QUESTION:
What's a realistic estime for the mass of fuel required to send a rocket to
Mars? Also, at about what rate would the fuel burn?

ANSWER from Andrew Petro on November 12, 1997:
To calculate the amount of propellant (which is fuel and oxidizer) needed
for a mission we usually base the calculation on the velocity change
("delta-V") that is required for the maneuvers.  In this case the payload
must be launched from Earth and then sent on a trajectory toward Mars then
it must land.  If you wanted to go into orbit around Mars first that would
require another maneuver.  So, a rough estimate of the delta v required to
launch from Earth into Earth orbit is about 10,000 meters per second (m/s). 
The delta v to escape from Earth orbit and head toward Mars is about 3600
m/s.  If you directly enter the atmosphere of Mars and land then you can
take advantage of the atmosphere to slow down but you will need a heatshield
and you will still need enough propellant to control your landing - about
1000 m/s. For each of these maneuvers you can use the Ideal Rocket Equation
to compute the amount of propellant that you will need.  One other piece of
information that you need is the specific impulse, or Isp, of the propulsion
system.  For the storable liquid propellants that are typically used on long
space missions the Isp is about 310.  The g in the equation is 9.80 if you
are using metric units. The form of the Rocket Equation that you can use is:

delta v / Isp x g 
Propellant Mass = Payload Mass x (e - 1)

If you work this out for each maneuver, you can compute an approximate
amount of propellant.  Start with the last maneuver and work backwards
because you have to include the propellant for later maneuvers in the
payload mass for the earlier maneuvers.  You should also include the mass of
the propulsion system and tanks in the payload mass.  You can do the
calculation once to get an approximate mass of propellant and then include
about 15% of that in the payload as propulsion system mass and do the
calculation again. Look up the Ideal Rocket Equation in some books for
further explanation. Good Luck.

Andrew Petro
Johnson Space Center