes at far to high an altitude. Theoretically, they could remain reefed that long, but that's what a drogue chute is for. Water vapor only minutely affects the density of the air (~2%). The tables for dry air, then, are accurate to better than 5%, which is good enough for KSP. (Patched conics anyone?) This doesn't necessarily make the calculations wrong, just "less right" than a more accurate model. A full atmospheric model could be implemented fairly easily, but again, a single equation is "good enough." This equation takes all of the variables in the standard atmospheric model and collapses it to: rho = rho_0 * exp[h/h_scale] where rho is SSL density and h_scale is the scale height. Again, not perfect, but we're not doing laboratory quality calculations here. I've done mission architectures for spacecraft design, and this is more than good enough to get a general idea of what is happening on reentry. Other factors will introduce far more error than the atmo model, such as your Cd vs. M curve and the weather. The lift/drag model is orders of magnitude more egregious (completely inaccurate) than the atmo model (first-order accurate).

Depending on how accurate FAR is, it looks like you're getting a "tip stall" condition. If you're going to use spoilers/speed brakes, you want them inboard. If you stall the tip of the wing first, usually one tip will stall first, causing the condition you see. Notice the "minor stalling detected" indicator. It could be an issue with FAR, but spoilers near the tip is bad aircraft design.

Spacecraft parachutes have ... devices (I don't know the real term) to control the opening of the chute. Left to its own devices, the chute would just snap open. Take a look at this video of a Dragon capsule landing for the actual opening sequence. As far as air density goes, the exponential atmosphere is first-order accurate. If you're using FAR, then your landing calculations have other errors that have far greater impact than the density. The exponential atmosphere misses some of the stratigraphy (troposphere, stratosphere, etc.), but we don't have weather, winds, or the coriolis effect going on anyways, so you're not missing much.

What is your reentry angle? Higher reentry angles mean that your shield must withstand higher temperatures. Ablatives can only ablate so quickly, and that speed is dependent on temperature. It is possible to overwhelm an ablative (or any kind of) heat shield.

The trick with running re-entry simulations with FAR is that your vessel needs a stable axis to re-enter on. For example, the capsules are stable entering rear end first. Coefficient of drag is then only velocity (Mach number) dependent, and a simulation could be performed in such a configuration. A lifting-body reentry could still be done using a linear CL vs. Alpha and CD vs. Alpha plot. Probably wouldn't be quite as accurate, but that's the point of having a lifting body for reentry: you can tweak the trajectory as you reenter. In order to take advantage of an entry profile like this with a more complex machine (rover/lander), you need to create an aeroshell with these properties. Playing with deadly reentry makes this a necessity for ANY atmospheric landing. Also, purely ballistic reentries are not all that accurate. For example, the Mars Exploration Rovers (Spirit/Opportunity) had "landing ellipses" that were about 20km wide by 150km long. Mars Science Laboratory (Curiosity), by virtue of its powered decent, was able to attain a much smaller landing ellipse, but it was still about 20km long as I recall.

Take a look at NACA TR1135. It has the cone shock solutions as well if you're interested. For slender bodies though, the linear methods usually suffice. Except once you understand that most KSP players don't have a clue about compressible flow theory.

Farram gives you enough information that you can be more refined with your control theory than a simple PID control system. I won't get into the theory here, but the "Dynamic Stability" tab gives the info necessary to define the dynamic nature of the system, and therefore the resulting time-dependent flight characteristics.

You found the closed form solution? The first time I wrote code to solve oblique shocks, I used a Newton solver, but I have since found a closed form solution. I have it in my compressible flow theory notes if you need it. You end up needing to solve a cubic in sin(β)^2.

Use the static stability graph in the SPH FAR CAS screen to check. Multiply the green line's maximum by 10 and that's your max L/D.

I agree, the wing leveler is extremely useful. I don't really need the others, though DCA can be useful for spaceplanes.

PWing abuse for sure. What was your max L/D? Also, how do you bring up the flight speed options? Anyways 82.54 + 3-point landing EDIT: Derp. Found the airspeed settings right in front of my face.

I'm loving the B9 pack; it makes playing with FAR tolerable for aircraft. I do have a quick question though. The turbofan engines make a TSFC of ~.03 lb/(lb-hr) at altitude. I could be mistaken, but I think this should be an order of magnitude higher, on the order of 0.3. Love the engines anyways, it's nice to have something that operates well subsonically.

You may purposefully eject parts, but whatever comes back to KSC must make a successful horizontal landing. Successful in the sense that you may not lose any remaining pieces of your aircraft. You may not, of course, eject any fuel tanks, and the whole point of the challenge is to use ONLY a Mk1 Fuselage worth of any type of fuel. That is, you are limited to only 150 units of LiquidFuel. Monoprop, Oxidizer, Xenon, SolidFuel, etc will disqualify your attempt. Also, no one else try it with FAR installed? I feel lonely.

I'll give it to you. The angle of attack of your wings just goes to prove how messed up stock aero is. Congrats! The flight report is usually a must, but your screen caps seem to indicate that you did it in good faith. Can't give you the runway landing though, sorry. It's only a unit of fuel anyways. Give it another shot sometime!

Alright, let's see it then. If you have the patience to infiniglide all the way around the planet, I will have the distinct honor of not including you in the leader boards. Congrats. Also gliding to KSC from KSC + 60km up would take a constant 63:1 glide ratio. Either you need absolutely massive wings or you need to pull 63:1 supersonically, which is, quite literally, impossible. Quite frankly, I clearly stated that this is an aerodynamics challenge. If you don't have anything constructive to do, go bother someone else.

Color me impressed. I guess I need to do separate StockAero/FAR categories.

I'm trying not to limit what people do beyond stock jets/intakes and 2 intakes max per engine. Part clipping is tolerated within what the game will allow without the Dev console, but remember the intake/engine max. Also, it is explicitly stated that hops above 60km are prohibited. I may lower that later if it becomes a problem.

A small request. The Rate of Climb indicator in the HUD is useful, but not for fine tuned maneuvering. Could you add a ROC line to the FAR Flight Data? Thanks.