4 ms·

Evacuating the barrel wouldn't do much. It's nice to skip 150m of air resistance, but when you emerge from the barrel, you still have 15 km to go.

by comicjk 11y ago

nickff 11y ago
The main problem with having an air-filled barrel is that when the projectile is going faster than the speed of sound, it is compressing all the air in front it, all the way to the end of the barrel. Once the air can no longer get out of the way, it is along for the ride (until outside the confined spaces of the barrel). Carrying 100m worth of air will heat up the barrel and projectile (as the air is being compressed), as well as adding a great deal of friction.
- talmand 11y ago
  I would be curious about any potential differences between the two once the projectile leaves the barrel. Would all that super-heated air being shoved out ahead of the projectile make a difference in the shock of it leaving the barrel?
  - nickff 11y ago
    There would be an incredibly loud sonic boom when the projectile left the unevacuated barrel, but since the projectile is going faster than the speed of sound, it would not benefit from the pressure wave; in fact, it would probably slow down, and possibly be destabilized when it penetrated the thick layer of (possibly plasma-phase) air. It is also likely that when the pressure wave left the barrel, the entire barrel would shake itself to pieces (unless it had some way of damping the axial waves going back and forth through the barrel, akin to a 'water-hammer' effect).
- arprocter 11y ago
  I'm not sure how/if it scales, but here's an article about different lengths of 5.56mm barrels, with graphs of pressure and velocity: http://www.sadefensejournal.com/wp/?p=1093 http://www.sadefensejournal.com/wp/?p=1093 In reference to the second graph on the page, mach 1 is apparently 1125.33 fps Do we know if the space guns are rifled or smooth bore?
  - nickff 11y ago
    Smooth bore as far as I know. Even tank cannons are mostly smooth-bore these days. The problem with those graphs is that they are not optimizing propellant quantity for each barrel length. Adding an axis for quantity of propellant shows that you get diminishing returns to increasing barrel length and propellant, but there is a limit to how much you can increase both without increasing the barrel's thickness (as well as the firing mechanism's strength). You also have to remember that a gun will require > Mach 25 to get to orbit, and air acts differently at that kind of speed (, and once past ~Mach 3).
    - arprocter 11y ago
      Thanks for the insight - I guess it doesn't scale! Is there a reason the space gun would be at a 45 degree angle and not pointed vertically? I know the reason rocket launches happen as close to the equator as possible, but they still go straight upwards...
      - nickff 11y ago
        Rockets only go up for a little while; most of the propellant is used to build up speed in the 'horizontal direction'. There have been some arguments that a strictly vertically-firing rocket would be best for going outside of Earth orbit, but the current strategy for all launches is to go up until outside the thickest part of the atmosphere, then go horizontal, to build up to orbital velocity.
        arprocter 11y ago
        After reading about the end of of Project HARP - it seems like they intended to use the gun to fire a rocket, and after a certain altitude the engine would ignite to get into space.
        nickff 11y ago
        Yes, and in the years since HARP, there have been a few rocket-assisted projectiles developed (though none which have been intended for space launch).[1] I should also mention that unless the projectile was fired at or above escape velocity (intended to leave the Earth's gravity well), it would require some onboard propulsion to 'circularize' its orbit (and avoid crashing back into the Earth on its first time around). [1] https://en.wikipedia.org/wiki/Rocket-assisted_projectile https://en.wikipedia.org/wiki/Rocket-assisted_projectile
ajuc 11y ago
Would shooting a powerful laser to heat the air in the path before launch help?
- devenson 11y ago
  Or perhaps a small rocket.
jasonwatkinspdx 11y ago
That 150m of air is constrained into a column by the barrel. It is a big deal and light gas guns commonly evacuate the barrel.
trhway 11y ago
evacuating barrel will help reach the speed. Evacuation is very easy to do, yet helps tremendously. The gun can be built somewhere in the Andes, at 4 km altitude. It can be pretty long there. Plus using either ablative head - like new Russian-Indian air missiles - or small forward jets - similar to Russian 200miles/h torpedo - will help to decrease atmospheric speed losses. The 6.5 Mach gets you to 180km altitude from powerful regular gun - https://en.wikipedia.org/wiki/Project_HARP https://en.wikipedia.org/wiki/Project_HARP : "On November 18, 1966 the Yuma gun fired a 400 lb (180 kg) Martlet 2 projectile at 7,000 ft/s (2,100 m/s)[1] sending it briefly into space and setting an altitude record of 180 km (590,000 ft; 110 mi);" That means that it was going at least 5 Mach after reaching 30km altitude. Doesn't looks like such a big loss of speed in the atmosphere - only about 1 Mach due to atmosphere as the other 0.5 Mach - due to gravitation.

