Re: Martian gravity penalty


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.

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

Rocket 1 does this with very powerful engines and achieves orbit in a
few seconds. The rocket spent energy just to go up a little and then to
get horizontal velocity. Very little gravity loss.

Rocket 2 has much less powerful engines and can only barely lift off,
it acheives orbit after sevral minutes. At first it is accelerating very
little, it is burning fuel mostly to avoid falling back down. So a part
of the fuel that is being burnt is used to accelerate the vehicle and
another part is used to have it hover above the moon. The energy of that
portion of the fuel which is wasted without giving an acceleration to the
vehicle is the gravity loss.

If you do the math, you will see that a vehicle accelerating slowly to
orbit needs to burn more fuel than one that is accelerating briskly. The
difference in energy requirement is the gravity loss.

At least that is what I understand about gravity loss.

An atmosphere may in some circumstances force a slower acceleration due
to aero-dynamic pressure. I would not expect this to be the case for any
reasonnable rocket departing from Mars.


Alain Fournier

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