Author Topic: Trajectories of recent satellite launches  (Read 92 times)

Offline bknight

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Trajectories of recent satellite launches
« on: March 19, 2017, 03:01:52 PM »
I watched the ULA launch yesterday and noticed that the trajectory reached ~270 miles and then the pitch was changed so the altitude decreased to ~ 200 miles while velocity continued to increase.  I remember several that had similar profiles.
I have done a lot of review of Bob. B's Saturn launch simulator and that trajectory was up and then "flat until orbital velocity was achieved.

My question is why this profile is planned and carried out with more current launches?
Truth needs no defense.  Nobody can take those footsteps I made on the surface of the moon away from me.
Eugene Cernan

Offline Peter B

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Re: Trajectories of recent satellite launches
« Reply #1 on: March 20, 2017, 08:11:37 AM »
I don't know the maths of it, but I'll have a couple of guesses.

1. Any altitude gained above the intended orbital altitude (that is, the climb from 200 miles to 270 miles) represents potential energy that can be cashed in for speed when the rocket dives back down to orbital altitude. My guess is that this potential energy is more cheaply gained when the rocket is going more slowly, and more efficiently spent when the rocket is going faster.

2. By climbing to the higher altitude the rocket is able to gain speed more efficiently by encountering less drag. Because drag increases at the square of the speed (or something like that), the high speed of a rocket means that even at orbital altitudes there are efficiencies to be gained by climbing even higher to where the atmosphere is even thinner.

How did I go?

Offline bknight

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Re: Trajectories of recent satellite launches
« Reply #2 on: March 20, 2017, 08:36:42 AM »
Yes, I thought about the gain of kinetic energy aided by Earth's gravity.  The drag may or may not be accurate the difference in drag at 200 miles seems to be small compared to 270 miles.
I don't know the maths either, so my perception may be way off, and that is why I asked the question.
Truth needs no defense.  Nobody can take those footsteps I made on the surface of the moon away from me.
Eugene Cernan

Offline Obviousman

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Re: Trajectories of recent satellite launches
« Reply #3 on: March 20, 2017, 04:12:57 PM »
The potential energy gain proposition seems reasonable. We certainly do that with aircraft a lot (climb to slightly above desired altitude then descend to pick up speed before trimming out).