purple100 Posted August 5, 2012 Share Posted August 5, 2012 I have a rocket with a lander designed for touchdown on Mun with a powered return to Kerbin. Once it drops into lower atmosphere, at 30ish km, it flips over and points nose-down and resists all attempts to change pitch. This is a problem because I can't use the engine's thrust to slow it down anymore. Why does it do this, and how do I design the vessel to avoid the problem? Link to comment Share on other sites More sharing options...
sal_vager Posted August 5, 2012 Share Posted August 5, 2012 I would assume it was due to the airflow over the lander, causing it to act a little like a shuttlecock, you may be able to alleviate this by jettisoning those side mounted empty tanks or by using pylons instead. Link to comment Share on other sites More sharing options...
purple100 Posted August 5, 2012 Author Share Posted August 5, 2012 The tanks were the simplest solution I could think of to extend the range of landing legs, but they do contribute a fair amount of drag. I'll see what I can do. Link to comment Share on other sites More sharing options...
semininja Posted August 5, 2012 Share Posted August 5, 2012 You might look into Vlad's airbrakes as another solution to this problem. You can find them here. Link to comment Share on other sites More sharing options...
purple100 Posted August 5, 2012 Author Share Posted August 5, 2012 Behavior persists even after removing the landing apparatus. The simple configuration of command module + tank + 1/2 tank + engine starts turning at 25 km and swings like a pendulum. Link to comment Share on other sites More sharing options...
Tigga Posted August 5, 2012 Share Posted August 5, 2012 When unpowered your craft should will be stable if the centre of mass is "above" your centre of pressure, unstable otherwise. That's "above" is not relative to the ground though - it's relative to your velocity. If you're trying to land tail down you'll want aero forces at the capsule end of the ship, mass at the engine end of the ship.The problem is that your centre of mass will vary massively depending on how those fuel tanks are filled. If you have a lot of fuel at the top you're going to be unstable, if you have a lot at the bottom you should be stable. Link to comment Share on other sites More sharing options...
purple100 Posted August 5, 2012 Author Share Posted August 5, 2012 My center of mass is too high up, then? Hmm, that presents a problem. This thing is coming down with the top tank empty and the bottom 1/2 tank about 25% full. Link to comment Share on other sites More sharing options...
Tigga Posted August 5, 2012 Share Posted August 5, 2012 My center of mass is too high up, then? Hmm, that presents a problem. This thing is coming down with the top tank empty and the bottom 1/2 tank about 25% full.In that case, I wouldn't have thought it would be in the second config. Dunno then, unless it's a Smart A.S.S. problem. Never used that myself. Link to comment Share on other sites More sharing options...
Festivefire Posted August 5, 2012 Share Posted August 5, 2012 you could try editing the deploy altitude and drag on a parachute to use it as a drogue to help stabilize your craft once it gets down to about 45000, which I've found is the highest a drogue can be effective. Link to comment Share on other sites More sharing options...
Kosmo-not Posted August 5, 2012 Share Posted August 5, 2012 The problem is that the CM and chute have a drag coefficient of 0.1 while everything else is 0.2. This puts the net drag force aft of the center of mass. Link to comment Share on other sites More sharing options...
semininja Posted August 5, 2012 Share Posted August 5, 2012 If you stick a few of those airbrake flaps around the decoupler, they should flip you the right way round as soon as you activate them. They're quite effective; I've landed using those once, although I kinda loaded the entire pod with them... Link to comment Share on other sites More sharing options...
purple100 Posted August 5, 2012 Author Share Posted August 5, 2012 Okay, so I modified an MK16 parachute to partially deploy at very high altitude with a drag of 10. It kept the rocket in retrograde orientation without SAS assistance and allowed me to use the engine to land. Looks like that's the best solution available at the moment. Link to comment Share on other sites More sharing options...
Festivefire Posted August 5, 2012 Share Posted August 5, 2012 thats why i said to use a drogue chute. increased drag at the top end, but not so much as to break the lander. note that i also said you could do a cfg edit to make it deploy at a higher altitude, so it is effective as a drouge chute in the upper atmosphere Link to comment Share on other sites More sharing options...
Clouds Posted August 6, 2012 Share Posted August 6, 2012 Adding some wings, like the cone on a shuttlecock or the feather/fins on an arrow, would cause a great amount of directional drag and might keep the intended end facing the rear. Be careful not to overdo it, or it'll also cause it to flip around during launch... Link to comment Share on other sites More sharing options...
TheDarkStar Posted August 6, 2012 Share Posted August 6, 2012 Try putting your parachute in a separate stage. The lower atmosphere will slow you down to 100-300 m/s, slow enough to avoid tearing off the parachute. Deploy at at least 400 meters above the ground to get 200 meters of braking (it takes about 200 feet between the parachute deploying and opening below 500). Link to comment Share on other sites More sharing options...
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