Rocket Doc Notes for Week of July 11, 2021
My View on Space Tourism
This week we witnessed the first truly commercial space tourists flying above the U.S. Karman line and getting their Astronaut Badges. The U.S. height to traverse from the atmosphere to space is 84 kilometers which is Von Karmen’s initial hypothesis back in 1957. Since then the official Karmen line has been defined by the Fédération Aéronautique Internationale (FAI), the international records keeping organization for aeronautics, as 100 kilometers (54 nautical miles; 62 statute miles; 330,000 feet) above Earth’s mean sea level. As you probably noted, the SpaceShip Two did not achieve an altitude of 100 km so his Astronaut Record will have an asterisk indicated that he did not achieve the official international standard. Not a big deal really. SpaceShip Two is shown in figure 1 below mounted on its mothership, White Knight Two.
Figure 1 – SpaceShipTwo mounted on White Knight Two
The SpaceShipTwo project is based in part on technology developed for the first-generation SpaceShipOne, which was part of the Scaled Composites Tier One program, funded by Paul Allen. The Spaceship Company licenses this technology from Mojave Aerospace Ventures, a joint venture of Paul Allen and Burt Rutan, the designer of the predecessor technology.
SpaceShipTwo is a low-aspect-ratio passenger spaceplane. Its capacity will be eight people — six passengers and two pilots. The apogee of the new craft was designed to be approximately 110 km (68 mi), 10 km (6.2 mi) higher than the Kármán line but as of July 2021, the maximum height reached was 89.9 km. The predecessor craft, SpaceShipOne's target was also 100 km but the last flight reached an altitude of 112 km (70 mi). SpaceShipTwo was designed to reach 4,200 km/h (2,600 mph), using a single hybrid rocket engine — the RocketMotorTwo. It launches from its mother ship, White Knight Two, at an altitude of 15,000 m (49,000 ft), and reaches supersonic speed within 8 seconds. After 70 seconds, the rocket engine cuts out and the spacecraft will coast to its peak altitude. SpaceShipTwo's crew cabin is 3.7 m (12 ft) long and 2.3 m (7 ft 7 in) in diameter. The wingspan is 8.2 m (27 ft), the length is 18 m (59 ft) and the tail height is 4.6 m (15 ft).
SpaceShipTwo uses a feathered reentry system, feasible due to the low speed of reentry. In contrast, orbital spacecraft re-enter at orbital speeds, close to 25,000 km/h (16,000 mph), using heat shields. SpaceShipTwo is furthermore designed to re-enter the atmosphere at any angle. It will decelerate through the atmosphere, switching to a gliding position at an altitude of 24 km (15 mi), and will take 25 minutes to glide back to the spaceport.
SpaceShipTwo and White Knight Two are, respectively, roughly twice the size of the first-generation SpaceShipOne and mothership White Knight, which won the Ansari X Prize in 2004. SpaceShipTwo has 43 and 33 cm (17 and 13 in)-diameter windows for the passengers' viewing pleasure, and all seats will recline back during landing to decrease the discomfort of G-forces. In 2008, Burt Rutan remarked on the safety of the vehicle:
This vehicle is designed to go into the atmosphere in the worst case straight in or upside down and it'll correct. This is designed to be at least as safe as the early airliners in the 1920s ... Don't believe anyone that tells you that the safety will be the same as a modern airliner, which has been around for 70 years.
In September 2011, the safety of SpaceShipTwo's feathered reentry system was tested when the crew briefly lost control of the craft during a gliding test flight. Control was reestablished after the spaceplane entered its feathered configuration, and it landed safely after a 7-minute flight.
Safety Issues with respect to current Space Tourism Options
The topic I wish to discuss this week is human safety in the context of the different vehicle configurations being put forward for Space Tourism. There are four concepts I want to discuss: the first is SpaceShipTwo, just presented; the second is Blue Origin’s New Shepard scheduled to launch next Tuesday with Jeff Bezos on board; the third is the Falcon 9 Inspiration4 Mission scheduled to launch September 15th with an all-civilian crew; and the fourth is the Space-X Starliner currently in test and forecast to carry passengers as early as next year.
The inherent safety varies widely among these vehicle systems. SpaceShipTwo is basically an airplane powered by a hybrid rocket engine. Hybrid rocket motors have fewer failure modes than liquid rockets but if the solid insert fails mechanically and a piece falls off and blocks the nozzle, a large explosion results with a high risk of loss of crew. I notice that Sir Richard Branson was wearing a parachute which improves his odds in case of a rocket motor failure. I would rank the risk of loss of crew for SpaceShipTwo to be acceptable (best guess is 0.3 percent).
Blue Origin’s New Shepard is a classic LOX-LH2 rocket with a separable crew module that lands via parachutes. See figure 2 below. Given the backups for the crew module, the only real risk is catastrophic failure of the rocket engine resulting in detonation of the propellants. As much testing as the BE-3 rocket motor has undergone and the successful separation testing of Crew Capsule leads me to believe that the risk of loss of crew is as low or slightly lower than the SpaceShipTwo.
Figure 2 – New Shepard at Launch
Space-X’s Falcon 9 with Dragon Crew Capsule is preparing to launch tourists into Earth orbit for the first time. Their configuration is shown in figure 3 below.
Figure 3- Falcon 9 Rocket with Dragon Capsule preparing for Launch
The Falcon 9 has nine first stage rocket engines and one second stage engine, so the risk of catastrophic engine failure goes up, but these propulsion systems have a lot of successful operating time behind them, and a very capable Dragon crew escape system, so I’m inclined to rate the safety as acceptable. Maybe not quite as good as SpaceShipTwo and New Shepard but acceptable for space tourism.
The Starliner departs from the script of the previous launch systems in that it currently shows no crew or passenger escape systems. The design of Starship is changing so rapidly right now that it’s hard to find a current picture. Figure 4 below shows the a near-current launch configuration based on a floating launch platform.
Figure 4 – SpaceX Starship Launch Configuration on Floating Launch Platform
No escape systems assume the Starship components would enjoy the same levels of reliability and fail-safe characteristics as current jet airplane components. Jet engines currently undergo about 7,000 cycles without failure and before overhaul. Airframes undergo roughly 10,000 cycles before major overhaul. Space-X engines and airframes are currently operating at 10 cycles without overhaul. We have almost three orders of magnitude disconnect in demonstrated reliability to work through here.
I can see SpaceX demonstrating 100 cycles without failure in the next couple of years but there is no chance of reaching a 1000 cycles before tourists fly. Hence, the Starship is going to have to incorporate a launch escape system before carrying significant passengers. That should not be all that hard. There could be a cylindrical escape pod mounted along the Starship center line with passengers stacked like waffles inside in recliners during launch and reentry. If something bad happens the pod rockets out the nose and lands on its side using parachutes. In case of a solar radiation event in space the escape pod serves as an emergency fallout shelter. Problem solved.
As you can tell I am very excited about Space Tourism finally coming of age. I have been actively pushing for this for a very long time. Each space tourism plan appears to be well thought out with a good shot at profitability. The SpaceX Starliner is a great start for space development, but they need work a bit on safety. Time will tell.
Thanks for Reading,