5 ms·

SABRE is designed for orbital flights, suborbital flight requires much less velocity. Some air breathing scramjet might be enough.

by 4ad 10y ago

vkou 10y ago
Suborbital flights require almost as much velocity if you actually want to go anywhere. Hopping up into space, and then back down to where you came from is only ~1,600 m/s. Hopping up into space from New York, and landing in London requires closer to ~6,000 m/s.
- credit_guy 10y ago
  Do you have a link with an elementary explanation of this type of numbers? I'm not doubting what you say, I'd just like to learn more about the topic.
  - vkou 10y ago
    Here's a breakdown of reaching orbital speeds, vs doing a vertical sub-orbital hop. [1] Here are some specs for an ICBM. It has a range of 6,000 miles, and has to reach ~7 km/s (Mach 23) to do so. [2] The mathematical reason for why suborbital flights that take you somewhere require so much velocity is because all ballistic trajectories are actually elliptic orbits. When you throw a baseball into the air, it is in an elliptic orbit - a very small orbit, with most of it passing through the space occupied by the earth. The Earth's center is one of the two centers of the ellipse. This means that in order to send a ballistic rocket to the other side of the world, you need to 'almost get into orbit'. At this point, it takes very little energy to turn your orbit into a circle... But it also means that you had to expend orbital-levels of energy to get to the point that your suborbital hop takes you somewhere. [3] Knowing that sub-orbital ballistic trajectories are ellipses, with one of their centers at the center of the Earth, you can calculate the velocity requirements yourself by using Kepler's second law. [1] http://www.orbspace.com/Background-Information/Suborbital-vs-Orbital.html http://www.orbspace.com/Background-Information/Suborbital-vs... [2] https://fas.org/nuke/guide/usa/icbm/lgm-30_3.htm https://fas.org/nuke/guide/usa/icbm/lgm-30_3.htm [3] http://lh3.ggpht.com/_1wtadqGaaPs/TF0zyBaNisI/AAAAAAAANLc/uU0kJHSubOQ/tmp438_thumb_thumb.jpg?imgmax=800 http://lh3.ggpht.com/_1wtadqGaaPs/TF0zyBaNisI/AAAAAAAANLc/uU...
    - credit_guy 10y ago
      Great explanation, thank you. From the links you provided (all very good), I started browsing and ended up with the book "Orbital Mechanics: Theory and Applications" by Tom Logsdon [1], which appears quite good. [1] https://www.amazon.com/Orbital-Mechanics-Applications-Tom-Logsdon/dp/0471146366/ref=sr_1_1?s=books&ie=UTF8&qid=1486922811&sr=1-1&keywords=9780471146360 https://www.amazon.com/Orbital-Mechanics-Applications-Tom-Lo...

SABRE is designed for orbital flights, suborbital flight requires much less velocity. Some air breathing scramjet might be enough.

by 4ad 10y ago

SABRE is designed for orbital flights, suborbital flight requires much less velocity. Some air breathing scramjet might be enough.

vkou 10y ago

Suborbital flights require almost as much velocity if you actually want to go anywhere. Hopping up into space, and then back down to where you came from is only ~1,600 m/s. Hopping up into space from New York, and landing in London requires closer to ~6,000 m/s.

credit_guy 10y ago
Do you have a link with an elementary explanation of this type of numbers? I'm not doubting what you say, I'd just like to learn more about the topic.
- vkou 10y ago
  Here's a breakdown of reaching orbital speeds, vs doing a vertical sub-orbital hop. [1] Here are some specs for an ICBM. It has a range of 6,000 miles, and has to reach ~7 km/s (Mach 23) to do so. [2] The mathematical reason for why suborbital flights that take you somewhere require so much velocity is because all ballistic trajectories are actually elliptic orbits. When you throw a baseball into the air, it is in an elliptic orbit - a very small orbit, with most of it passing through the space occupied by the earth. The Earth's center is one of the two centers of the ellipse. This means that in order to send a ballistic rocket to the other side of the world, you need to 'almost get into orbit'. At this point, it takes very little energy to turn your orbit into a circle... But it also means that you had to expend orbital-levels of energy to get to the point that your suborbital hop takes you somewhere. [3] Knowing that sub-orbital ballistic trajectories are ellipses, with one of their centers at the center of the Earth, you can calculate the velocity requirements yourself by using Kepler's second law. [1] http://www.orbspace.com/Background-Information/Suborbital-vs-Orbital.html http://www.orbspace.com/Background-Information/Suborbital-vs... [2] https://fas.org/nuke/guide/usa/icbm/lgm-30_3.htm https://fas.org/nuke/guide/usa/icbm/lgm-30_3.htm [3] http://lh3.ggpht.com/_1wtadqGaaPs/TF0zyBaNisI/AAAAAAAANLc/uU0kJHSubOQ/tmp438_thumb_thumb.jpg?imgmax=800 http://lh3.ggpht.com/_1wtadqGaaPs/TF0zyBaNisI/AAAAAAAANLc/uU...
  - credit_guy 10y ago
    Great explanation, thank you. From the links you provided (all very good), I started browsing and ended up with the book "Orbital Mechanics: Theory and Applications" by Tom Logsdon [1], which appears quite good. [1] https://www.amazon.com/Orbital-Mechanics-Applications-Tom-Logsdon/dp/0471146366/ref=sr_1_1?s=books&ie=UTF8&qid=1486922811&sr=1-1&keywords=9780471146360 https://www.amazon.com/Orbital-Mechanics-Applications-Tom-Lo...