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But, while we are waiting patiently, I thought it might be interesting to ponder one of the more obvious outcomes of continued intensive development of civilian sub-orbital spacecraft, the executive spacecraft. You know it's only a matter of time. Let's start with the mission critical stuff first, the interior. Sadly, the first generation of executive spacecraft won't have the weight allowances for a case of Dom Perignon, nor the storage space to tuck away a bag of golf clubs. No galley, no lav. Probably two passenger seats (pods, really) and two crew. No flight attendant. The seats aren't even going to have leather upholstery--too heavy. Think Spartan, though not a Frank Miller Spartan. A key development will be the oxygen mask that efficiently whisks away the evidence of passenger vomiting during the zero gravity portion of the flight. Avionics and automation issues Continuing forward to the flight deck, cockpit designers will specify many of the usual avionics boxes. Obviously, we will still need a gyrocompass, artificial horizon, and the usual suite of basic flying instruments, modified or enhanced for sub-orbital flight. We will need a fast, precise 3-D navigation system that can handle the typical flight profile, probably GPS-based, but we'll need a back-up method, too--manually tuned ADF is out of the question. At the peak speeds we anticipate (Mach 5 at a minimum), an integrated flight management system seems required, which implies an autopilot and auto-throttle, if we use the conventional paradigm. Given the short duration of these flights, crew tedium isn't likely to be an issue. But, because of how quickly things can go wrong with a rocket-propelled hypersonic spacecraft, I still prefer having the pilots in the loop driving the spacecraft instead of simply monitoring the systems, waiting for the computers to call for help. At the very minimum, I'd like a highway in the sky (HITS) flight director interface. And if we could just project all of the relevant symbology onto the pilot's helmet visor, completely eliminating the conventional panel, that would help. Traffic management On the flip side of the avionics coin are the air navigation and air traffic control systems. Pilots of aircraft today have to deal with a number of different air traffic control facilities, switching frequencies as they leave one area of control and enter another. I would prefer to avoid that with sub-orbital spacecraft. I want one channel per spacecraft talking to one controller who follows the flight from beginning to end. It isn't like the sky is crowded with sub-orbital spacecraft, after all. But, if something happens during the flight it will almost certainly require immediate action by the crew or the controller or both. It makes a lot of sense to have a single point of contact for everything ground related given the sparseness of sub-orbital flights, their short duration, and the small window of opportunity to cope effectively with an in-flight problem. There will be a huge international airspace planning effort to establish corridors for sub-orbital spacecraft launches and descents. This might become somewhat complex if the sub-orbital craft are mixed in with conventional aircraft at large airports. This, I believe argues for either separate spaceports, which I do not care for, or for a dedicated spacecraft runway attached to the airport. While I do prefer a dedicated runway at an existing airport, I do recognize the limits for expansion and accept that sometimes spacecraft may well be integrated into conventional air traffic. Noise Planning for the noise-abatement takeoff corridor will make airspace planning slightly more complicated, but it really is necessary. No one enjoys a sonic boom at low altitudes. Speed control during second segment climb will also be a major factor in the airframe and engine design. To be acceptable to regulators for operations near densely inhabited areas, the executive spacecraft will almost certainly need to fly efficiently at subsonic speeds at least to 36,000 feet, getting above the tropopause. Higher is better, of course. Departures over water might be fine for early supersonic accelerations, depending on the location and direction of travel. Approaches and landings should be very quiet as the spacecraft will be subsonic well before it gets below 10,000 feet, and without engine power, it's a glider. There has also been a fair amount of research in the area of sonic boom suppression in the past twenty years or so. Gulfstream, for example, continues to push ahead with this sort of thing in hopes of finding a magic solution that would enable them to build a supersonic business jet. Their collaboration with NASA on Quiet Spike won an award back in '08. Northop-Grumman did the Shaped Sonic Boom Demonstration (SSBD) back in '03 and '04, in hopes of creating either a business jet or a “global strike platform.” Boeing has been working the same issues for their HSCT, probably since 1972 or so. This is a very difficult problem to solve, and so far, there are no good solutions, though there has been some success at minimizing the shock wave by spreading it out. SSBD claims a reduction in sonic boom noise of about 1/3. The XCOR Lynx configuration appears to have benefited from the SSBD work. The other noise issue will be the engines. Engine exhaust noise is driven by shear between the air and the exhaust plume. The noise is proportional to the sixth power of the velocity difference, which means that small