Tuesday, February 20, 2007

Pushing The Limits

We frequently hear about aerospace projects struggling with weight gain, cost overruns, insufficient payload capacity or insufficient performance. When we hear of these kind of problems (or experience them ourselves) we often don't think too much of it - It just seems to be an inevitable part of the business. However it's worth asking ourselves every now and again, why does this happen so frequently and why don't the project managers simply start out with easier goals and less strenuous performance requirements? To answer the question, I've come up with the following list of legitimate reasons why aerospace projects so frequently push the limits of performance and capability:

-A military system must be designed to be operationally superior to other systems which it may face off against in the event of a conflict. For example, the F-22 Raptor should be superior to the Sukhoi Su-35. As an alternative example, the F-117A Nighthawk should have very good survivability against most established radar based defence systems.

-A commercial system should at least be equivalent in price and performance to competing systems, for example the Boeing 777 as compared with the Airbus A340.

-The system will be entered into contests, eg races. Any formula One or Indy 500 car is a good example of this outside of the aerospace industry.

-The company or organization with the task of developing the system was given a requirement including a 'maximum' or 'minimum' word, eg lowest cost, maximum possible payload, etc.

-The design team found that for some reason, the most marginal solution in one respect is clearly the optimum solution in almost every other way. In practice this will virtually never happen.

-The requirements 'happen' to specify a system which the technology will only marginally allow for, given the available budget.

Clearly there are some cases where pushing the envelope is desirable or necessary, but no sensible engineering manager would take on that kind of risk without good reason. It involves extensive test and validation of design concepts throughout the design process. It may also involve engineers squirrelling away weight and performance reserves in various subsystem designs, so that when the design budget is inevitably maxed out, they can dust off design B which might have significant penalties in other areas such as material and manufacturing cost. Significant rework, unexpected development cost increases and compromises in manufacturing and operating costs in order to meet the required performance parameter may also be unavoidable. Does this sound familiar?

Fortunately there are cases where pushing the envelope is unnecessary and therefore practically speaking a Bad Thing. Usually this is the case when the project is the only one of its kind. There are currently two good examples of this within the New Space industry. First is the Bigelow space habitat project. There are two ways in which this project is comfortably overengineered. The first is the choice of launch vehicle. The Dnepr vehicle has a payload capacity of 4500kg, which is more than double the mass of the Genesis I. The second is the design for the outer skin, which is 15cm on the Genesis I and will be about 40cm thick for the Nautilus. I suspect the bugs inside the next prototype will be safer from collisions than I am in my (800cc) GM-Daewoo Matiz on a Korean expressway!

The second example is Scaled Composites' SpaceShipOne. It was only required to reach 100km above sea level but had a demonstrated capability of reaching about 112km. This extra margin may have helped play a part in the success of the initial X-Prize flight in which it barely reached the required 100km altitude due to control issues.

Now let's consider a design that is marginal - NASA's Ares 1 launch vehicle and Orion capsule combo. At the current stage of development, with the system still in the design stage, the weight for the Orion vehicle is estimated at an effective 50,231 lb (with the launch escape system not included entirely because it is jettisoned well before the payload is placed into orbit), however the absolute weight limit for the Ares 1 to deliver a payload to the ISS at an orbital inclination of 51.6 degrees is 52,405 lb, with a margin of 2175 lb. That's only 4% and at this stage of the design process. Additional weight gain is quite possible. Scott Horowitz , NASA's Associate Administrator for Exploration Systems Missions Directorate (ESMD), in a recent media briefing talked about various ways they could reduce weight and improve performance, in a way that seems very similar to what I just described.

From my perspective, it seems strange that the Ares/Orion design is pushing the limits to the extent it does. Certainly there are no known competitors. The origin of the requirements for the Constellation program is a complex story and is outside the scope of this post. However it seems certain that NASA at least partly responsible for the requirements interpreting President Bush's Vision for Space Exploration, so they cannot with 100% legitimacy claim that an inconvenient set of requirements was hoisted on them.

Nevertheless, Scott Horowitz and Jeffrey Hanley, the Program Manager for the Constellation Program seem to be treating the entire situation as entirely normal and business as usual.

In my opinion there are two reasons for NASA's approach - one kind of legitimate and the other not. First, from my perspective, frankly NASA's development paradigm appears to be a poor one. There is no reason why any development program, even in aerospace, needs to push any boundaries unless there is a particular need for a superlative system, for the reasons listed above. Surely NASA projects are no exception.
Second, with a bit of mind bending, one can see that NASA in fact has a competitor- not a present day project under the control of another government in a race to land a crew on the Moon first. As far as I know, there is no other credible crewed lunar exploration project in existence. Their only competitor is in fact the Apollo era, 1960's NASA-past. If NASA of today can't deliver a much cheaper, better exploration program than Apollo they risk seeming, to the rest of the world, to have been unable to make any observable progress in opening up the space frontier since the end of Apollo. Of course they can lose as much credibility from a blown out budget as from poor public perception of technological progress since the 1960's. It's a risk they seem willing to take.

If I had more time I would have liked to join all the other bloggers in looking at Ares alternatives, but since I don't, I will merely comment that from my point of view it seems that NASA would have been better off finding a solution for Ares that is at least 20% bigger or made Orion 20% smaller from the very start.

On the other hand, at the end of the day the ultimate engineering test for an engineer is how well he or she delivers. If Ares/Orion works out, NASA will have quite a capable system with just enough performance to fulfill its main purpose, which is all they need to outdo the ghosts of the past.


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