Should We Go to the Moon with SLS, Falcon Heavy or Starship?
How Falcon Heavy has changed the game, and ought to change NASAs’s strategy for getting to the Moon
On April 11, 2019 SpaceX launched is Falcon Heavy rocket for the second time, sending the Arabsat-6A satellite into orbit. All three boosters managed to land. This rocket exists, it works, as has been proven a second time, and it is cheap.
These facts cannot be ignored by NASA and US senators anymore. The Space Launch System (SLS), got started around 2011 and is still not finished. That is about the same time Falcon Heavy was announced, which already flew in 2018. SpaceX spent around $500 million to develop it. NASA has spent around $15 billion on SLS! And it is still not finished!
You would think they are making a vastly more capable rocket at 30 times the cost. But they are not. The first version of SLS will lift 95 tons to LEO. Falcon Heavy can lift 63 tons to LEO if not reused. That is about 65% of what SLS can do.
And it does this at a cost $150 million for a non-reusable launch. It can get 23 tons into LEO at $90 million. And this is what SpaceX is charging, not necessarily the actual cost. As you can see in my table Falcon Heavy can make around 13 launches for the cost of one SLS launch. SLS is predicted to cost of 1.5 to 2.5 billion dollars. The development cost each year is around 2 billion. So every year wasted developing the SLS NASA could have launched 13 Falcon Heavy’s in expendable mode. That is
13*63 = 819 tons of payload into LEO each year.
The development costs thus far of the SLS could have bought 100 launches or about 6 million tons of payload into LEO.
Okay hopefully I’ve hammered home the point that SLS is insanely expensive, but that does not answer the question of whether Falcon Heavy could do the jobs SLS was meant for. Or more specifically can Falcon Heavy be used for going to the moon.
At first glance it may not seem that way, because the Saturn V, delivered 140 tons to LEO, and Falcon Heavy (FH) can only do 63 tons. If the powerful Saturn V only got a tiny moon lander with 2 astronauts on top of the moon. How can the much less capable FH pull it off?
Moon Transport System Architectures
To answer that question we got to look at transport system architectures. What on earth is that you ask?
It is one thing to design some rockets and launch them, but you also have to devise a system for how these rockets are going to be used to actually reach you destination.
You start by building a powerful booster to put lots of payload into orbit. But then what? What exactly should you put into orbit?
When the Apollo program was conceived there was many discussions of how to get to moon:
- Fly directly with a spaceship, land and fly straight back to earth.
- Lunar orbit rendezvous. What was done in the Apollo program. A space craft remained in orbit around the earth while a moon lander detached and landed on the moon. Upon return the moon lander docked with the space craft and crew transferred to the space craft.
- Earth Orbit Rendezvous. A bit opposite approach. The docking and crew transfer happens in low earth orbit.
The simplest approach to explain is the first one. That is essentially how SpaceX Starship will operate: A booster push a spaceship into orbit. This space ship will then go to desired destination such as the moon, land, launch into orbit again. Return to earth and land. There it will possibly be refueled.
The challenge with this approach is that it requires a lot of fuel, which potentially requires a very large booster rocket to push into orbit. Still you may not have enough fuel, so the SpaceX solution is to have options to refuel in many locations. SpaceX e.g. imagines keeping Starship in orbit, and launch one or more refueling ships similar to Starship but only containing fuel. These will refuel Starship so it can go to the moon with fully fueled.
That means it will have enough fuel to land a large heavy spacecraft straight on the moon surface. The SpaceX idea for Mars was to have sent one-way mission to Mars with fuel production equipment which would produce fuel on Mars which could be used to refuel later arriving Starships so they could take off and return to the Earth. In the case of the moon, it is less obvious how that would work.
Starship is a methane fueled spacecraft because it was designed for Mars travel. On Mars you heat up ice in the ground to acquire water. This water can then be electrolyzed, producing hydrogen and oxygen. Hydrogen can be combined in a Sabatier Reactor with CO2 from the Mars atmosphere to produce methane. Methane is superior to hydrogen as a fuel, because it can easily be store long term. Takes much less space and does not require cryogenic storage.
But here is the catch: There is not carbon dioxide or carbon on the Moon, so we cannot make methane. Unless we bring carbon in some form, which is probably impractical.
However the moon does have ice, so we can produce hydrogen there.
