The Aries 1B moon ship in 2001: A Space Odyssey is a clear descendant of the lunar landers described by Arthur C. Clarke in The Exploration of Space and their realization as the Apollo Lunar Module that landed on the moon in 1969.
The Aries 1B landing at Clavius base – still from the film.
In the movie, the ship takes Dr. Heywood Floyd from the space station in Earth’s orbit to Clavius base on the moon.
The question to be answered, is: “Does the design work with chemical rockets, or does it need something more potent?”
In “2001: The Lost Science”, and echoed in “The Spaceship Handbook”, it is stated that the ship uses chemical rockets, probably LH2/LOX. This might be considered surprising as both the Orion spaceplane and the interplanetary ship, Discovery, both use nuclear engines. In the novel, Clarke writes about a plasma exhaust that implies a power plant that is not chemical. he wrote:
There was none of the power and fury of a takeoff from Earth – only an almost inaudible, far-off whistling as the low-thrust plasma jets blasted their electrified streams into space. The gentle push lasted for more than fifteen minutes, and the mild acceleration would not have prevented anyone from moving around the cabin. But when it was over, the ship was no longer bound to Earth, (…)
To answer the question, we need to determine the likely mass of the Aries 1B, its fuel payload and likely engine performance.
The most difficult is to estimate its mass. Given its luxurious interior, it perhaps most resembles a spaceship version of an executive jet. Executive jets typically assume passenger and luggage weights around 110 kg, and when compared to their structure weights, the passengers are in the ballpark of 15% of the empty airplane. The movie version has the Aries 1B carrying 24 passengers, with a crew of 4. Using this data, the Aries masses about 20,500 kg, and 23,600 kg with a full complement of passengers and crew.
The fuel she can carry is determined by the Aries’ fuel tank volume, which is dependent on her dimensions and the location of the tanks. “<em>2001: The Lost Science”</em> indicates that the Aries has a diameter at the “equator” of 45 feet, about 13.6 m. The passenger section takes up this midsection. Above the passenger deck is the galley, and then the flight deck. Assuming that all the fuel is located between the passenger deck and the engines, I estimate that there is about 320 cubic meters of space for the fuel or propellant tanks. Given this volume constraint, the mass of fuel will therefore depend on its density.
Starting from Earth orbit, the ship needs to add nearly 3.2 km/s velocity to completely escape Earth’s gravity, and another 2.4 km/s to land on the moon, a total of about 5.6 km/s. If the ship only refuels on the moon, then it needs to be able to meet a total velocity change of 11.2 km/s. If the ship can refuel in earth orbit as well, then only 5.6 km/s is needed for each leg of the journey. Intriguingly, “<em>2001: The Lost Science”</em> suggests that there are heat shields to protect the Aries when reentering Earth’s atmosphere. The Aries isn’t obviously designed to return to earth like an Apollo or Soyuz ship, so I take it to imply that she may use aerobraking to shed her return velocity to return to LEO (low Earth orbit). If so, she would only need 8 km/s of extra velocity for a 2 way journey between refueling stops.
If we use Clarke’s description of the departure from Earth orbit, 15 minutes of engine burn would need about 1/3rd of a g to reach the extra 3.2 km/s for escape velocity. This would also be adequate performance to land on the Moon too.
Passenger section. Business jet roominess. – still from the film.
Passenger section. Flight attendants discussing their mysterious VIP passenger – still from the film.
Galley section is zero-g. Elevator in the back accesses the passenger section, while the flight deck is accessed from one of the floors.
Zero-g Flight deck in the “nose” of the ship and orientated perpendicular to the passenger section.
The best chemical propellants have an exhaust velocity of 4500 m/s. This means that the ship needs to have a mass ratio of about 3.5 for a one way trip, and about 12 for a two way trip if it only refuels on the moon.
LOX (liquid oxygen) has a density of 1140 kg/m3, while LH2 (liquid hydrogen) has a low density of 70 kg/m3. This averages to 427 kg/m3. Therefore the this high performance fuel cannot mass more than 137,600 kg in the available space. Using the rocket equation, we get a maximum velocity change of 8.6 km/s.
This allows the Aries to make a 1 way trip to the moon or back, with refueling at each end of the journey, like a modern airliner. If we allow aerobraking, then it could make a 2 way journey, refueling at either the Moon or Earth orbit. However, Aries cannot make a 2-way journey purely using rocket propulsion.
Unless aerobraking is accepted, it is probably best to assume that the Aries is refueled at each destination. Today, with the discovery of possible water at the lunar south pole, it would make sense for the water to be shipped to Clavius base for processing. The resulting LH2 and LOX would be used to fuel Aries on the moon, and perhaps shipped to Earth orbit as well for refueling spaceships.
We can probably discount Clarke’s suggestion that the Aries has a plasma jet rocket engine. Today, such engines are low thrust and require a lot of electrical power. The mass of a nuclear power plant to power the engines for short periods would not make sense. The movie version of Aries has no obviously large radiators to dump its waste heat.
A more conventional nuclear rocket, like Nerva, typically gets its high performance by heating low mass hydrogen. This gives such rockets a high specific impulse. However it comes at a cost of large tanks, as LH2 has a low density as we have seen. In the context of the Aries’ limited volume for fuel storage, this low density results in a fuel mass of just 22,400 kg.
Assuming about 900 specific impulse, the velocity change is about 6 km/s,which is enough for a one way journey, refueling at each end of the journey. Like the chemical rocket, it cannot achieve a 2 way journey with one refueling. It is also not sufficient to make a 2 way journey with aerobraking. This performance looks disappointing as it appears less useful than chemical rockets. However, there is an advantage in that the cost to deliver just 22,400 kg of fuel to LEO is a lot less than the 137,000 kg of LH2 and LOX for the chemical rocket. In addition, any lunar water can now supply oxygen to Clavius base, rather than burning it as oxidant.
As the low density of the liquid hydrogen prevents Aries from achieving the convenient 11.2 km/s velocity change that allows it to manage a 2 way journey, can it be solved using other, denser propellants? The problem with denser propellants is that the exhaust velocity is dependent on the mass of the molecules in the exhaust. Using water, or even liquid argon, requires a specific impulse that exceeds that of tested nuclear rockets. Only hypothetical liquid or gas core reactors would have the necessary performance to make this work.
The most likely propulsion system for Aries is chemical rockets using LH2/LOX. This requires refueling at the end of each journey, at the space station and at Clavius. If aerobraking at Earth is possible, then a 2 way journey is within reach. Replacing the engines with nuclear thermal rockets and liquid hydrogen as the propellant, provides slightly reduced performance, allowing 1 way journeys only. They do have the advantage of being cheaper to supply with propellant, although replacing the nuclear fuel core would also have to be considered. Changing the propellant does not provide any major performance improvements withing the design specification of the Aries as given.
- 2001: A Space Odyssey, Stanley Kubrick & Arthur C. Clarke (movie)
- 2001: A Space Odyssey, Arthur C. Clarke
- 2001: The Lost Science, Adam Johnson
- The Spaceship Handbook, Jack Hagerty