[AR] Re: Turning after launch
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 16 Apr 2018 13:34:49 -0400 (EDT)
On Mon, 16 Apr 2018, John Dom wrote:
Even space launches would start turning almost immediately -- look at
the videos of Apollo LM ascents from the Moon -- were it not that their
final velocities are so high that they have to get completely clear of
the atmosphere fairly early.
Isn't that called a gravity turn considering orbital/space launches.
Not exactly -- a gravity turn is a specific type of turning trajectory
which happens to be a good approximation to the atmospheric part of
launcher trajectories. The LM ascent trajectory, with no atmosphere to
worry about, was *not* a gravity turn, because gravity turns are not in
fact optimal for orbital launch. (They're not too far from optimal, but
the LM needed every little bit of advantage it could get.)
Those are slow
The ones normally used for launchers are slow. Gravity turns don't have
to be slow. And the turn the LM used was not; it was done fairly quickly,
after a brief vertical ascent to assure terrain clearance.
and have little to do with the speedy trajectory switch observed
with SAMs or cruise missile launches.
As I indicated, the difference is because of different requirements.
SAMs aren't going to accelerate to 8km/s, so they don't need to gain lots
of altitude rapidly to clear the atmosphere. Cruise missiles don't want
to gain altitude at all.
This quick turn was not done say 10 years ago looking at YouTube.
Doing such a sharp turn so early generally requires vectored thrust,
because a vertically-launched missile doesn't have enough airspeed then to
make such a hard turn with fins. Early vertical-launch missiles -- mostly
converted from designs built for more traditional launch setups -- didn't
have suitable thrust vectoring, and so had to delay their turns somewhat;
the need was only identified after experience with early vertical-launch
missile arrays.
Henry
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