Lunar orbit rendezvous
What the Apollo program did was that they launched as payload into earth orbit a lunar lander and a space craft to take the crew from low earth orbit to lunar orbit. In lunar orbit the Apollo astronauts transferred to the lunar lander, which landed on the moon. The lander would then take them back into lunar orbit, dock again with the orbiting spacecraft and return back to earth leaving the lunar lander behind.
The reason for this whole complicated setup, is to save fuel. You avoid having the land a relatively heavy space craft with a capsule attached for returning to the surface of the earth. That would require more fuel to land and more fuel to ascend from the lunar surface.
Earth Orbit Rendezvous
This is a different approach to solving the weight problems. One of the first concrete plans to use this approach was the Constellation program which NASA started working on after the Space Shuttle got canceled during the Bush Presidency.
Constellation tried to learn from the mistakes of the Space Shuttle. One of them was to combine a space craft for carrying crew and cargo into one. It may seem practical to just have one vehicle but it just complicated matters a lot. A crewed vehicle has different needs then something carrying cargo. When you launch cargo you don’t need to care about G forces to the same extent and you can relax safety concerns. When launching people into orbit, safety is a huge concern and you have to design everything for that.
The space shuttle orbiter was attached to the side of the booster rocket, which meant there was no option for escape in case of failure, unlike with the Saturn V. Sold boosters while great at giving a strong kick to push a vehicle off the ground, are also not very safe. You cannot turn them off if something goes wrong. You can with a fluid based propellant.
The way Constellation solved this problem was by making two different rockets Ares I and Ares V. Ares I would be a smaller rocket for lifting a crew into orbit. Ares V would be designed for lifting cargo into orbit. The idea was that a big space craft would be launched with the powerful Ares V into low earth orbit. However before that, the crew would be launched by the smaller Ares I into orbit. The crew would dock with space craft launched from Ares V, which would take them all to the moon.
Now these plans are dead. There is no Ares I. The SLS is essentially a continuation of Ares V. But SLS is supposed to fill the role of both Ares I and Ares V.
The reason why it is useful to mention Constellation is because this approach as many similarities with the Moon Direct transport system architecture proposed by Robert Zubrin, well known for writing the Mars Direct book. This is a great book detailing how to go to Mars on the cheap.
Moon Direct is basically applying many of his ideas in Mars Direct to moon exploration. As a Norwegian I like how Robert Zubrin makes an analogy to Norwegian polar explorer Roald Amundsen. Norwegian polar explorer decided on an entirely different strategy from British polar explorers. The British would travel to the poles in large expeditions with large crews and lots of supplies.
The Norwegian approach was to send small crews and take minimal supplies, and instead rely on living off the land. This approach was cheaper and more successful.
This is also Zubrin’s approach. Smaller expeditions, which instead rely on living off the land. That means setting up fuel production on the target destination and use produced fuel for return flights. If you don’t do that, you need a much larger and more powerful spacecraft to be able to carry all the fuel needed for return flight.
What I like about Zubrin’s thinking on both the Moon and Mars is that he thinks about how to actually get stuff done once you arrive. He care about getting maximum surface time for astronauts at low expense.
So Zubrin ads a number of twists to the Constellation setup: Instead of using Ares V for launching the heavy space craft, he choses to use Falcon Heavy, which is already in operation and cheap. Instead of Ares I, he uses Falcon 9. This solves several problems:
- All these rockets already exist and have flown. No new boosters to develop.
- Falcon 9 cannot launch enough payload to get to the moon itself. However it is getting human certified and can push a crewed dragon to low earth orbit.
- Falcon Heavy is not human certified and probably never will be. However it can get a lot more cargo into orbit.
Like with Constellation docking will be done in low earth orbit. However there are some differences. The Constellation guys imagine their space craft taking the crewed Orion (corresponding to Dragon) and attach it to the larger space craft carrying the moon lander and fly everything to the moon. Once at the moon Constellation would basically operate like Apollo. Keeping Orion in orbit while the lunar land lands. Upon return they dock with Orion again.
This approach is not practical with the Moon Direct approach, and it is also not desirable. First of all Dragon is not made to go outside of low earth orbit, so it does not make sense to attach it to any spacecraft and go to the moon. Instead Zubrin proposes to leave Dragon in earth orbit. Upon return from the moon the astronauts will dock with the Dragon again and descend to earth’s surface.
