austin1812 Posted August 17, 2012 Share Posted August 17, 2012 Hey Kerbonauts! I am suffering from biggest is best syndrome, a very severe case. I will go to the VAB with the thought to build a sensible munar rocket but end up building sun launcher. I am using mods mixed with stock parts and I think I can suppress my problem if i understand properly an engine strength against the weight pressing on it. Please help a fellow kerbonaut, I've been talking to the engine scientist too much. Link to comment Share on other sites More sharing options...
Paprika Posted August 17, 2012 Share Posted August 17, 2012 is one of your mods MechJeb? There's a bit on that that tells you your thrust to weight, I think it's in the vessel information. I tend to do pre launch 'launches' just to check that and make sure it's going to take off ok.If not, there's an equation to work it out in http://www.youtube.com/watch?v=Aw0NyPWWP4U&feature=plcp < That video. Link to comment Share on other sites More sharing options...
quarthinos Posted August 17, 2012 Share Posted August 17, 2012 It is in vessel info. On my display, I keep it open and leave it so the title is hidden under Mechjeb's main panel. That keeps thrust, mass and TWR always visible. Link to comment Share on other sites More sharing options...
bsalis Posted August 17, 2012 Share Posted August 17, 2012 Hey Kerbonauts! I am suffering from biggest is best syndrome, a very severe case.Perhaps someone should start a Big Rockets Anonymous (BRA) support group. Not that I need it. Link to comment Share on other sites More sharing options...
Atimed Posted August 17, 2012 Share Posted August 17, 2012 (edited) Hey Kerbonauts! I am suffering from biggest is best syndrome, a very severe case. I will go to the VAB with the thought to build a sensible munar rocket but end up building sun launcher. I am using mods mixed with stock parts and I think I can suppress my problem if i understand properly an engine strength against the weight pressing on it. Please help a fellow kerbonaut, I've been talking to the engine scientist too much.One of the basic formulas i use is:TtW=Thrust / Mass*10- For Stratosphere flight build rockets witch thrust is 1.5-2.5.. if lower than 1 it wont lift of, if higher than 2.5 you'll reach terminal velocity soon and waste fuel..- Over 16'000m you can go as high as you want i usually go up-to 5, but not lower than 0.8 or you will have a hard time reaching orbital velocity..- In Vacuum everything works.. but for the sake of your nerves don't go under 0.33, unless you wanna take a nap while you do a injection burn..Hope it helps Edited August 17, 2012 by Atimed Link to comment Share on other sites More sharing options...
quarthinos Posted August 17, 2012 Share Posted August 17, 2012 Umm. How do you go "terminal velocity" going up? Isn't terminal velocity where drag == acceleration due to gravity? Link to comment Share on other sites More sharing options...
sal_vager Posted August 17, 2012 Share Posted August 17, 2012 quarthinos, I believe the correct term, which everyone forgets, is Max-Q Link to comment Share on other sites More sharing options...
Atimed Posted August 17, 2012 Share Posted August 17, 2012 (edited) Umm. How do you go "terminal velocity" going up? Isn't terminal velocity where drag == acceleration due to gravity?I meant, due your high acceleration you reach the max speed at that altitude.. while fighting air and gravity drag u start wasting fuel.. I don't know the proper terminology (English is not my main, nor is advanced rocketry ) but max dynamic pressure might be it, however i'm not 100% ..Hope it clears some things up.. Edited August 17, 2012 by Atimed Link to comment Share on other sites More sharing options...
Ziff Posted August 17, 2012 Share Posted August 17, 2012 Yes, you are talking about Max Dynamic Presuure, or Max-Q. Read about it here -> http://en.wikipedia.org/wiki/Max_Q Link to comment Share on other sites More sharing options...
togfox Posted August 17, 2012 Share Posted August 17, 2012 You can reach terminal velocity going down and you can reach terminal velocity going up. Those links offer interesting reading. Link to comment Share on other sites More sharing options...
closette Posted August 18, 2012 Share Posted August 18, 2012 (edited) Terminal speed and max Q are not the same thing at all. I think the OP was using the result that the most efficient ascent speed (at a given altitude) happens to be equal in magnitude to the local terminal speed, i.e. when the drag force and gravitational force are equal in magnitude. Unlike in the real world, if you follow the optimal ascent speed vs. altitude, the instant of "max Q" (maximum drag force) occurs soon after launch for most spacecraft in KSP, due to the weird mass-dependence in the drag model. As the rocket ascends, it not only encounters thinner air but also loses mass as fuel is used up. Even though the drag is proportional to speed-squared, those two effects more than offset that. (For my Goddard Challenge rocket when following this fuel-efficient ascent profile, max Q occurred at a mere 250 m altitude on the way up while at full thrust, and at about 8000 m when falling back down from 33000 m).Most craft will experience the greatest effect of drag (i.e. drag force per unit mass) on descent from orbit, around the 8-12km altitude mark, when their re-entry speeds get slowed from a few thousand m/s down to a few hundred very quickly. Edited August 19, 2012 by closette Link to comment Share on other sites More sharing options...
hubbazoot Posted August 18, 2012 Share Posted August 18, 2012 To OP:If you don't want to use mechjeb and want to do it manually, I usually do a test flight to see what engine's capability for lift is. Attach a small pod to however many fuel tanks you think that engine could lift off the ground (not necessarily accelerate, just lift). If it can lift it, attach more, if it can't, attach less. Most of the stock engines can lift 3~4 of their diameter fuel tanks. One large grey tank is equal to four of the 'bigger' small white tanks, in both mass and fuel content. If something can hold itself off the ground, it can generate one gee of thrust, and all you need is some solid rocket boosters to get it up to terminal velocity. (Short version: keep your rocket at ~100m/s until you get to 9000m, then accelerate freely)Also, for efficiently lifting off from the pad, I've found the Asparagus-style rocket to be insanely efficient and easy to build. Pretty much all of my rockets use this style of staging now in some form or another.Another tip is to make sure you have, for yourself, properly defined your payload and where you want that payload to be. For instance, if I'm making a mun lander, my first payload is the entire system that will get me to the mun and back. Inside that payload is the system that will get me back from the mun. Link to comment Share on other sites More sharing options...
rkman Posted August 18, 2012 Share Posted August 18, 2012 To OP:(Short version: keep your rocket at ~100m/s until you get to 9000m, then accelerate freely)Recommended ascent velocity at 10000m is ~260m/s.A bit more or less doesn't matter much, but 100m/s at 9000m is really slow. It causes ineffiency due to gravity drag. Link to comment Share on other sites More sharing options...
hubbazoot Posted August 20, 2012 Share Posted August 20, 2012 I've lost my terminal velocity chart, and like I said, that's the SHORT version. Link to comment Share on other sites More sharing options...
closette Posted August 21, 2012 Share Posted August 21, 2012 I've lost my terminal velocity chart, and like I said, that's the SHORT version.You mean this chart? http://kerbalspaceprogram.com/forum/showthread.php/6664-Mini-challenge-max-altitude-with-this-supplied-spacecraft?p=100912&viewfull=1#post100912 Link to comment Share on other sites More sharing options...
Zephram Kerman Posted August 21, 2012 Share Posted August 21, 2012 Closette's ascent speed profile has saved my Kerbals so much fuel, Bill made a copy of it on a yellow sticky and stapled it to Bob's helmet visor.Here's a graph. Coincidentally, the curve of speed vs altitude sortof resembles the gravity turn curve of range vs altitude. Link to comment Share on other sites More sharing options...
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