zaterdag 18 januari 2014

Low Cost Lunar Missions part 2: A lunar highway

If we ever want to make space exploration affordable, we'll have to get some kind of "Highway" between destinations, to make transportation between those destinations safe and affordable. One of the first destinations that might be of use here is the moon; it's an easy to reach destination, being close home, and has a lot of value scientifically, for further development of our exploration capabilities and possibly even economically (I generally consider lunar/asteroid resources to make money humbug, but still something to keep in mind). Establishing a "highway" between the Earth and the moon would be vital to make visits to the moon affordable, and lately I've been contemplating how it might be done. The main focus in this blog will be European launchers and hardware, as my original focus was designing a European lunar architecture that could be achieved with existing or near-term launch vehicles. 
Like last time, the European Ariane 5 and 6
 launchers form the core of the architecture

Unlike the previous article on Low Cost Lunar Missions, this is focused on getting as much capability out of as little recurring cost as possible. With "affordable", I mean low recurring cost. The total development cost of this program might be in the billions of dollars. The previous article was about reducing development cost.

Getting the crew to cislunar space and back

The initial destination I decided on was the Earth Moon Lagrange point, as this provides global lunar access and requires the lowest amount of delta V to reach out of all the possible staging points. L1 could also be an option, as could DRO, but L2 was the reference point here.

The mission mode that gives the highest "payload" out of all options is the following:

1. Send the spacecraft to LEO
2. Let the craft go to L2 by its own propulsion
3. Let the craft return with its own propulsion
4. Use aerobraking in Earth's atmosphere to slow down into LEO
5. Refuel and repeat.

The ∆V to reach L2 from LEO is about 3150 m/s for TLI with another 240 m/s for circularizing. Returning then takes 360 m/s, along with another 200 m/s for post-aerobraking maneuvers. Finally, I assumed 100 m/s of RCS ∆V at L2, in order to rendezvous at a station or lander pre-placed at L2. The engine assumed here is Vinci, with a specific impulse of 464 seconds. The heat shield is assumed to weigh 10% of total entry mass at aerobraking. Assuming an Ariane 5 ME with 23 metric tons to LEO,  we get the following mass breakdown:

1. Parked in LEO: 23 tons.
2. After TLI: 11.5 tons (∆=11.5t)
3. At L2: 10.9 tons (∆=0.6t)
4. After rendezvous: 10.56 tons (∆= 0.34t)
5. After return burn: 9.76 tons (∆= 0.8t)
5. After aerobraking: 9.33 tons (∆=0.43)

Main Propulsion propellant (H2/LOX): 13.33t
Main Propulsion empty mass: 2.17t 
RCS propellant: 0.34t
Heat shield mass: 0.98t
Crew Cabin total mass: 6.18t

This crew cabin's total mass of 6.18 tons can be compared to the Soyuz Orbital Module, which massed in at only 1.37 tons on the TMA variant. This cabin houses sufficient space for a crew of four. A breakdown of such a cabin and it's mass could look something like this:

Crew: 500 kg

Consumables: 280 kg (sufficient for 14 days)
ECLSS system mass: 740 kg (based on Soyuz)
Power: 250 kg (two solar arrays, ATK ultraflex derived, ~30 kW total, only ~15 kW needed)
Cabin empty mass: 4.41 tons
Habitable volume: 11 m^3
Habitable volume per crew member: 2.75 m^3

The habitable volume is based off a value of 200 kg/m^3, considered a pessimistic value, which is based on ISS modules). It is then multiplied by 0.5 to give the habitable volume. Per crew member, this volume significantly bigger that of the Orion crew capsule. This spacecraft should give a fairly comfortable environment for the crew for a trip of up to 14 days. The reason this craft is so much lighter than the similarly capable Orion capsule at 9 tons is because it is a simple cylindrical shape rather than a capsule shape, which frees up far more volume for the same mass. This is possible because of a "parachute" shaped heat shield in front of the craft. The heat shield doesn't have to be nearly as capable as Orion's heat shield because it only has to bleed off about 3km/s, while Orion has to bleed off over 11 km/s when reentering the atmosphere.

