Slashdot Mirror
Suborbital Spaceflight Picks Up Speed
RocketAcademy writes "The race to develop low-cost, suborbital spaceflight is heating up. On Thursday, Virgin Galactic's SpaceShip Two successfully completed its second powered test flight, reaching a speed of Mach 1.4 and an altitude of 69,000 feet. Meanwhile, XCOR Aerospace has begun posting daily reports on the progress of its Lynx spaceplane, which is expected to begin flight tests sometime around the end of this year. This means one of both companies are likely to begin commercial service by the end of next year. XCOR still plans to move its headquarters to Midland, Texas later this year, but Midland may not be the only suborbital spaceport in the Lone Star state. On Wednesday, the Houston Airport System revealed renderings of its proposed spaceport at Ellington Airport, near Johnson Space Center just south of Houston. Citizens in Space (also based in Texas) has begun training five citizen astronauts to fly as payload operators on the XCOR Lynx and evaluating biomedical sensors for use on the flights. Details of those astronaut activities were also released this week."
Play around with Kerbal Space Program and you realize just how big the gulf is between suborbital and orbital flights. Getting enough boom to get yourself up to 100km is trivial. You really appreciate the difference in design when you're doing it yourself and seeing just how much more boom it takes to achieve orbit.
I read the internet for the articles.
A single stage to orbit vehicle is very difficult. The amount of fuel a rocket needs to carry increases exponentially with the ratio of the required velocity change to the exhaust velocity. The options are very limited.
Solid propellants have much too low exhaust velocity for this to be workable.
Kerosene / Oxygen is a typical rocket propellant, but the exhaust velocity is low enough that a single stage to orbit would require an unreasonable ratio of fuel to rocket mass. I don't remember any design concepts for kerosene / O2 rockets for single stage to orbit.
Hydrogen / Oxygen has higher exhaust velocity but liquid hydrogen is very low density so the fuel tanks become enormous, and heavy. There have been design studies for Hydrogen / oxygen single stage to orbit, but it doesn't look very practical - you basically have an enormous flying fuel tank.
The more exotic chemical fuel mixes don't improve things a lot and are too toxic and expensive for atmospheric use. Fluorine / Beryllium / Hydrogen tri-propellant has good specific impulse but is insanely deadly in multiple ways.
Nuclear thermal (like NERVA) could probably do it, but people are understandably unwilling to put 100GW class nuclear reactors in rockets and launch them.
Airbreathing rockets (like ram-jets / scram jets) don't need to carry their own oxidizer and in principal do much better. The problem is that hypersonic ram-jets are very difficult to build and wind up heavy and inefficient. (Mach ~8, only 1/3 of orbital speed, 1/10 orbital energy) is the highest speed ramjet that I am aware of. For various fundamental reasons you expect the performance of ramjets to drop with velocity. Since ramjets also don't work at low speeds, you wind up needing a 3 stage rocket: conventional, scram-jet, conventional, and it ends up not having any advantage over just conventional rockets.
The basic technology for getting things into orbit hasn't changed in over 50 years, and really isn't likely to change - just no clear path. The best approach is probably what Space-X is doing, optimizing the design, and working to make each of the stages recoverable to reduce costs.
The "tourist" sub-orbital rockets don't seem to me to be developing technologies that are applicable to orbital flights. They may attract a few people who are willing to spend $100K for a few minutes of zero-G, but I suspect that most wealthy thrill-seekers will quickly find that a ride in a MIG-29 is a lot more fun and less expensive.