togfox Posted March 23, 2012 Share Posted March 23, 2012 ... I can\'t work out why a craft\'s acceleration DECREASES over time. As a craft punches through the atmosphere and reaches a higher altitude with lower gravity combined with the weight loss of spent fuel, a craft should accelerate over time.It\'s super easy to test with no mods.Stick a capsule on a LFT/LFE and launch. At 10 seconds, pause the game and note your velocity down the bottom. Mine says 95 m/s. Continue the launch and pause at 20 seconds (2 x 10). If acceleration were constant, your speed would be 95 x 2 = 190m/s. It\'s not. It is less than this meaning acceleration is DECREASING.Are my laws of physics totally flawed? S:| Link to comment Share on other sites More sharing options...
KerbalStorm Posted March 23, 2012 Share Posted March 23, 2012 I too, have wondered this. Thinner atmosphere, less mass, slightly less gravity... what\'s slowing it down? Link to comment Share on other sites More sharing options...
Steve5451 Posted March 23, 2012 Share Posted March 23, 2012 I\'ve never noticed this.For me I feel like I accelerate as I gain altitude. o:Maybe the fuel tanks are screwed up and actually addweight as you use it? Link to comment Share on other sites More sharing options...
Mynorus Posted March 23, 2012 Share Posted March 23, 2012 Nope. Basically, if a stranded ship were stuck there, floating in space, he would still have some force pulling on it; that\'s what\'s causing orbiting, along with speed. So, if you were on escape velocity, this rule would mean that earth, mars, Kerbin or the mun will still be pulling on you like 'Hey, @%$hole, I\'m not done with you!' That would make you gradually slow down. If you are in deep space, it\'s like slowing down at your apoapsis to 0 m/s, you just fall down. I may be wrong, I\'m only a 7th grader.EDIT: The air resistance isn\'t helping, is it? Link to comment Share on other sites More sharing options...
Jarnis Posted March 23, 2012 Share Posted March 23, 2012 ... I can\'t work out why a craft\'s acceleration DECREASES over time. As a craft punches through the atmosphere and reaches a higher altitude with lower gravity combined with the weight loss of spent fuel, a craft should accelerate over time.It\'s super easy to test with no mods.Stick a capsule on a LFT/LFE and launch. At 10 seconds, pause the game and note your velocity down the bottom. Mine says 95 m/s. Continue the launch and pause at 20 seconds (2 x 10). If acceleration were constant, your speed would be 95 x 2 = 190m/s. It\'s not. It is less than this meaning acceleration is DECREASING.Are my laws of physics totally flawed? S:|Your example is caused by air resistance. Under 15km altitude it is a fairly dense soup that seriously drags you down once you go much beyond 100m/s.Once you are above 45km or so, it should no longer play a major part and you\'ll probably see rapid acceleration.Best way to see this effect is to experiment with straight up altitude with, say, 4xLFT + tricoupler + 3xLFE that has tons of thrust early on compared to weight. You get a lot higher with the fuel you carry if you moderate your thrust early on so you don\'t go much above 100m/s until you are out of the lower atmosphere. Link to comment Share on other sites More sharing options...
togfox Posted March 23, 2012 Author Share Posted March 23, 2012 None of these theories explain why I\'m decelerating. Under constant thrust, in a straight line, my acceleration should increase as air density and gravity decreases.The other theory is the launch pad offers a boost as it is an immobile object and when that immobile object is gone, my acceleration decreases as I lose the benefit of that. I\'m not sure if that is sound physics but its the only explanation I have. Link to comment Share on other sites More sharing options...
Steve5451 Posted March 23, 2012 Share Posted March 23, 2012 Possibly. When advancing in stages and firing my rockets it pushes my discarded stage back realistically. Link to comment Share on other sites More sharing options...
Causeless Posted March 23, 2012 Share Posted March 23, 2012 None of these theories explain why I\'m decelerating. Under constant thrust, in a straight line, my acceleration should increase as air density and gravity decreases.The other theory is the launch pad offers a boost as it is an immobile object and when that immobile object is gone, my acceleration decreases as I lose the benefit of that. I\'m not sure if that is sound physics but its the only explanation I have.Acceleration is reduced because the faster you go, the bigger speed difference between you and the surrounding air, and thus more drag. Despite the thinning air, you\'ll be going so quickly that the air around you effectively turns into a sludge.It\'s why it\'s much easier to move your arm slowly through water, instead of as quickly as you can, and the same reason the Bugatti Veyron Super Sport, despite having 200 more bhp than the default Bugatti Veyron, can only manage around about 20 km/h more. Link to comment Share on other sites More sharing options...
KerbalStorm Posted March 23, 2012 Share Posted March 23, 2012 Actually, I think Jarnis is right. The engines provide a constant force, and the faster it moves through the atmosphere, the force of friction on the spacecraft will increase. Thus, the net force will decrease faster than the loss of atmosphere and gravity can counteract it. At a certain altitude it will reach a tipping point where friction is low and atmospheric density is low, and the increase in friction due to acceleration will be slower than the loss of friction due to lower atmospheric density, and it will start to accelerate more again. Link to comment Share on other sites More sharing options...
togfox Posted March 23, 2012 Author Share Posted March 23, 2012 Ah - I see now. Whilst air density is decreasing, the friction is increasing (due to my speed), so that my net acceleration is decreased. Thanks! Link to comment Share on other sites More sharing options...
Cykyrios Posted March 23, 2012 Share Posted March 23, 2012 That\'s right, and this is also the reason you decelerate greatly when re-entering the atmosphere at high speeds (and may soon be the reason your ship blows up =P)EDIT: Scratch that, I didn\'t read your message right.Friction decreases with air density, but increases with speed for a given air density.For instance, when re-entering with your command module only, your speed will fall quickly due to the air becoming thicker and thicker (even though gravity becomes slightly more powerful), and your speed stabilizes at some point, as the only forces are gravity and friction. Link to comment Share on other sites More sharing options...