reductions in the relative velocity between layers gives big benefits to the noise folks. Conversely, small increases make the exhaust much louder. There is no exhaust plume more problematic than that of a rocket. It'll take some work and ejector nozzles look promising. If we are carried by a conventional jet mothership to altitude before being dropped, then no problem, we are well above the houses when the rocket fires. If we use a multi-mode rocket engine for takeoff, it will be horrifyingly loud (worse than a B-52F on takeoff), unless a lot of work is done to minimize the exhaust noise, which will cost a lot of money. Alternatively, we can add a conventional turbofan engine for takeoffs and accept the weight penalty, or admit that Rutan is right and go with the mothership system. The need for speed The entire point of a sub-orbital executive transport is to put someone on the ground far, far away very, very quickly. One does not take a sub-orbital flight from New York to Philadelphia, nor even San Francisco to Los Angeles. The point of sub-orbital flight is to go several thousand miles in about an hour from boarding, generally continent to continent. A flight from Chicago to Mumbai (8,054 miles) would make perfect sense, as would one from Bentonville, Arkansas to Shanghai, China (7,237 miles). This requirement for raw speed is where the spacecraft excels. At a minimum, we get Mach 5. With a bit of work, Mach 6 or 7 can be achieved, making even the Mumbai flight no more than two hours long. Better engines and fuels could get us to Mach 10 or 12, dropping the flight time to around an hour. Contrast this with the time needed for a conventional jet of around fourteen hours. This speed allows for an executive to day-trip to the other side of the world, returning before the end of the business day, task accomplished. That has considerable value. Raw speed in flight is one thing, but minimum total trip time is another important aspect of our executive spacecraft. I think it is reasonable to expect a one hour turn time. Turn time being the amount of time it takes from the passengers getting out till the spacecraft is ready to depart again. This encompasses only the technical issues associated with the spacecraft itself, refueling, replenishing oxygen, cleaning the cabin, minor maintenance, etc. Part of that turn time is the need for additional systems or infrastructure. If we are flying something similar to SpaceShip One or SpaceShip Two, then we would have to have another mothership on hand to carry us to altitude. If our design can takeoff and climb out under its own power, then we have more freedom of action and fewer things to coordinate, allowing a faster total trip. Cross-range performance Another key to the utility of this executive spacecraft is what the militarists called "cross-range performance." That is, how far the craft can maneuver away from the end point of its ballistic path. There is an energy consideration. Using just the tiniest bit of vector math, we need the energy to go straight up, potential energy, plus the energy to create velocity towards our destination, kinetic energy. The total energy needed is easy to calculate; it is the sum of two parts. The potential energy is simply a function of the height and the mass of the spacecraft. The kinetic energy is a function of the mass and change in velocity of the spacecraft. That change in velocity, called delta V, is often used all by itself in orbital mechanics work, but for us, energy is the important quantity because that tells us how much fuel we have to have to make the mission. Different flight profiles have different required fuel loads. My point is that with good to excellent cross-range performance, the spacecraft needs less energy because the ballistic end point will be short of the actual destination, and the spacecraft's cross-range performance will be utilized to glide to the airport (or spaceport). Therefore, the better the cross-range performance, the less energy (fuel) required for any given mission. Essentially, the better the cross-range performance, the longer the effective range of the spacecraft with full tanks. There are things to watch for, of course. Cross-range performance depends largely on being shaped like an airplane. In general, the more it looks like an airplane, the better the cross-range performance, but airplanes usually have poor shapes for rapid supersonic acceleration through the dense lower levels of the atmosphere. The better the shape is for cross-range performance, the more difficult it will be to accelerate through the dense lower atmosphere. There will have to be trade-offs. The engine it would be best if the engine were built by a company that specialized in sub-orbital spacecraft engines. I understand we are at the Wright brothers stage of civilian sub-orbital flight. No, I do! But, have we learned nothing in the past century? Engine development will go much faster when a dedicated engine company is in charge. The difficulty we face right now is that there is no single rocket engine design being pursued. Each spacecraft company is designing their own, from scratch. And they have no alternative. There is no company out there from whom they can buy (at anything approaching a reasonable cost) a tested, reliable rocket engine that matches their requirements. I prophesize that the first really successful manufacturer, be it XCOR or SCALED or whomever, the engine development will have to be spun off into its own company. When the resources of four or five or more spacecraft