# Aerospace SpaceX: Interplanetary Transport System

1. Sep 28, 2016

### Staff: Mentor

Elon Musk presented SpaceX's plans for its manned missions beyond Earth orbit yesterday. SpaceX is well known for missing their deadlines (like most in the space industry, to be fair), so we'll see how long it will take to develop it. The question how it will look like is clearer already, here are the key points. I rounded some numbers, and they are all preliminary anyway:

The system will have three components:
- a booster
- an interplanetary spacecraft ("ship" in this post)
- a tanker spacecraft ("tanker")

All three components are designed to be fully reusable with propulsive landing - they hope to use the booster up to 1000 times*, the tanker 100 times and the interplanetary spacecraft 10-15 times. They all use methane and liquid oxygen. A propellant that is easy to handle, and one that is not hard to produce on Mars. It also means they can get rid of the helium bottles to pressurize the tanks. Both accidents with Falcon 9 were linked to those helium tanks.

A typical Mars mission would launch the ship on top of the booster. The booster accelerates the spacecraft up to ~2 km/s, then separates and flies back to the launch site while the ship uses its fuel to reach Earth orbit. The booster directly lands at the launch site again. It gets re-fueled, a tanker is put on top, the system launches again. The tanker docks to the ship** and transfers its remaining fuel, then de-orbits with its heat shield and lands somewhere close to the launch site. This process is repeated until the ship is full - something like 5 flights are necessary. The ship then flies to Mars.
At Mars, the ship first uses its heatshield to slow down and then uses its engines to land vertically. It then deploys its payload and tools to produce fuel (energy from solar cells) or (later) directly gets fuel from some storage. Once it is full again, it launches towards Earth. As Mars has a lower escape velocity, no booster is needed here.

The rocket is going to be huge with a total mass of 10,000 tons, more than the Eiffel tower. The re-usable mission as described above will deliver 150 tons of dry mass plus 300 tons of payload to low Earth orbit (LEO), an expendable version could deliver as much as 550 tons. Payload to Mars can be 450 tons if the tankers also deliver some more cargo.
As comparison: The most powerful rocket built so far, the Saturn V, could deliver 140 tons to LEO.

The booster is basically two huge tanks with 42 Raptor engines at the bottom. Overall 77m height, 12m diameter. While the large number of engines gives a lot of redundancy (multiple engines can fail with the payload still making it to orbit), it also gives a lot of possible failure modes where the engines influence each other. N1 is an example - 30 engines, and none of the four attempts made it to orbit, at least two of them failed directly as consequence of the many engines. On the other hand, SpaceX has the Falcon 9 with 9 engines already, and plans to launch the Falcon Heavy with 27 in the next months, so they have experience with many engines.

The interplanetary spacecraft is the most complex component. 50 meter long/high, up to 17 meters diameter. The lower part is like a smaller version of the booster with two fuel tanks and 9 engines, the middle has space for various unpressurized payloads, the top is for the passengers and pressurized cargo. No launch escape system (it is too large), no way to separate passengers from the fuel tanks. But at least it can land everywhere under nearly every condition apart from disintegration in a huge explosion.

The tanker is a simplified version of the interplanetary spacecraft with larger tanks and without all the infrastructure necessary for humans. It can deliver 400 tons of fuel to LEO, which means about 5 trips to fill the 2000 ton propellant storage of the ship.

Cost: SpaceX expects all those components to cost about $200 million each, a bit less for the tanker. They key point is the reusablility: Use the booster 1000 times and the initial build cost just contribute$ 200,000 to each launch. The ship is the most expensive part. It can do only one mission to Mars every 26 months due to orbital mechanics, and it has very fast atmospheric re-entries stressing the heat shield, so its costs per flight are expected to be about 40 millions while booster plus tanker only account for $20 millions. If each flight can transport 100 passengers, that leads to a price of roughly half a million dollars to go to Mars (together with 3 tons of payload). Something a significant fraction of the population can afford. SpaceX didn't reveal plans about using the ship for LEO-only missions. If it can be used more often there, holidays in space could get affordable ($40,000 for an extended stay if the ship can be used 100 times in LEO like the tanker).

Beyond Mars:
Jupiter moons could be reached both from Earth orbit (with reduced payload) and Mars (with in-orbit refueling). If they have carbon, oxygen and hydrogen, fuel can be produced there. A ship starting from a Jupiter moon can reach the Saturn moons if the planets are in the right place. Uranus, Neptune and some Kuiper belt objects are accessible as well.
Venus is easier to reach than Mars, but you can't launch a rocket from the surface.

* Sounds like a lot, but commercial aircrafts can do tens of thousands of flights. Rockets are faster, but for most components this does not matter. The vertical propulsive landing is a key point - you couldn't do this with parachute landings in water.

** due to orbital mechanics, the same booster can be used only once every 90 minutes for a given spacecraft, or twice every 24 hours if the fuel efficiency is maximized. If there are more regular flights, multiple boosters can accelerate the process.

Full presentation
Compilation of all technical slides
Animation of the system
Page at spacex.com

2. Sep 28, 2016

### mheslep

The new motor, Raptor, with triple the thrust in the same size as the existing Merlin, and already with a successful test, is remarkable. The actual build of an enormous, ~5M diameter liquid-gas O2 capable carbon fiber tank is remarkable. These accomplishments with 5% of the company's resources are .. yeah.

Last edited: Sep 28, 2016
3. Sep 28, 2016

### mheslep

The suggestion of dual use for earthbound fast commercial transport was intriguing, another alternative made by propulsive landing and booster reuse. Ten minutes across the Atlantic would be quite the ride. I'm not sure who or what customer needs 40 min lift anywhere on the planet badly enough to pay the freight. Military transport comes to mind, and I suspect Musk's too, though I doubt he'll publicly solicit that business as I doubt he wants his vehicles associated with the military as the only or primary customer.

4. Sep 29, 2016

### Staff: Mentor

That's what I thought as well. ~2.5 millions per trip if the ship can be used 100 times. Better land somewhere with access to liquid oxygen and methane, otherwise the trip costs ~200 millions.

The system could also be used for large-scale manned LEO spaceflight. Every ship has the same interior volume as the whole ISS (~1000 m3). Add some connectors and you can build a large space station for a few billion dollars (mainly driven by ship construction costs). Something like 5 millions per launch (booster+ship), or \$50,000 per passenger.
Add something for supplies, staff and so on for a few weeks of holidays in orbit. Where can I sign up?

For a trip to Mars, packing 100 passengers in the vehicle will be quite crowded. 10 m2 per passenger... not sure if SpaceX really wants to go for this. Even for paid astronauts, all space agencies plan more volume per person.