Buzz Lovell Posted March 23, 2012 Share Posted March 23, 2012 Part of the drag equation - drag is in part a function of velocity. Link to comment Share on other sites More sharing options...
witeken Posted March 23, 2012 Share Posted March 23, 2012 Acceleration is reduced because the faster you go, the bigger speed difference between you and the surrounding air, and thus more drag. Despite the thinning air, you\'ll be going so quickly that the air around you effectively turns into a sludge.It\'s why it\'s much easier to move your arm slowly through water, instead of as quickly as you can, and the same reason the Bugatti Veyron Super Sport, despite having 200 more bhp than the default Bugatti Veyron, can only manage around about 20 km/h more.Yep. Actually this is the inverse of falling with high speed: The air makes you slower. There was a test about 50 years ago, someone was dropped from 100km to test something idk. The guy reached a top speed of about the speed of sound: 1000km/h, or 300m/s! When he was at 10 km, he slowed down to 300km/h.Once you\'re out of the first color layer (about 15km?), your acceleration will increase dramatically. Link to comment Share on other sites More sharing options...
boolybooly Posted March 24, 2012 Share Posted March 24, 2012 yup initial acceleration from standing start includes 0-100 m/s of low drag, once over 100m/s you are on the bottom of a parabolic curve of optimum speed / drag, this was explored in the 'max altitude with supplied spacecraft' challenge, the numbers are here. Link to comment Share on other sites More sharing options...
RC1062 Posted March 24, 2012 Share Posted March 24, 2012 If you launch a rocket and adjust your thrust so that your acceleration is equal to 0. You should see an increase in speed as you get higher. Link to comment Share on other sites More sharing options...
Tiberion Posted March 24, 2012 Share Posted March 24, 2012 Ever listened to a Shuttle launch, where they say \'throttling back for MaxQ'?That\'s related; MaxQ is the point where a ship undergoes its maximum aerodynamic pressure from the combination of acceleration and friction; They did so to limit the forces the ship encounters during this time; too high and it might shear a wing off or something equally catastrophicMaxQ also happens to be the point where your overcome friction and begin positive acceleration gains againhttp://en.wikipedia.org/wiki/Max_QI\'ve seen rockets in KSP actually replicate this, up to part connections failing due too the drag forces getting too high. Link to comment Share on other sites More sharing options...
Zephram Kerman Posted March 24, 2012 Share Posted March 24, 2012 I found this immensely helpful: (from http://kerbalspaceprogram.com/forum/index.php?topic=7161.msg106460#msg106460) Basically, control your thrust to keep the ship close to terminal velocity for its current alititude. The reason for this is the 'Goddard Problem'; the dilemma of finding the happy medium between exponentially increasing aerodynamic drag, and wasted fuel resisting gravity.vT ~ 97.3 exp(+altitude / 10183m), so for example:Altitude (m) Target speed (m/s) - rounded to nearest 5 m/s0 97.3- you\'d better get a move on!500 105 - you should be catching up by now!1000 1102000 1203000 130 - usually the first benchmark I have time to look at5000 1608000 21510000 26013000 35015000 42516000 470 - most of us are at least thinking about staging and pitchover maneuvers by now!..32000 2250 - this is equal to the orbital speed, so by now you should be pushing over hard for orbit, and air drag is not as important above this altitude.[scribbles this on a yellow sticky, and staples it to the inside of Bill\'s visor.]By following this profile, any design suddenly becomes much more efficient! Link to comment Share on other sites More sharing options...
Randox Posted March 24, 2012 Share Posted March 24, 2012 Your example is caused by air resistance. Under 15km altitude it is a fairly dense soup that seriously drags you down once you go much beyond 100m/s.Once you are above 45km or so, it should no longer play a major part and you\'ll probably see rapid acceleration.Best way to see this effect is to experiment with straight up altitude with, say, 4xLFT + tricoupler + 3xLFE that has tons of thrust early on compared to weight. You get a lot higher with the fuel you carry if you moderate your thrust early on so you don\'t go much above 100m/s until you are out of the lower atmosphere.Just an add on to this, the air resistance is proportional to the SQUARE of your velocity. The faster you go, the more resistance increases, and the more work you need to do to maintain the same acceleration rate. Right at the beginning, the velocity is so low that the engine can accelerate you really quickly, but as your speed increases, your engine needs to do a lot more work to maintain that acceleration. Since the thrust of the engine is finite, the result is that the faster you go, the slower you accelerate.For a rocket, the air also gets thinner as you gain altitude, and eventually its thin enough that your engine has enough power to regain a positive jerk (change in the rate of acceleration). Link to comment Share on other sites More sharing options...
Elladril Posted March 25, 2012 Share Posted March 25, 2012 LOL... reading through this thread, I felt like I was actually - in the workshop - as we reinvented the wheel. Link to comment Share on other sites More sharing options...
Guest GroundHOG-2010 Posted March 25, 2012 Share Posted March 25, 2012 Basicly, its like falling. As something falls, it doesn\'t just continue to speed up even though the force is constant (gravity is an accelaration, I know that). Thats friction. Link to comment Share on other sites More sharing options...
Warringer Posted March 25, 2012 Share Posted March 25, 2012 Okay, people, I would like to publish the data that was put out with a MaxQ_Test craft (capsule, parachute, decoupler, ASAS, 2xLFE, 1xLV-T45, 1x Elderberry Ni Antenna) pointing straight up and taking the telemetry of the craft with my Elderberry Ni software.Here\'s my results, a chart for Altitude and Velocity. Link to comment Share on other sites More sharing options...
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