Hi, I'm Will.
And I'm Pierre-Yves.
Welcome to the Electric Propulsion Lab at
Princeton University, where we test plasma
rockets.
We've seen a lot of headlines about trips
to Mars using future plasma rockets and powered
by yet-to-be-developed nuclear power supplies.
But assumptions about the future will always
make things easier, and neither of us is very
patient.
So what would a cargo mission to Mars look
like today, using existing and near-term technology?
We begin our journey at low-Earth orbit, aboard
a Falcon 9.
Our total mass is 23 metric tons.
The goal of this mission is to use plasma
thrusters to deliver the maximum possible
payload to Mars in a timeframe comparable
to that achieve using chemical propulsion.
We achieve this goal using 4, 50 kilowatt
Hall thrusters.
This type of thruster has been tested at a
wide range of power levels and produces approximately
50 millinewtons of thrust per kilowatt of
power.
This means that more power results in shorter
transit times, and for this mission, we require
200 kilowatts.
Producing this much power is by far the most
challenging part of our mission.
Solar power is much lighter than existing
nuclear power sources for this mission, however
a self-assembing solar array of this size
has never been tested.
Exactly how big is it?
It spands nearly a football field in length.
Its largest dimension is comparable to the
width of the International Space Station,
and it dwarfs the self-assembling arrays of
the Juno spacecraft.
Various types of arrays of this scale are
already in development, including telescopic
solar arrays, Megaflex (or mouse-ear) arrays,
and the Roll-Out Solar Array, the last of
which was tested outside the International
Space Station in July of 2017.
We fire our thrusters, gradually increasing
speed.
Our spacecraft slowly spirals beyond low-Earth
orbit.
After 5 and 1/2 months, we finally achieve
a high enough speed to escape the pull of
the Earth.
This places us into orbit around the sun.
We continue to fire our thrusters for the
first leg of our trip around the sun.
5 weeks later, we turn off our thrusters.
We have generated enough momentum to coast
almost all the way to Mars.
After almost 6 months of coasting, we require
one final burn to match the velocity of Mars.
We arrive at our destination 16 months into
this mission.
Let’s compare electric propulsion to a traditional
chemical rocket.
The chemical rocket fires briefly at low-Earth
orbit, coasts the entire way to Mars, then
fires a final time to match the velocity of
Mars.
This maneuver is known as a Hohmann transfer.
The total duration of this trip is 8 and 1/2
months.
While our plasma thruster spends a similar
amount of time traveling around the sun, the
months it spends raising its orbit around
the Earth nearly doubles the length of the
plasma mission.
Now let’s briefly examine how trip duration
varies with available power.
As power increases, we narrow the time gap
between chemical and electric propulsion missions.
Shorter missions are possible for both rockets,
but such missions would require shedding excess
velocity at the destination, and this would
substantially reduce our payload mass.
As we increase the available power, the mass
of the power supply becomes greater, reducing
our payload mass.
Above 400 kilowatts, we can no longer deliver
a payload to Mars.
However at 200 kilowatts, our payload is still
more than 7 and 1/2 metric tons, more than
double the payload of a chemical rocket.
To visualize this quantity, imagine we are
delivering automobiles to Mars.
The chemical rocket delivers the mass of 3
and 1/3 1965 Ford Mustangs.
However the plasms rocekt delivers nearly
7.
So far, we have assumed that the power supply
is dead weight, but all missions require power
supplies at their destination.
If we include the 200 kilowatt power supply
as part of our payload, we can double again
the delivered mass.
So while near-term technology will not allow
us to reach Mars faster than a chemical rocket,
plasma thruster can deliver substantially
larger payloads in comparable times, and this
is especially true for missions requiring
large amounts of power.
