Engineering simple solutions to complex problems
by Eric R. Hedman
|The ability to come up with simple creative solutions to complex problems is an all too rare skill.|
When the basic configuration of the space shuttle was decided upon, it was a compromise based on competing demands. It was supposed to be the workhorse for both NASA and the military. This is not unlike many other engineered products. A car is a compromise of competing demands. A low center of gravity improves stability while higher driver’s seats provide better visibility and situational awareness. Crumple zones protect the passengers, but also make repair costs higher in moderate energy impacts.
One way of allowing a simpler solution is to simplify the mission specifications. When the shuttle was being designed, space was perceived as a very expensive and optional endeavor and it was unlikely that more than one vehicle was going to get funded for development. I remember reading an article in Popular Science that touted all the advantages of the do-everything space shuttle. It was supposed to fly fifty to one hundred times a year and dramatically reduce the cost of delivering a pound of cargo to orbit. Expensive satellites were supposed to be regularly and affordably serviced in orbit or returned to Earth for major repairs. The argument that it was going to radically reduce costs is probably what got it through Congress. It is easy to call the sales pitch naive after the fact. Monday-morning quarterbacking is an easy-to-do national pastime.
People have a bad habit of comparing space travel with air travel. That’s like comparing Formula One racing with go carts. Space travel is still experimental and will be for a while to come. Nearly fifty years after the first manned flight out of the atmosphere, the total number of all manned spaceflights is still in the hundreds. Fifty years after the first flight at Kitty Hawk, there had already been millions of takeoffs and landings. Fifty years into aviation there were far more fatal plane crashes, killing cumulatively thousands of people, than there have been flights into space. Aviation has become very safe due to countless test flights and rigorous crash investigations. It is easy to forget the price that has been paid to get to our current state of safe air travel.
All the problems with Discovery’s flight tell us we haven’t yet done what it takes to fully understand the flight envelope. One of the reasons we haven’t worked out all the issues aviation has is there have been far fewer designs flown and tested. I am not alone in suspecting that the new arrays of sensors to monitor the shuttle are detecting problems that existed all along in the shuttle program. We just were not aware of the seriousness and frequency of the problems.
|The shuttle hatch swings out and requires complicated mechanisms to seal it for flight. The SpaceShipOne hatch has no moving parts and opens inward. It is held in place by the cabin pressure.|
Quite often, but not always, the simplest design that will work is the best solution. At the EAA I had a chance to ask both Mike Melvill and Brian Binnie how they find engineers to work at Scaled Composites that do not over-engineer solutions and fit within their culture. Mike Melvill said that they try to find people that are not yet set in their ways of doing things. After sixty days they review each new employee and decide if they fit. If they don’t, they part ways. He also said that they do not have many hard and fast rules on how things should be done. I can imagine that is, in part, what leaves room for creativity. He did say they look more for generalists than specialists. A generalist that can understand more of the system being developed will have a better understanding of how what they do affects the rest of a vehicle.
Brian Binnie told me that an example of how they try to come up with simple solutions is the hatch on SpaceShipOne. He compared it to the multimillion-dollar hatch on the shuttle. The shuttle hatch swings out and requires complicated mechanisms to seal it for flight. The SpaceShipOne hatch has no moving parts and opens inward. It is held in place by the cabin pressure. He said it probably cost no more than a couple hundred bucks. Comparing it to the hatch on the shuttle is in part comparing apples to oranges, but does bring up a valid point about the value of a simple solution.
As advanced as we think we are, there are still significant portions of the flight regime that are not as well understood as they need to be. There are no wind tunnels that can be used to run long sustained tests in the Mach 10 to 24 ranges. We currently cannot fly a chase vehicle to watch closely as another vehicle enters or leaves the atmosphere. Computer models using computational fluid dynamics can only be validated by indirect inferences. If the overall results on vehicle performance through these flight regimes matches calculated values within in a certain degree of accuracy we have to trust that the models are accurate enough. Unlike wind tunnel test for airliners, there is no way to confirm exactly if or how vortices and shock waves form off of wing tips or other geometries. Weather forecasting models are continuously improved by comparing actual readings with predictions. Complete, accurate readings of hypersonic flight are still not possible to get.
