3 ms·

What surprised me the most, the difference between being in orbit and starting to reenter is only 362 km/h but it takes a three minute burn to decelerate which

by danbruc 10y ago

What surprised me the most, the difference between being in orbit and starting to reenter is only 362 km/h but it takes a three minute burn to decelerate which I guess is a 0.5mv² thing with big m and huge v. EDIT: My intuition was wrong, this has nothing to do with a large kinetic energy due to a large velocity.

mikeash 10y ago
The reentry burn is done with the teeny little OMS engines rather than the big main engines. If you look at the back of the Shuttle (such as https://i0.wp.com/amazingstoriesmag.com/wp-content/uploads/2014/05/Space-shuttle-main-and-OMS-engines_a.jpg https://i0.wp.com/amazingstoriesmag.com/wp-content/uploads/2...), the OMS engines are the two little ones in the upper corners. Once you're in orbit, there's usually no hurry to make changes, so you don't need much acceleration. For getting to orbit, you want to stop fighting gravity as fast as possible, so powerful engines are a must.
- danbruc 10y ago
  Tiny is pretty relative [1]. I expected them to be more powerful and - mistakenly - thought the small acceleration is due to the large kinetic energy due to the large velocity. [1] https://en.wikipedia.org/wiki/AJ10 https://en.wikipedia.org/wiki/AJ10
  - mikeash 10y ago
    Right, rockets provide the same acceleration regardless of speed, since there's no way for the engine to "know" how fast the vehicle is moving. Acceleration is basically exhaust velocity times mass flow divided by vehicle mass. With an 84cm diameter and weighing about 200 pounds, I'm OK calling them "tiny."
    - danbruc 10y ago
      I missed that, I only looked at the image and there it looks way more massive and larger than what the dimensions table says. Even taking into account that the image depicts an entire stage and not just the engine, 0.84 m and 100 kg is a lot less than I would have expected or guessed based on the image.
      - mikeash 10y ago
        Agreed. I first saw the image and thought wow, that really is big. Then I saw the specs. I guess it's misleading because the engine is much closer to the camera than the background, but nothing makes that obvious. There's a tiny little person behind it, and if you don't realize the perspective difference, that would look like the engine is pretty huge.
Retric 10y ago
Rocket exhaust starts with the same velocity as the shuttle which makes the v^2 part of that equation less meaningful. They use a much smaller engines to among other things add precision.
- danbruc 10y ago
  Just realized that myself by doing the actual calculations, the Space Shuttle would need the same three minutes to come to halt from 362 km/h. In comparison, a Bugatti Veyron will do that in less than 10 seconds. So the OMS Engine are really not that powerful, at least relative to the mass of the Space Shuttle.

by danbruc 10y ago

mikeash 10y ago

The reentry burn is done with the teeny little OMS engines rather than the big main engines. If you look at the back of the Shuttle (such as https://i0.wp.com/amazingstoriesmag.com/wp-content/uploads/2014/05/Space-shuttle-main-and-OMS-engines_a.jpg https://i0.wp.com/amazingstoriesmag.com/wp-content/uploads/2...), the OMS engines are the two little ones in the upper corners. Once you're in orbit, there's usually no hurry to make changes, so you don't need much acceleration. For getting to orbit, you want to stop fighting gravity as fast as possible, so powerful engines are a must.

danbruc 10y ago
Tiny is pretty relative [1]. I expected them to be more powerful and - mistakenly - thought the small acceleration is due to the large kinetic energy due to the large velocity. [1] https://en.wikipedia.org/wiki/AJ10 https://en.wikipedia.org/wiki/AJ10
- mikeash 10y ago
  Right, rockets provide the same acceleration regardless of speed, since there's no way for the engine to "know" how fast the vehicle is moving. Acceleration is basically exhaust velocity times mass flow divided by vehicle mass. With an 84cm diameter and weighing about 200 pounds, I'm OK calling them "tiny."
  - danbruc 10y ago
    I missed that, I only looked at the image and there it looks way more massive and larger than what the dimensions table says. Even taking into account that the image depicts an entire stage and not just the engine, 0.84 m and 100 kg is a lot less than I would have expected or guessed based on the image.
    - mikeash 10y ago
      Agreed. I first saw the image and thought wow, that really is big. Then I saw the specs. I guess it's misleading because the engine is much closer to the camera than the background, but nothing makes that obvious. There's a tiny little person behind it, and if you don't realize the perspective difference, that would look like the engine is pretty huge.

Retric 10y ago

Rocket exhaust starts with the same velocity as the shuttle which makes the v^2 part of that equation less meaningful. They use a much smaller engines to among other things add precision.

danbruc 10y ago
Just realized that myself by doing the actual calculations, the Space Shuttle would need the same three minutes to come to halt from 362 km/h. In comparison, a Bugatti Veyron will do that in less than 10 seconds. So the OMS Engine are really not that powerful, at least relative to the mass of the Space Shuttle.