Ballistic Reentry vs Aerodynamic Reentry

oh it's Scott Manley here with another Kerbal space program demonstration after the Soyuz M s 10 failure there was a lot of confusion about ballistic re-entries how were ballistic re-entry is any different from well just falling will despite their superficial resemblance to things like rocks space capsules with a flat bottom do actually get aerodynamic lift and you can actually see this in stock Kerbal space program Here I am falling down and you can see that I'm using the one board reaction control wheels to try and adjust my sideways velocity I'm just trying to move it back towards the runway I initially moved it away to show that we had it and yeah what happens is because the thing has a flat bottom then wind or the air hitting it at that angle will push it in that direction now the thing about real spacecraft is they don't have these amazingly powerful reaction wheels that they do in Kerbal space program so it's not something that can be done in real life the way that I'm doing this right now so if I want to make this work the way it does in real life I'm gonna go into the game and modify the parts to make them at what more realistic now obviously there are mods that do this but I want to show you how this works I'm going into the parts menu and I'm looking for the mark 3 pod and I'm gonna edit it now what I'm gonna do is I'm gonna add an entry called C or M offset that's the center of mass offset in real spacecraft that fly like this the center of mass is deliberately not exactly at the center and this means that when it's falling down under the force of gravity the aerodynamic forces will actually force the spacecraft to a slight angle now I'm just trying to set it up here get it stable but there look you see it's about 25 degrees offset in this case now what I'm gonna do now is just maintain this what what I've got here is that my positioning vector is slightly higher than my velocity vector so in this location I'm actually generating aerodynamic lift and in this case I've got Kerbal engineer installed you can see that in the window at the right and about halfway down there's a value showing g-force there's two value so there's the current one and there's a maximum one and if when I'm using the era's mamak lift it is lifting me up and stopping me falling down into the denser parts of the atmosphere and by using that I've kept the g-force to just about 2.1 g the rate of deceleration of the spacecraft is a function of the speed and how dense the atmosphere is so if I can keep their spacecraft up higher longer I can reduce the g-forces so now what I do is I rotate the spacecraft in the opposite direction they should realize that I'm just using my role here on a real spacecraft you would have roll control but you would not have enough thrust in the pitch control to be able to maintain this angle you just have to rely on the natural stability of the denser of the Arizona mix so yeah look at this one we've already gone past g-force of two we're up to three GS were again at max our data just over four G's and what because all we've done is we flip this thing around and it pushed us down deeper into the atmosphere faster so we were going at a higher speed when we hit the denser parts of the atmosphere and therefore assured in much higher g-force it becomes much easier to see this effect if you put the two videos side by side but because one is pushing the spacecraft down and because it's falling faster it takes a lot less time to get down to the surface the one on the left is carefully gliding through the atmosphere trying to keep as much lift as possible trying to stay as high as possible the one on the right is already ten kilometers below because it has been using its aerodynamic properties to push itself down into the thickest parts of the atmosphere it experiences the higher G loads and before the spacecraft on the Left has even reached maximum g-force the spacecraft on the right is ready to open its parachute assuming its crew is still conscious after those high G's this technique has been used for a really long time this is a extract from a video regarding the Apollo program the entire reentry was actively flown by the flight computer it could adjust his altitude in this design here they actually talked about doing a skip outwards and then falling back in the idea was to have two moments of heating so they could use reduced the overall heating this didn't end up getting used but they did actively fly a flight corridor and not only did they control the altitude but they could actually turn their spacecraft left and right and therefore they would be able to control where they ended up landing the spacecraft so they could minimize the area that had to be searched by the recovery forces now see there's a computer failure in that case you don't necessarily know which way is up which way is down which we will maximize or minimize your heating so the spacecraft can do a ballistic reentry and what it does is it just rolls and that averages out the velocity for the in the lift and it will essentially following something very similar to ballistic trajectory and if you look at this example which again has exactly the same starting conditions we max out the g-force at a bit 3GS which is roughly halfway between our minimum and our maximum corresponding to the two extrema of the aerodynamic lift configurations now consider what happens during a launch failure where you are sub-orbital you're not able to set a very gradual descent instead you're falling down steeper or steeply through the atmosphere remember what I said that the deceleration you experience is a function of your speed and how dense the atmosphere is well if you are any suborbital trajectory you may not be going at orbital speeds but because you don't have that speed and you don't have the very gentle descent you may actually fall down through the atmosphere much faster and therefore you get to the thicker parts of the atmosphere more quickly so in this case we did a rolling entry because we would assume the computer may not be able to configure itself for every abort scenario and yet it came out to have a maximum g-force of about 4.1 which is equivalent to the worst case aerodynamic reentry but from an initial velocity which was 70% of our orbital velocity so you can see that during an abort scenario where you have not got into orbit where you are falling very quickly down into the thicker parts of the atmosphere you can actually experience much higher G loading than you would from our re-entry from orbital velocities and that is what we've been talking about in terms of ballistic vs. aerodynamic re-entries I hope this makes a bit more sense I'm Scott Manley fly safe [Music] [Music] you

Channel: Scott Manley

Views: 356,299

Rating: 4.948072 out of 5

Keywords: kerbal, apollo, spaceship, soyuz, aerodynamics, space, rockets, nasa, roscosmos

Id: HgTNzDCc0gk

Length: 7min 47sec (467 seconds)

Published: Tue Oct 23 2018