In order to get the crew at the spacecraft a small crew "shuttle" would be used. It probably wouldn't be a real shuttle, rather just a small capsule with sufficient space to carry four people for a few hours. Basing this off of Soyuz, Dragon and Apollo, the mass of such a system could be as little as 7-8 metric tons. Given the payload of Ariane 5 ME at 23 metric tons, there would be 15-16 tons of payload left. That's enough to fully refuel the craft that's waiting in orbit. On a typical mission, the crew would lift off in an Ariane 5 ME, rendezvous with the spacecraft in orbit and the craft would refuel. Then, the crew would set out on their mission. The tanker module would reenter and the shuttle would stay in orbit, waiting for the crew to return. Once the crew returns to LEO, they would rendezvous with the shuttle waiting for them to reenter.

The "shuttle" would look a lot like a Dragon capsule. It would be fully reusable, with no service module to speak of. All the power systems, RCS and propulsion systems for its not very demanding mission would be located in the spacecraft and would reenter and land with the craft.

The launch cost of bringing the crew to cislunar space this way would be €150 million, which is the price of Ariane 5 according to CNES. Other options include sending up two Ariane 6 launchers (€140 million total), or even two Skylon spaceplanes (as little as €14 million total), which would both allow for about 7 tons of additional cargo to be carried with the craft. The high costs of Ariane 5 are, in my opinion, offset by the reduced mission complexity and higher reliability achieved by fewer launches. With Skylon's possible reduction though, it would definitely be worth it.

Landing the crew on the surface

In order to get the crew to do anything useful at the moon they would probably have to descend to the surface. Get the crew down there, let them explore, do science, enjoy the view, plant a flag and sing an anthem, whatever they are required to do there.

And again, a key factor here would be to reduce cost. Now again, the launch vehicle here is the Ariane 5 launch vehicle, and it's assumed that the lander is parked at L2. With a 23 ton tanker, powered by a Vinci engine, it would have a mass of 10.9 tons. Assuming a 0.86 PMF (pessimistic, similar to the crew vehicle) it would hold 7.7 tons of propellant. A lander carrying this much propellant would be able to get 7.0 tons of payload down to the lunar surface (empty lander mass 1.3 tons).

If you want to use it as a taxi, you'll have to ascend again, and this would mean you can get only 1.7 tons of payload back up to the L2 point. Not a lot. But there's no reason you'd have to take all of your fuel down there.

ESA's depiction of an ISRU plant for the moon.

Lunar soil consists for a very large part out of metal oxides. These materials could be refined to create oxygen on the surface, which could be used as an oxidizer for the rocket motors. Liquid oxygen is about 85% of the mass of hydrogen/oxygen propellant, so being able to make that on the surface would save a massive amount of payload. If the oxygen could be made on the surface, it would be possible to get about 6 metric tons down to the surface, with 1 ton of hydrogen fuel stored in an extra tank and the lander would still be able to get the 6 ton payload back to L2. After that, the lander could be refueled and reused, functioning as a "lunar taxi".

Now, it would be kind of dangerous to immediately send the crew down to the surface without oxidizer for the return trip. Before any crew would land there, a robotic orbiter and small surface lander/rover would scout for a suitable and interesting landing spot, one where resources such as water ice and metal oxides might be common, or places that are interesting scientifically. Then, a cargo lander would be sent. It would land all the In Situ Resource Utilization (ISRU) equipment at site, as well as small robots to set it up and start testing the equipment. When it's functional, a crew can land. A 6 ton crew cabin could have a volume of about 20 m^3, more than the Altair lander, and have sufficient consumables to support a crew for stays of about a month on the surface. The crew could land there and other cargo landers could land near it to supply power, habitable volume, additional consumables and surface equipment. Every lander can land about 6.8 tons of cargo and has sufficient hydrogen fuel to fly back if it can be supplied with oxygen from the ISRU plant. The cargo would be sent to L2, where it would be attached to the lander. At the surface, the cargo module would be detached and the lander could ascend back to the L2 service point where it could be refitted for the next mission.

