donderdag 7 november 2013

Low Cost Lunar Missions; To the moon with Ariane 6

Lunar exploration has always been a huge interest of mine. The moon is our closest neighbor and could teach us tremendous amounts about the history of our solar system and our planet. It could also function as a place to gather resources to explore further into the solar system. In short, there's plenty of reason to go there, but how? Many earlier plans to got there have yielded nothing but powerpoints and pretty animations. Getting there in a low-cost manner would be critical.

(For info on the plan discussed, scroll to the bottom.)

Constellation: How not to go there

The Constellation program was initiated by NASA as part of Bush's Vision for Space Exploration policy in 2005. It had the goal to return Americans to the moon by 2020 and give America independent manned access to space by 2014 after the Shuttle's retirement in 2010. However, the way they wished to accomplish this was doomed to fail from the beginning. 
The first fatal flaw of the program was the way two different LVs were used for achieving one goal. Using two different vehicles delayed Heavy Lift capability and costs a lot of additional money for developing the extra vehicle. Even if the smaller vehicle is a lot lower in cost than the big one, which Ares 1 certainly wasn't, you still wind up having to pay more. Using two or more launches of the same launch vehicle, like is the plan with SLS, significantly reduces the costs and allows the mission to take place much earlier, and doesn't require a vehicle as big as Ares V, which by itself was way too big and expensive.

The second fatal flaw in the program is that it's very ambitious. Landing four people on the moon for at least 7 days, anywhere on the surface, is a very big requirement, which ends you up with a spacecraft and lander which are both extremely huge and very expensive to develop. It also caused Ares V to be huge; up to 188 metric tons to LEO according to some sources. Even the biggest version of SLS won't go over 130 tons, and that is still years into the future. Ares V would have used a new, bigger core, new engines, new boosters and would have nothing in common with the space shuttle. It was a completely clean-sheet design and it would have cost over 20 billion dollars to develop. The whole constellation program would've cost over $40 billion dollars just for the first lunar flights. NASA doesn't have the funds for that.

Lastly, another big flaw in the program was Ares 1 itself. The vehicle was designed from the start as "it must use a space shuttle solid rocket booster as the first stage" and that is were the problems started. Even when using the powerful RS-25 engine for the upper stage, it was underpowered and provided almost no margin for Orion. The slightest grow of Orion or performance reduction of Ares would have made the LV useless.

Early Lunar Access

ELA lander docking with EDS. Credit: NASA, Wikimedia
By far one of the better ways to do "budget moon flights" is the Early Lunar Access architecture designed in the early 1990's. It was based on the basic "Faster Cheaper Better" spirit at NASA at the time. It requires only two launches of Medium Lift Vehicles; one space shuttle and one Titan IV or Ariane 5. The total mass in LEO is only 52 tons. It would use only near-term hardware and could be ready by the year 2000, and would bring two people to the surface of the moon. While less capable than constellation, the use of smaller expendable LVs allows more missions to be done for a lower cost, more than making up for the smaller capability. The idea is great, but slightly outdated. Many modernized versions of this architecture are possible, using newer launchers like Falcon Heavy, Delta IV, Ariane 5 or SLS. This time, however, I'll focus on an architecture using Ariane 6, Europe's new, low cost launcher.

Ariane 6

Ariane 6. Credit: ESA

Ariane 6 is the successor to Ariane 5. It is expected to become operational after 2021, and cost approximately €70 million a piece, which is about $93 million dollars. It can get 6.5 tons to GTO, but its LEO performance has not been released yet. The vehicle is designed for GTO and that's where its market is at, so LEO performance isn't as important. However, it is possible to estimate the performance of the vehicle.

Using the LV performance calculator, which should be added is just an estimation tool, I've been able to model the performance of 6.5 tons to GTO using the following numbers:

Stage 1, 2 and boosters: 135 tons of propellant, 11.8 tons empty mass, 4500 kN of thrust, 280 second specific impulse. 

Stage 3: 28 tons of propellant, 4.94 tons empty mass, 180 kN of thrust, 464 second specific impulse. 

