Re: Martian gravity penalty

On Tue, 21 Aug 2007 12:29:45 -0400, Alain Fournier
<alain.fournier@xxxxxxxxxxxxxxxx> wrote:


Hop David wrote:
To get off earth's surface & achieve orbit, a rocket must first get
above the atmosphere. I've heard this takes about 2 km/sec and is known
as the gravity penalty.

I would guess a ship leaving the surface of Mars would also need to get
above Mars' atmosphere before doing the big burn. Does anyone know what
the Martian gravity penalty would be?

Thanks in advance,

Hop

I think you are confusing two things here. The gravity penalty has
nothing to do with the atmosphere it is about gravity.

As has been pointed out, the gravity penalty has quite a bit to do
with the atmosphere, as the atmosphere strongly encourages one to
climb near-vertically, against gravity, for much longer than one
would otherwise want to.

Indeed, it is practically impossible to seperate the gravity penalty,
the drag penalty, and the back-pressure penalty. The combined penalty
is, as Mr. David correctly notes, about 2 km/s.


Suppose two rockets want to get to lunar orbit from the moon surface.

The question is about Mars, not the Moon. Mars has an atmosphere, the
Moon doesn't. If you truly believe the atmosphere makes no difference,
why the change of subject?


In any event, the answer to the original question is that the combined
gravity/drag/backpressure loss for a Mars ascent vehicle is likely to
be about 800 m/s for a launch thrust/mass ratio of about 7 N/kg. This
results in total delta-V for launch to a 500 km equatorial circular
orbit of 4,300 m/s.

Numbers from Larson & Pranke, Human Spaceflight Mission Analysis and
Design.


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