companies are scattered about, developing their own particular engine, their progress must, of necessity be relatively slow. If there were a single company funded with all of the allocated resources of all of those same spaceship companies, progress would be much faster. We could have the sub-orbital equivalent of the Liberty L-12, designed for high power to weight and for mass production. I would add high reliability and high availability, too. For the non-aviation junkie, the Liberty L-12 was the standard aviation engine for the US during World War I. It was designed to be modular, powerful, and easily manufactured with the technology available. More than 20,000 were built by a wide array of companies. The Liberty was the obvious choice for airplane designers for years even after the end of the war. They were powerful, proven, available, and affordable. That last characteristic will be important for us since we aren't a nation state. Flight profile The engine and fuel are really important because of another critical requirement for our executive spacecraft. We can't put our passengers in g-suits and ram them into space at four and five gravities for extended periods of time. Our passengers are astronauts in name only. We're talking older, out of shape males who may be flying with a much younger and quite athletic assistant. Two to three gravities of acceleration with them loaded into carefully designed and adjusted acceleration couches is optimal. We are carrying CEOs, not fighter pilots, and burning a bit more fuel is a perfectly acceptable price to pay for a flight profile that maximizes their comfort and odds of survival. As I mentioned above, the more energetic the fuel, and the more efficient the engine is at converting that fuel into thrust, the better the performance of the spacecraft. If the engine were multi-mode, i.e., air-breathing while down low, and going on oxidizer only when there isn't enough air, that would be another huge coup, reducing the amount of oxidizer that would have be loaded on board. This kind of engine might have an interesting flight profile, where it starts as a rocket until it gets enough speed where it can function as a ramjet, which it uses to very high altitudes, where it switches back to rocket mode. The air-breathing aspect is very advantageous when flying a low-g profile because it allows much more economical operation. A number of different multi-mode rocket engine configurations have been studied since the mid '60s, some quite elaborate. I think our executive spacecraft needs an engine like this. Industry challenges Back to the delta V and energy for a moment. The amount of fuel on board is a function of how energetic it is and how efficiently the engine uses that fuel to produce thrust as well as the desired range. With any given engine/airframe combination, the more fuel on board, the greater the potential delta V and therefore, the greater the potential range of the spacecraft, up to the point where the spacecraft gets to be too heavy and underpowered. Much design work is put into closely matching the airframe to the engine's capabilities and matching both to the desired mission. While the fuels we have currently have an acceptable energy density, a great deal of time and effort will have to be spent to develop new fuels, ones that are not just more energetic, but also sustainable, non-polluting, non-hazardous to handle, etc. We really need new fuels as well as new engines. Our executive spacecraft will have to be designed and built for less than we pay for it. Otherwise, there is no profit and without profit there are no investors and without investors, we can't get enough cash together to do the next iteration and without the next iteration, the market dies and our extremely sexy and high status sub-orbital spacecraft becomes a lawn ornament for a junior college in Terre Haute. Properly, our very useful executive spacecraft is simply one model in a series of models. Think of airliners. Don't get bogged down into the historical details of the Fokker F.VII versus the Ford Trimotor, jump to Boeing in the latter part of the 20th century with model after model of airliner, each a technical improvement on the previous, each designed for a specific market, each easily customized ('easily' in the industrial sense of the word) for the specific operational needs of the operator. This is what we want: a design that is a derivative of, and an improvement on a very successful previous design. We want to buy something that is itself quite new, but is based on proven technology. We want a product built by a stable company with a good track record. In short, we need a stable, profitable sub-orbital spaceship industry. And that's what Virgin Galactic is working on. I wish them the very best. Terry Drinkard is currently consulting on an aviation start-up. His interests and desire are being involved in cool developments around airplanes and in the aviation industry. Usually working as a contract heavy structures engineer, he has held positions with Boeing and Gulfstream Aerospace and has years of experience in the MRO world. Terry’s areas of specialty are aircraft design, development, manufacturing, maintenance, and modification; lean manufacturing; Six-sigma; worker-directed teams; project management; organization development and start-ups. Terry welcomes your comments, questions or feedback. You may contact him via terry.drinkard@blueskynews.aero Other recent articles by Terry Drinkard:
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