3 comments

• Edward

  July 8, 2018 at 4:07 pm

  Robert,
  I see additional differences between Virgin Orbit and Virgin Galactic.

  1) Virgin Galactic is producing a manned rocket, which requires far more safety assurances than for Virgin Orbit. This makes it harder to do and would be expected to take longer. After their accident in 2014 they are most likely being even more careful with safety issues.

  2) Virgin Galactic also chose, early on, a hybrid rocket engine, which comes with a problem that is inherent in solid rockets: it can act like a pipe organ in that there is a natural frequency for the gasses within the rocket body. Virgin Orbit is using a liquid fuel rocket.

  The hybrid rocket engine’s natural frequency changes as the fuel is expended and the combustion cavity increases in size or diameter. The engine may or may not vibrate at this natural frequency, but that vibration would be transferred to the payload and other sections of the rocket. This kind of potential vibration was one problem that the Constellation Program’s Ares rocket had.

  This can be a major safety issue, because human bodies also have natural frequencies of different parts of the body. Individual internal organs can vibrate at around 20 or so hertz. Strong vibrations from 8 to 80 hertz can be bad for humans, as body functions can be impaired, bones can break or separate from each other, and organs can mash against each other until they are mush. The spinal column is also vulnerable. Certain sounds, such as helicopter rotor chop, can be strongly felt in the chest and body; even sonic vibration can induce sympathetic vibrations in the body.
  https://physics.stackexchange.com/questions/37543/does-the-human-body-have-a-resonant-frequency-if-so-how-strong-is-it (Please note that the values in the figure have not been fact-checked, but it gives an example of a model.)

  http://www.donmar.com/Tech/VIBRATION-BODY.pdf

  The structure of the rocket can also be badly affected by vibrations, as even the rocket parts have their own natural frequencies. Either these frequencies must be avoided or the rocket parts must be beefed up (read: “weight added”) in order to be strong enough to withstand the forces associated with these vibrations.

  Virgin Galactic has considered injecting helium gas into the rocket engine in order to change and control the natural frequency of the engine, but that reduces the maximum altitude that they can achieve. I hope that in the three years since the accident that they have redesigned their engine so that it can perform as desired without the vibration problem.
• Robert Zimmerman

  July 8, 2018 at 4:28 pm

  Edward: Your points are well taken. However, it is my belief that some of Virgin Galactic’s problems come less from engineering and more from some management decisions early on that have hampered the spacecraft’s development. For one thing, it is my impression that their choice of the hybrid engine came not because of engineering considerations but because Richard Branson wanted to build a “green” spacecraft. If true, this decision was not based on picking the best engine system for the spacecraft, but Branson’s personal whim.

  Once they decided to begin building Virgin Orbit, the engineers could tell Branson to go jump in a lake. So could his backers, who needed to build this fast and well to recover some of their investment.
• Edward

  July 8, 2018 at 4:47 pm

  “… Richard Branson wanted to build a ‘green’ spacecraft. If true, this decision was not based on picking the best engine system for the spacecraft, but Branson’s personal whim.”

  I recall some amount of pride Virgin Galactic had in their choice to change fuel types from what was used in SpaceShipOne. It reminds me of Congress setting design constraints on the SLS designers, similarly resulting in delays in reaching first launch.