A few days ago, I mentioned that some of my friends at ULA had passed me copies of some papers they were going to present at the Space 2008 conference a few weeks ago. The paper I want to discuss in this blog article, which can be found in Word Document form here, revolves around using mid-air recovery to reuse the propulsion section from the Atlas V first stage.
I first heard about this concept back around the beginning of the year, but other than a somewhat elliptical reference back in my second Orbital Access Methodologies post, I had held-back on posting until my friends had had a chance to publicly present their concept.
The basic logic behind concept is pretty simple. The RD-180 is derived from the Russian RD-170/171 engines that were designed for use on their Buran and Zenit launch vehicles. They were designed to be reused multiple times, and during the qualifcation test programs were tested to many times the length of a normal stage burn. The RD-180 represents most of the cost of the Atlas-V first stage hardware. It’s also made by a country that’s becoming less than friendly with the US, and it’s one of the key limiting factors for higher flight rates for the Atlas V. The tanks on the other hand are relatively cheap, bulky, and heavy. What this means is that, you can get a lot of cost savings from reusing the RD-180, even if you end up tossing the tankage each time around. The other great thing is that this is a modification that doesn’t cost a lot of capital to develop, and can be profitable at much lower projected flight rates than building a fully reusable vehicle of this size would. This makes it much more realistic in the near-to-medium term.
In order to reap those potential savings, you have to recover the RD-180 in such a way that minimizes the amount of refurbishment work that needs to be done between flights. That’s where mid-air recovery comes in.
Their proposed scheme would partition the propulsion section of the Atlas-V (which is already largely packaged in this manner anyway) into a recoverable module that could separate from the main tanks using some sort of linear shaped charges. This module would include a hypercone-style inflatable hypersonic decelerator, and some parachutes. At the end of the stage burn, the module would close itself off from the tanks, some purges would be performed to make sure there aren’t any contaminants in the engine, then the shaped charges are blown, severing the tank and separating the module. The hypersonic decelerator slows the vehicle down and keeps heating and reentry forces on the engine module itself down to a minimum, until the vehicle is slow/low enough for the main parachutes to open. As the stage is coming down, a helecopter that is positioned just outside the potential landing zone moves to rendezvous with the slowly falling stage. Using 3GMAR (Third Generation Mid-Air Recovery) techniques developed and refined in work that Vertigo, Inc did with ULA, the helicopter can gently capture the propulsion module by the parachute, and then haul it back to a specially designed recovery cradle located either on land or on a barge out at sea.
By doing this, the recovery loads on the propulsion system can be kept much lower compared with splashdown recovery, and sea water contamination concerns can be greatly reduced or eliminated. This means that most of the stress the engine sees in operation is actually due to the engine firing, not recovery or ground handling. This makes it much easier to reuse the relatively complex staged-combustion engine without the need for extensive refurbishment. In the paper the ULA guys explain a bit about what refurbishment would be needed, and based on that they estimate that you get the most bang-for-the-buck by going for 3 reuses per engines.
There are all sorts of other interesting technical details in the paper, so I’d highly recommend reading it yourself.
Ramifications and Further Thoughts
There are several potential benefits from the proposed scheme:
- Lower Cost: The obvious benefit of this scheme is that it helps reduce one of the key hardware cost items in an Atlas-V launch. While for a single-core Atlas-V launch, the savings would be relatively modest, they would be much larger potentially for Atlas-V Heavy flights or Atlas-V Phase II flights (if either of those ever become a reality).
- Higher Sustainable Flight-Rate: Another benefit is that this allows for higher flight rates of the Atlas-V without requiring drastically higher engine production levels from the Russians. I’ve heard hints that the Russian’s would have a hard time cranking out much more than 10-15 RD-180s per year without spending a decent amount of cash on expanding their production facilities and hiring on more people. If each engine can be used three times, all of the sudden engine manufacturing rate completely supply ceases to be a constraint on launch frequency.
