r/KerbalSpaceProgram • u/sand500 • Jul 31 '14
xkcd What If?: Letter to Mom (third to last paragraph)
http://what-if.xkcd.com/107/7
Jul 31 '14 edited Feb 12 '19
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u/gyro2death Jul 31 '14
I think depressed trajectories are like what we see in Scott Manley's video about getting around Kerbin in the fastest time. You launch up to an altitude that gives you a sufficiently lowered air resistance that you can go a bit faster, after that pitch down to keep yourself on a sub-orbital trajectory as the speeds you reach would normally take you in a ballistic arc (You would want to stay ideally parallel to the earth at all points forming a perfect circle). This is terribly fuel inefficient and very impractical but saves time on waiting for gravity to bring you down normally.
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u/giltirn Jul 31 '14
Surely that is identical to how normal passenger planes operate?
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u/ASBusinessMagnet Jul 31 '14
No, they push UPWARDS to keep themselves in flight. He probably means that your velocity is so great that you're on an escape trajectory, and have to push yourself DOWNWARDS to keep yourself in flight.
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u/gyro2death Jul 31 '14
Came back to say exactly this. Planes use upward lift to keep the flight plane level. Rockets on the other hand in this case are going so fast the have to thrust downward just to keep at the same altitude due to the curvature of the earth falling away
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u/UmbraeAccipiter Jul 31 '14
I have done this thousands of times, and now I know it has a name... I wonder how many other complex rocket terms I don't know that I know what they are and how to do them.
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u/gyro2death Jul 31 '14
While ksp has simplified physics it really is rocket science in the end ;)
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u/starmartyr Jul 31 '14
It's faster in the sense that the missile arrives at it's target sooner. The actual speed is slower due to air resistance. It arrives sooner because even at reduced speed the distance traveled is lower.
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u/sand500 Jul 31 '14
Pretty much what wikipedia says. Probably relates to KSP because of this:
An alternate, non-military, purpose for a depressed trajectory is in conjunction with the space plane concept with use of air-breathing engines, which requires the ballistic missile to remain sufficiently low inside the atmosphere for air-breathing engines to function.
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u/Jarnis Jul 31 '14
Here is an example of a depressed trajectory. A video with Orbiter.
https://www.youtube.com/watch?v=ROgKCxozau4
19min 58sec flight from Kennedy Space Center to Ascension Island using XR2 Ravenstar. Note how ~6min into the flight (2min in video, it is 3x speed) the vehicle is going substantially faster than escape velocity (MECO is at 11.5km/s), with the craft angled downwards to keep it within the atmosphere.
At the end of the flight the vehicle does a propulsive braking to get the velocity below orbital speeds (followed by a more "normal" aerobraking re-entry).
Some serious flying.
(If you want to try something similar in KSP, you will definitely need FAR - the stock atmosphere & aerodynamic model just won't work for something like this)
If you want to watch similar flight profile in more detail, here is another video that attempts to re-fly the profile - recorded in normal speed;
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u/Phredward Jul 31 '14
I couldn't quite tell, but did it seem like at some point the craft was pulling over 10g's to slow down?
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u/BHRnR Jul 31 '14
Does that mean Randall Munroe is reading r/kerbalspaceprogram? I'm feeling so important. :)
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u/Oishi2 Jan 03 '15
Before reading the KSP bit, I got really excited that depressed trajectories could be useful in KSP.
I need to go outside
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Jul 31 '14
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u/KnownSoldier04 Jul 31 '14
"time to cross the distance between you and your mother could be reduced to as little as six minutes" This Seems like it could be another one
He must have enjoyed writing these
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u/evilkim Jul 31 '14
What do you do if you dislike someone? Send them a letter. Using a missile.