Another twist is that Zubrin proposes to skip the whole lunar orbit and docking. Instead Falcon Heavy push into low earth orbit a cargo transport vehicle holding what Zubrin calls a Lunar Exploration Vehicle (LEV), replacing the Lunar Lander.
So what is the difference? The classic Apollo style lunar lander is not reusable in any way. It knocks off its lower part to get into lunar orbit and dock. That is it. You cannot use it to land on the moon again.
Zubrin’s LEV is an entirely different approach. It is placed on the ground with the cargo transporter, on the first trip. However it is designed for reuse. When astronauts want to leave the moon, they take the LEV straight to low earth orbit. No mucking about in lunar orbit to dock like Apollo and Constellation.
In LEO, there is a Dragon capsule waiting for the astronauts to take them back to earth surface. Alternatively a Falcon 9 is launched with a dragon capsule with a new crew and extra fuel. It docks with the LEV and refills it with fuel. Crews are exchanged. The LEV flies straight back to the moon and lands.
At this point it has no more fuel. But Zubrin’s idea is that the first astronauts would have setup propellant production on the moon. An unmanned Falcon Heavy would deliver a cargo ship to orbit around earth, which would carry fuel producing equipment to the lunar surface. Robots would possibly do a lot of the setup. Whatever they cannot do, the first astronauts will finish, If they fail doing that, no problem. A cargo ship lands the LEV initially, thus the LEV is fully fueled and ready to return for the first crew.
However if propellant production get online, it means that on subsequent missions the LEV can fly itself to the moon without any cargo transport. It spends its fuel on the travel and landing. But that is okay, because it just refuels on the moon. When it get to low earth orbit, it just refuels with fuel launched from a Falcon 9 as mentioned earlier.
In short this is a great setup which allows shuffling new crew in and out of the moon for the cost of one Falcon 9 launch. That is no more expensive than exchanging crew at the ISS.
So what do we need to make this happen. We already have the following:
- Falcon 9
- Dragon (close to being rated for human crews)
- Falcon Heavy
Here are things we don’t have or may only partially have:
- The Lunar Excursion Vehicle, LEV. This will be a hydrogen-oxygen propellant based space craft at about 8 tons. XEUS being developed by ULA is perhaps the closest thing, but it is too heavy fill the role. It is mostly likely easier to develop than something like dragon because it does not need heat shields and ability handle going through the atmosphere.
- Cargo transporter. This will be launched on Falcon heavy and will cary a variety of things from earth orbit to the moon such as the LEV, habitat and fuel production facilities. This will be similar to a rocket stage. It has no crew compartment or other special needs. It does not need to be Hydrolox (hydrogen-liquid-oxygen propellant). It can use whatever engine is cheap and practical as it is not being reused or refueled anywhere.
- Moon fuel production equipment. This will be a combination of solar cells, microwaves and other equipment to extract ice from lunar creators, turn it into water and electrolyze it.
- Habitat module. A place for astronauts to live. Better for long term occupation than the LEV.
None of this with perhaps the exception of point 2 involved anything that has not been made before in some similar fashion. One has made lots of habitat modules for ISS already. Making one for the moon should be much easier since one does not need to deal with zero gravity. It can also be covered by lunar soil for radiation protection. Inflatable Biglow habitats may be a possible solution.
The suggested LEV does not have capabilities far beyond what the Apollo Lunar lander hat, which was based on significantly older tech.
Implications for the SLS
So why am I getting into all these details? SpaceX has not expressed any kind of interest in this approach. They are hard at work with Starship, which they believe will completely replace Falcon Heavy and Falcon 9.
Ideally Starship is the best solution to colonize the moon or explore it. However NASA cannot make decision today based on its existence. That is where moon Direct comes in.
It means that whatever happens to Starship we already have a much cheaper alternative than the SLS on the table. By canceling the huge money sink SLS is, NASA could refocus its resources on building fuel production facilities, habitats etc on the moon. Stuff that will be needed regardless of when Starship comes online. They can make a LEV and cargo transporter as a backup. Even if they will not be needed in the future. They are much cheaper backup solution to Starship than the SLS is at 15 billion dollars and counting.
But perhaps more importantly perhaps these alternatives could serve as a way of embarrassing the American political elite, so they can accept that NASA cannot keep operating the way it currently does. It is building things slow at magnitudes slower speed and magnitudes higher price than they should be.
There needs to be a fundamental re-evaluation of how NASA does things.