- Less Foreign Dependence: Right now, ULA stocks something like 2-3 years worth of RD-180s in the US at all times. The idea being that if the Russian Government decided to stop allowing ULA to buy RD-180s, that inventory would give the US enough breathing-room for Pratt & Whittney Rocketdyne to bring an Americanized version of the RD-180 into production. Honestly, I’m not sure how seriously Russia takes that threat. There’s enough tricky metallurgy and other issues with making an Amercanized oxygen rich staged-combustion engine like the RD-180, that I’m not sure the Russians really believe we could have an in-sourced version ready to fly before we’d run out of engines. Now, if that 2-3 year stockpile became a 6-9 year stockpile because of the ability to reuse the engines, all of the sudden the threat of in-sourcing becomes more real. The incentives then make it much more likely that Russia would continue to sell us RD-180s at a reasonable price, because the threat of us in-sourcing production is more realistic.
There are also some ways of continuing this line of thought that I think might be worth investigating down the road. A big one is the potential for using mid-air recovery to reuse the Centaur upper stage. The 3GMAR technique is capable of snagging payloads of up to 25klb with existing US helecopters, and the Centaur stage only weighs about 4500lb dry. Now, there are all sorts of caveats on that idea. For one, recovery of a first stage on a suborbital trajectory is much less demanding than recovery of an orbital stage. You need much more serious TPS for the upper stage, landing dispersions will likely be bigger, and the stresses on the stage may be more severe, while additional dry-weight on the upper stage has a much larger impact on payload performance. Not to mention the fact that recovering a modified Centaur stage that goes into GTO (which is quite common) will be even harder, if not impossible.
That said, the idea isn’t entirely without precedent, or even that wacky when you think about it. As it is, ULA was already studying with SpaceHab the ARCTUS concept which would’ve had a Centaur-derived COTS cargo delivery system that could be recovered from space using something similar to the hypercone/MAR concept proposed for the Atlas-V propulsion system. In fact some of the tests they discussed in the paper were tests for that very program. Also, SpaceX is planning on trying to recover the Falcon 9 upper stage. When I spoke with Elon back in January, he hinted that the approach they were looking at at the time was actually pretty straightforward–using ablator panels on the front and some of the sides of the upper stage, followed by parachutes and splashdown recovery. Now, there’s a huge string of if’s between where we stand and SpaceX successfully recovering a Falcon 9 upper stage, but between that and the work done on ARCTUS before the COTS re-bid process was over, it at least looks feasible to scale that up to recovering a Centaur stage.
The Centaur is one of the major cost items on an Atlas V flight. If both it and the first stage propulsion system could be reused a couple of times each, it could make the Atlas V substantially cheaper for moderate flight rates. Also reusing the Centaur looks even better if they ever go to the larger Phase 1 Centaur upper stage concepts. The largest one of those dicussed, a six-RL10 version with ~180klb of propellant has a predicted dry weight of about 15klb. That would leave quite a bit of weight for a recovery system. Of course, that’s assuming that recovering such a big fluffy object wouldn’t pose other problems for the mid-air recovery effort–which probably isn’t a safe assumption.
What does this all mean?
Probably not very much unless either NASA is forced by Congress to ditch Ares-I and focus on making Orion flyable on an EELV, or unless Bigelow manages to close a deal with them for a large number of flights. The current flight rates are so low that the savings provided wouldn’t pay for the added development cost. But what it does mean is that if demand starts increasing, whether due to ISS resupply, Bigelow, propellant depots, or orbital tourism, or several of the above, Atlas-V still has plenty of room for evolution to keep competitive. Would such a recovery/refurbishment scheme get rid of the need for real RLVs? No, but neither would the existence of real RLVs get rid of the need for medium lift launch vehicles.
I’ll get into it more in a later article, but I really think small RLVs and “recoverable” launch vehicles (including both the Atlas concept discussed as well hopefully as Falcon 9) are actually complimentary technologies. But more on that later…