All these values are based on Vega's P80 lower stage (which forms the basis for A6's P135 main stages) and Ariane 5's upper stage information (propellant mass fraction was improved, because A6 doesn't have the same volume restrictions as A5). For LEO, a mission to a 200x200 km orbit with an inclination of 6ยบ, it has a payload capacity of 17.3 tons. This seems a little optimistic to me, so I took out a chunk of payload by including a 12.5% performance margin for error, reducing payload to 15.1 tons to LEO. This is a lot more realistic for this vehicle, and it's the LEO payload I went with for the rest of this article. 15 tons is a lot less than the 23 tons Ariane 5 ME can get into orbit, but it does so at a much lower price; only $93 million instead of the estimated $210 million for Ariane 5. That's a 32% decrease in cost per kg (and pretty close to the claimed 30% cost decrease for A6). For this reason, Ariane 6 is the vehicle I went with. That, and it's European, and the European space program is a great interest of mine.

The mission 

Launch 1: The first Ariane 6 launches the Lunar Landing and Return Vehicle. This vehicle consists of a capsule (3.7 tons fully loaded), a propulsion module carrying an engine (Aestus 2, storable propellant, 55.4 kN and 340s Isp) and landing legs and a propellant load of approximately 9.3 tons.  Total mass of the system in LEO is 15 metric tons. The hypergolic Aestus 2 engine was chosen because it allows fuel to be stored in space for much longer. The Aestus 2 is by far the most efficient upper stage engine available for this purpose right now. It is launched without a crew: to prevent another Ares 1 fiasco, it's probably better not to launch humans on a solid powered rocket without any significant margin, especially not because it doesn't have a Launch Escape System yet. This part of the ship has 3226 m/s of ∆V.

Launches 2,3 and 4: Carry small propulsion modules. They each have a mass of 15 tons and a structural index of 10%, meaning that 10% of their mass is non-propellant. Thee of them give the stack, including the LLRV ∆V, a total of 8697 m/s, which is just enough for the total round trip from LEO back to Earth. 

Soyuz can provide crew transport to the LLRV
Launch 5: Should Ariane 6 not be capable of launching humans, which it probably won't be, a 5th launch would be necessary. This would be a Soyuz, launched from Kourou Space Center in French Guyana. It would bring a crew of 2 to the loitering spacecraft in LEO. The Soyuz facilities at GSC are made so that they can easily be adapted to human launches, so this shouldn't be a big problem. Landing locations after the flight are a different story, but they aren't supposed to reenter in Soyuz anyway.

Major mission events include a 3100 m/s burn to a trans-lunar trajectory, a 2700 m/s burn to land, surface exploration by the two astronauts, an 1872 m/s burn to enter LLO and a 1025 m/s burn to return to Earth. In order to save ∆V, the spacecraft sent on a trajectory straight to the moon and does not enter LLO before landing. This saves about ~300 m/s compared to entering LLO first (only 200 m/s was assumed here). However, in order to allow some more global access to the moon, the craft does enter LLO on the return trip

The launch costs of this mission are not insignificant. 4x Ariane 6 is a total of $372 million dollars, and a Soyuz adds another 40-60 million. In the worst case it's $432 million total. However, this is still a lot lower than the cost for two SLS launches (which I previously estimated at $1.4 billion a piece, for $2.8 billion total). It also helps kick up the flight rate of Ariane 6. In order to get a low price out of the vehicle, a low price is required. It is supposed to launch 7-15 times a year, and reach the 70 million euro goal at 7 launches a year. Costs can only go down by flying the vehicle more often, and four extra launches will certainly help with this. 

Sources and other interesting information:

2 opmerkingen:

  1. Thanks for that. I agree a manned return to the Moon can be done much more cheaply by going small rather than by using a super heavy lift vehicle, such as the SLS.
    Note such a small size mission could also be done by using 2 Ariane 5 ME's at a 24 metric ton (mT) per launch capability, plus a Soyuz or other manned-rated vehicle to carry the crew to LEO.

    Bob Clark

    1. I did consider Ariane 5. However, it would require two or three launches rather than four, and one Ariane 5 launch costs about $200-220 million, while two Ariane 6 launchers would cost about $185 million. There's a significantly lower recurring cost using A6, and recurring costs will be very important in a sustainable lunar program.

      The 15 ton payloads also have the advantage of being light enough to be lifted into orbit by Skylon, should that ever turn into reality. This would bring costs down even more. 24 ton payloads would require a redesign in order to work with Skylon, and considering ESA's interest in Skylon, I think it's a possibility real enough to take into account.