Wednesday, October 20, 2010

Designing RLVs with the Lowest Life-Cycle Cost

This was the Space Shuttle we wanted:
The Shuttle parked in the hanger.  Integration for the next mission was supposed to be comparable to Southwest Airlines loading my luggage (maybe I exaggerate a little).  This is the Space Shuttle we got:

The Shuttle requires between 200,000 and 400,000 human maintenance hours between each flight! You can barely see the shuttle in the picture above because of the scaffolding surrounding and incasing the vehicle.

Shuttle experts can (and have) elaborated more eloquently than I could on the reasons why the Space Shuttle reusability goals fell so short. But as we prepare for suborbital RLV operations (and hopefully orbital operations) in the not so distant future, I wanted to discuss the implications of an interesting paper by SpaceWorks Engineering (Michael J. Kelly, et al) and its implications for the costs of RLV design & operations.

The paper is called, What’s Cheaper to Fly: Rocket or TBCC? Why?  In it, SpaceWorks compares two hypothetical RLV designs (one rocket-based and one turbine-based) and discusses the expected operational costs of both systems. Both designs made the following RLV performance assumptions:
  • Fleet of three unmanned RLV vehicles
  • Fleet flies monthly (12/yr)
  • Every 10 flights, RLVs spend 6-mo in offsite heavy maintenance facility
  • 100 nautical mile LEO orbit
  • Payload 20K lb.
What I found interesting was what ratio the paper’s authors leveraged from the Space Shuttle program to include in their analysis.  The Shuttle utilizes seven support personnel for every one technician in their maintenance and integration efforts. For every one technician preparing the Space Shuttle for its next mission, there are seven individuals supporting that technician. This support staff consists of mission specialists, engineering support personnel, etc. Using this 7:1 ratio, the SpaceWorks paper estimated the need for RLV technicians and then extrapolated the number of support personnel needed.

Using the SpaceWorks rocket-based RLV as an example, below are the costs associated with preparing the rocket for its second flight:

Ignore the exact dollars but pay attention to the percentage. 91% of all “between flight” costs is labor using the 7:1 assumption. Stop worrying about fuel cost – start creating low-maintenance designs.  Of course there are other costs that go into the price of an RLV launch: range costs, fixed cost amortization, development cost amortization, etc. But you can see how critical life-cycle costs become in RLV design discussions.

Quoting the paper, “Any program that can do better than 7:1 will probably save significant money over a program that cannot.” And “In addition to considering operational impacts when selecting engines and TPS materials, vehicle designers should strive to eliminate the need for centralized hydraulics, and for auxiliary power units.”

For example, here is what maintenance and integration costs could look like at various improvements to the Shuttle’s 7:1 support personnel to technicians ratio (all other assumptions unchanged):

I end this post with a quote from Byron Ellis, Executive Director of the Jethro Project, on life-cycle cost and Government Acquisition (just as applicable for RLV designers as Government acquisition agents):

“Executive Order 13123 requires government agencies to use life cycle cost analysis (LCCA) to minimize the government’s cost of ownership. Unfortunately, many stakeholders do not understand the concept of cost and proceed to minimize project acquisition (first) cost, rather than total project cost. However, over the life of the project, facility management cost is often two to three times higher than acquisition costs. Therefore, it is essential to design for minimum facility management cost.”


  1. All very nice, but during shuttle development back in the 1970's, NASA was specifically directed by OMB on several occasions to reduce the pace of its shuttle-related R&D program -- by several years, as it turned out -- and to chop about a quarter of its projected development costs. That this would raise operational costs was explicitly recognized, but OMB was determined to reduce the Federal budget in the 1970's rather than worry about the 1980's and later. Yes, Shuttle operations cost a bunch, but the choice was made by YOUR national leaders, not by crummy engineers.

    This isn't exactly secret knowledge, y'know?

  2. Mike:

    See this post less as a Shuttle critique and more as encouragement and direction for a new generation of RLV designers. The company or Govt that improves on the 7:1 ratio will see significant benefits.

    The Shuttle situation is fascinating though…in a sad and twisted sort of way. It must have been very frustrating for those Shuttle engineers with excellent ideas for ways to save costs to see those ideas unimplemented.


  3. Agree with your main point, but your second table is a little over-simplified. In order to build a vehicle that requires less servicing between flights, you'll likely have to give up some performance. So that vehicle may have to fly 1.3 to 2 times more to accomplish the same job. Still significant savings, but a little less so and at a cost of schedule and increased risk.

  4. Not really. Yeah, we knew the shuttle was going to be less than ideal, but it was seen as a stopgap in the first place. Clearly, the early 70's weren't going to be wonderful for space programs but things would inevitably improve with time. By 1980, folks would be back to working on interplanetary flights and a second generation shuttle would be close to hand and ....

    If we'd really been able to foretell the future, we'd have all quit engineering for banking jobs.

  5. Sean:

    Good point. All things being equal, a vehicle designed for fast turnaround would probably offer less payload than a vehicle designed to maximize payload. So I could see a scenario where such a fast-turnaround vehicle would need to fly more to deliver the same lbs to orbit. But not in all cases. The SpaceWorks paper discussed steps like removing hydraulics in favor of other solutions that significantly reduce maintenance while not increasing vehicle weight – such examples hint that the pie may not be fixed. Some solutions (not all) can both reduce maintenance and maintain lbs to orbit.

    But I also don’t think lb to orbit will be the only measure that customers use to evaluate which vehicle to use. Faster turn around times means customer fly more often, perhaps means the company could operate with fewer vehicles (costs savings), faster integration cycles may mean customers can also fly with less notice (last minute), etc.

    All of this to say, yes I agree the chart is overly simplistic. Perhaps the best takeaway from the chart in question is that if two companies compete with comparable RLV’s, the company with a better ratio for maintenance and integration labor will probably be able to make money at a lower price point.

    - Colin

  6. I think it is also important to note that the shuttles we are still flying were prototypes and never intended for the use they received. As an example of this emptying the waste tanks involves dismantling half the vehicle. If you look at the maintenance issues that come from a vehicle that was never designed to be maintained a simple redesign of the shuttle with no new technology could have reduced the rebuild costs considerably. With new technology there is no excuse for a modern RLV to cost any where near what these prototype shuttles cost.

  7. Mike:

    Forget banking. We need as many engineers as we can get! :)

    - Colin