Round trip or one way?
The next choice that NASA makes in planning a mission is whether it will be one-way or round trip. There are two reasons why we might want the mission to return: One is that it would be nice to have a Mars mission return to Earth with samples of the Martian “soil.”
A second reason for a round-trip is to bring people back. The first time we send people to Mars, we will probably want them to return to Earth. (As you will see later in this unit, there is an alternative approach that may turn out to make a lot of sense; that is to send people to build a colony and make the return trip only when they are able to produce their own fuel on Mars.)
As of November 2003, all the trips to other planets have been one-way. Why?
It turns out that it is a lot more expensive and many times more complicated to have a round-trip mission rather than simply a one-way mission. The reason should be fairly obvious. A spacecraft on Mars needs to have fuel to take off and return to Earth. Since the fuel sitting on Mars has to come from Earth, it takes a lot of fuel and a large, expensive rocket to send the fuel to Mars for the return trip back to Earth.
We’ll now do an exercise to visualize why transporting fuel has a really big effect.
To begin, we will assume that we have a way of getting a lot of material (fuel, supplies, and equipment) into a low Earth orbit (LEO), like the orbit the space shuttle uses. To get anything from Earth’s surface into space – whether this “anything” is a spaceship, an astronaut, or some fuel for the journey – takes several tons of fuel to use as propulsion per ton of “stuff” launched into LEO. The exact amount of fuel depends on a number of things, including where the launch site is located and in what direction the ship is launched. If the rocket is launched from a location near the equator it will start with about 1.5 km/s directed eastward from Earth’s daily rotation. By current day space prices it costs about $10,000 for a pound of material to reach LEO (or $22,000 per kg). Even though this is very expensive, we will ignore the launch from Earth in computing the cost of our journey, but remember: When we include launching stuff from Earth it will probably make the trip cost about 3 or 4 times as much as what we will compute.
Instead of computing the cost of the trip to Mars in dollars or some other unit of money, we’ll compute it in tons of rocket fuel. You can translate this into your favorite currency by looking up the price of rocket fuel. If you can’t find a number, you can probably get a ballpark estimate – a halfway decent guess – by considering the price of gasoline. $1/gallon translates to about $500 /ton. You can reasonably assume rocket fuel is at least 5 to 10 times more expensive than that, or $2500 to $5000 per ton. So let’s start the planning.
Step 1: Since we’ve assumed that our Mars spacecraft is already in LEO, we need to get it into a high Earth orbit. That requires a Dv (“delta-vee” or change in velocity) of about 4 km/s. If our rocket exhaust velocity is 3 km/s then this requires 1.3 tons of fuel per ton of spacecraft boosted into the high Earth orbit (HEO). To simplify the computations, we’ll round this off to 1 ton of fuel per ton of spacecraft in the following.
Step 2: When a spacecraft is in a high Earth orbit, the space ship moves very slowly relative to Earth, so we may assume that it moves at approximately the same speed as Earth when Earth travels around the Sun. The Earth moves at about 30 km/s in its annual journey around the Sun. The speed needed to put the spacecraft into the HTO to Mars is 33 km/s, so we need to increase the speed of the spacecraft by Dv = 33 – 30 = 3 km/s. Thus, to get anything from High Earth Orbit to the Hohmann Transfer orbit to Mars takes approximately another ton of fuel for each ton boosted (the spacecraft, fuel, and all equipment), and this fuel also needs to be launched from Earth.
Step 3: The trip through space to Mars is essentially “free” from a fuel standpoint. Some energy will be needed to power the spacecraft’s systems, but there will not be any need to burn fuel until the spacecraft gets closer to Mars. When the spacecraft reaches Mars, it will be moving about 21.6 km/s and Mars will be orbiting at 24.3 km/s, so the spacecraft needs to change its speed by 2.7 km/s (which is again close to 3 km/s) to match Mars’s speed. To speed it up to match the velocity of Mars takes another ton of fuel per ton of material and spacecraft.
How many times more fuel than payload? --------------------------------
Altogether, to get the mission to an orbit around Mars – one way from LEO to Mars orbit - requires about 7 tons of fuel for each ton of payload we send! Given the high cost of getting material into space from Earth – not to mention all the costs of designing and building a system – you begin to see why space exploration is expensive.
Suppose our mission was just going to orbit Mars for a while and then return. We’d need to leave Mars’ orbit for the Hohmann transfer orbit again – using another ton of fuel per ton of spacecraft – and return to HEO – yet one more ton of fuel needed per ton of spacecraft. This return mission means we increase the total fuel by 4 times:
How many times more fuel than payload? --------------------------------
Breaking this down, this is what we get:
Compare this with a one-way trip that only required 7 tons/ton of payload. The bottom line of all these calculations is that you can get about eight times as much stuff to Mars for the same amount of fuel if you send it one-way instead of trying to bring it back! The reason this is so much more is the cost of transporting the fuel on the outbound trip that will be used for the return trip.
The situation gets more complicated if our mission actually lands on Mars and then flies back to Earth. In the "Landing on Mars?" section we discuss ways of landing without using up a lot of fuel, but there is no way to leave Mars without using fuel to get back into space. So anything that lands on Mars and then returns is a very expensive item indeed!
A few clever tricks for saving fuel:
1. Throw away what you don’t need as soon as you no longer need it.
NASA uses this trick a lot. The fuel tanks used for leaving Earth are tossed away as soon as they are empty (although they can be rescued and reused in the case of the lower launch stages). On the Apollo Moon landings, the lunar landing module went to the surface of the Moon, but only half of it left the Moon, and that half was discarded when the astronauts returned to Earth.
For Mars: We could save fuel if we send a mission with a lot of instruments etc. but only return a small sample carrier, getting the rest of our data by radio from the instruments on Mars.
2. If we aren’t willing to make a one-way trip, we might try to get our fuel on the surface of Mars. If we could manufacture the fuel for the return trip on Mars, then we would be able to save a lot payload space, because we’d basically only need fuel to get to Mars. Of course, we’d first have to ship out all the equipment needed to make more fuel, but that could make a one-way trip and potentially be re-used for several missions.
Can you think of any other clever ways to save fuel?