Reducing the cost even further

The British/European Skylon spaceplane might allow for a 10x further reduction in cost

For every lunar mission, we have so far managed to get it down to just two Ariane 5 launches, with every cargo launch requiring another Ariane 5. But I think it's possible to get it down even further. It's possible to send cargo and fuel to the L2 service point with even lower mass in orbit, making the use of the cheaper Ariane 6 launcher (and possibly Skylon) even easier.

With an Ariane 6, the payload to LEO is about 15 metric tons. It's possible to use a Solar Electric Propulsion spacetug to get propellant and cargo to L2. If we assume a ∆V of 7 km/s to go from LEO to L2 with low thrust propulsion, the payload at L2 is about 10.5 metric tons, and the spacecraft's "empty" mass is 1.3 tons. I say "empty" because it still carries about 300 kg of propellant to return to Low Earth Orbit. When in LEO, it can be refueled for the next mission. 

Doing this, it's possible to refuel the lunar lander and send up cargo blocks with just a single Ariane 6 launch, costing less than half of what an Ariane 5 launch costs. It takes longer to get the cargo there, which might form a problem with propellant boil-off. However, it's possible to get the 7.7 tons of propellant there with about 8.5 total tank mass using normal tanks. This leaves a lot of margin for sunshields, additional insulation and additional propellant to make up for boil-off. The 10.5 tons of cargo is actually enough to place a small manned service platform at L2, where the lander, cargo and crew vehicle can dock and transfer propellant, undergo maintenance and get outfitted for other missions. 

Using an electric space tug allows for a lunar landing of four people for a month on the surface to be undertaken in just three Ariane 6 launches, or just 45 tons into Low Earth Orbit. For comparison, a similar mission with conventional propulsion, a normal mission approach and no ISRU on the surface would require about 200 tons into Low Earth Orbit. The mass savings from this kind of approach would be immense. The total launch cost is about €210 million, or about $285 million. With Skylon, this goes down by a factor of 10.

Advantages and possible problems with the architecture

The big advantage to this architecture is that all components are fully reusable. The crew shuttle, as well as the L2/LEO shuttle, the lander, and the electric space tug, are all fully reusable and can be refit and refueled after every mission. The only real cost in the architecture is the launchers, the expendable fuel tanks and the occasional expendable cargo vehicle, as well as crew shuttle maintenance. Unlike conventional architectures, no new heavy lift vehicles are needed. Instead, existing launch vehicles can be used to support the architecture, also increasing the flight rate and therefore reducing unit cost on these vehicles. 

A possible downside could be that using two 23 ton launchers or three 15 ton launchers has reliability disadvantages compared to using a single 50-ton class launcher. If these vehicles have a success rate of 98%, two vehicles result in a total success rate of 96%, and with three launchers it would be 94%, while a single launch would result in a success rate of 98%. More launchers might result in lower reliability.

However, using a system more often increases the the reliability of the vehicle as you can more easily filter out problems with your system. You get a vehicle reliable by flying them often and taking out mistakes in the process, so a vehicle that flies more often is likely to be more reliable.

Another one could be that, at high mission rates, the selected launchers might be under capacity to support the program. Ariane 6 is required to fly 12 times per year. If it does 9 commercial flights per year, like it is supposed to, you'll only get a single crew landing per year. Even at 15 flights per year, you can only land 2 crews on the surface. 

A possible option is to keep operating Ariane 5 alongside Ariane 6 to do the lunar missions until a rapid reusable vehicle like Skylon is available. If Ariane 6 does 9 to 12 commercial flights per year, Ariane 5 is still capable of at least 8 flights and thus 4 crew landings or 8 cargo landings alongside it.