Evacuating the barrel wouldn't do much. It's nice to skip 150m of air resistance, but when you emerge from the barrel, you still have 15 km to go.

by comicjk 11y ago

Evacuating the barrel wouldn't do much. It's nice to skip 150m of air resistance, but when you emerge from the barrel, you still have 15 km to go.

nickff 11y ago

The main problem with having an air-filled barrel is that when the projectile is going faster than the speed of sound, it is compressing all the air in front it, all the way to the end of the barrel. Once the air can no longer get out of the way, it is along for the ride (until outside the confined spaces of the barrel). Carrying 100m worth of air will heat up the barrel and projectile (as the air is being compressed), as well as adding a great deal of friction.

talmand 11y ago
I would be curious about any potential differences between the two once the projectile leaves the barrel. Would all that super-heated air being shoved out ahead of the projectile make a difference in the shock of it leaving the barrel?
- nickff 11y ago
  There would be an incredibly loud sonic boom when the projectile left the unevacuated barrel, but since the projectile is going faster than the speed of sound, it would not benefit from the pressure wave; in fact, it would probably slow down, and possibly be destabilized when it penetrated the thick layer of (possibly plasma-phase) air. It is also likely that when the pressure wave left the barrel, the entire barrel would shake itself to pieces (unless it had some way of damping the axial waves going back and forth through the barrel, akin to a 'water-hammer' effect).
arprocter 11y ago
I'm not sure how/if it scales, but here's an article about different lengths of 5.56mm barrels, with graphs of pressure and velocity: http://www.sadefensejournal.com/wp/?p=1093 http://www.sadefensejournal.com/wp/?p=1093 In reference to the second graph on the page, mach 1 is apparently 1125.33 fps Do we know if the space guns are rifled or smooth bore?
- nickff 11y ago
  Smooth bore as far as I know. Even tank cannons are mostly smooth-bore these days. The problem with those graphs is that they are not optimizing propellant quantity for each barrel length. Adding an axis for quantity of propellant shows that you get diminishing returns to increasing barrel length and propellant, but there is a limit to how much you can increase both without increasing the barrel's thickness (as well as the firing mechanism's strength). You also have to remember that a gun will require > Mach 25 to get to orbit, and air acts differently at that kind of speed (, and once past ~Mach 3).
  - arprocter 11y ago
    Thanks for the insight - I guess it doesn't scale! Is there a reason the space gun would be at a 45 degree angle and not pointed vertically? I know the reason rocket launches happen as close to the equator as possible, but they still go straight upwards...
    - nickff 11y ago
      Rockets only go up for a little while; most of the propellant is used to build up speed in the 'horizontal direction'. There have been some arguments that a strictly vertically-firing rocket would be best for going outside of Earth orbit, but the current strategy for all launches is to go up until outside the thickest part of the atmosphere, then go horizontal, to build up to orbital velocity.
      - arprocter 11y ago
        After reading about the end of of Project HARP - it seems like they intended to use the gun to fire a rocket, and after a certain altitude the engine would ignite to get into space.
        nickff 11y ago
        Yes, and in the years since HARP, there have been a few rocket-assisted projectiles developed (though none which have been intended for space launch).[1] I should also mention that unless the projectile was fired at or above escape velocity (intended to leave the Earth's gravity well), it would require some onboard propulsion to 'circularize' its orbit (and avoid crashing back into the Earth on its first time around). [1] https://en.wikipedia.org/wiki/Rocket-assisted_projectile https://en.wikipedia.org/wiki/Rocket-assisted_projectile

ajuc 11y ago

Would shooting a powerful laser to heat the air in the path before launch help?

devenson 11y ago
Or perhaps a small rocket.

jasonwatkinspdx 11y ago

That 150m of air is constrained into a column by the barrel. It is a big deal and light gas guns commonly evacuate the barrel.

trhway 11y ago

evacuating barrel will help reach the speed. Evacuation is very easy to do, yet helps tremendously. The gun can be built somewhere in the Andes, at 4 km altitude. It can be pretty long there. Plus using either ablative head - like new Russian-Indian air missiles - or small forward jets - similar to Russian 200miles/h torpedo - will help to decrease atmospheric speed losses. The 6.5 Mach gets you to 180km altitude from powerful regular gun - https://en.wikipedia.org/wiki/Project_HARP https://en.wikipedia.org/wiki/Project_HARP : "On November 18, 1966 the Yuma gun fired a 400 lb (180 kg) Martlet 2 projectile at 7,000 ft/s (2,100 m/s)[1] sending it briefly into space and setting an altitude record of 180 km (590,000 ft; 110 mi);" That means that it was going at least 5 Mach after reaching 30km altitude. Doesn't looks like such a big loss of speed in the atmosphere - only about 1 Mach due to atmosphere as the other 0.5 Mach - due to gravitation.