An example of an unanticipated circumstance in a flight regime happened at the EAA with the mated pair of White Knight and SpaceShipOne. After attending the opening day on Monday July 25th I decided to return on Friday the 29th when White Knight was supposed to take to the air to open the afternoon air show. When I arrived, though, the flight had been canceled for a couple of reasons. One was a torrential rain storm that happened on the evening of the 25th. That evening I listened to a presentation put on by Mike Melvill and Scott Crossfield in the Theater in the Woods. The theater has a roof but no walls. During the presentation, the skies opened up with an intense long-duration rain storm. After the presentation was over I walked past the mated pair parked in the open. It was an incredible sight, lit up in the intense rain by two banks of spotlights put in place for security. A Hollywood lighting director couldn’t have done a better job of illuminating the vehicle in a more dramatic fashion. Due to the storm, the lack of a tripod, and my fear of close-by lightning, though, I was unable to get a picture.
|Burt Rutan said the SpaceShipTwo will probably have fifty test flights before commercial passenger service is offered. They intend to get both White Knight Two and SpaceShipTwo FAA-certified for commercial service.|
Later in the week when they opened up the vehicles they had to drain nearly 40 liters of water out. Water had penetrated between the layers of glass in the windows of both vehicles. After draining the water, the windows of both vehicles were still fogged over. The fogging was one of the reasons they canceled the demonstration flight. Mike Melvill told me that on the flight to Washington after the show that they were going to carry a hammer in the cockpit. If the windows fogged over they were going to smash one or two out so they could see to land. White Knight and Spaceship One were designed and flown from the desert in Mojave, California, where torrential rain is not a factor. I don’t know where the rain penetrated the vehicles. One possible place could have been the hatches which are designed to seal when pressurized, not when parked on a tarmac.
One simple solution to one part of the flight regime used by SpaceShipOne is its technique of bending up its wings to get the vehicle to be aerodynamically stable during atmospheric entry. This has an added advantage in that it slows the vehicle quickly while the vehicle is still high in very thin air and eliminates the need for complicated controls for stabilizing the vehicle. This also minimizes the heat soak the vehicle is exposed to and eliminates the weight and expense of a heat shield. I asked Brian Binnie if the atmospheric entry method would work with a vehicle returning from orbital speeds. As I suspected, he said that they had not figured out a way to make that technique work for such high speeds with the need to bleed off so much more kinetic energy.
Burt Rutan said the SpaceShipTwo will probably have fifty test flights before commercial passenger service is offered. They intend to get both White Knight Two and SpaceShipTwo FAA-certified for commercial service. Shortly after I returned on Friday to the area where SpaceShipOne was parked, Secretary of Transportation Norm Mineta, arrived ,wearing a straw hat with the FAA logo on it and flanked by a couple of Secret Service agents. He spent about 45 minutes in an intense conversation with Burt Rutan. I asked Brian Binnie if he was lobbying the secretary. He said, “Probably. Is Burt smiling or looking concerned?” “A little of both.” was my answer.
Later that day I had a chance to talk extensively with a pilot at the Cessna display. He was showing me the CJ3 business jet. He explained in great detail how the FAA aircraft certification process worked. He explained some of the standards an aircraft had to meet to get part 23 and part 25 certifications. They are why aviation is as safe as it is. I asked him how he thinks that would translate to certifying vehicles for commercial passenger carrying spaceflight. He wondered that, as good as the FAA is at validating the safety of the jet he was showing me, if they even had people that would know what standard to set for spaceflight. I can imagine that was part of the discussion Burt Rutan was having with Norm Mineta.
|The path taken with the shuttle, while not ideal, is probably the only one that was politically acceptable at the time it was started.|
Vehicles designed after the shuttle have an advantage in that there is now a vast bank of knowledge in the public domain of what works and what doesn’t work. The shuttle, while being complex and fragile, has many components that are robust and work very well. The solid rocket boosters have worked flawlessly since the redesign of the O-rings and the clevis and tang joints that failed in the Challenger disaster. If vehicles are given a chance to mature, solutions emerge that make the vehicles safer and safer. Even so, after a certain age a vehicle does need to be retired and replace with one that builds upon the lessons learned from its predecessors.
Considering what the Air Force A-10 pilot said to me and after thinking it over, I am in favor of letting the shuttle complete its mission of building the International Space Station and retiring gracefully. The return to flight is proving that it is still an experimental and risky vehicle and it will run into problems. The path taken with the shuttle, while not ideal, is probably the only one that was politically acceptable at the time it was started. The path being laid out for the shuttle’s replacement and the new missions for it are promising. The designs being proposed are simpler and take advantage of the robust proven components and capabilities of the shuttle. I for one am optimist about the future of both government and private space programs in this country. The fact that we will have both improves our odds of success.