5 ms·

Not to mention, this is capable accelerating a projectile to Mach 7 within the friction of Earth's atmosphere. Without one, perhaps it would be capable of faste

Not to mention, this is capable accelerating a projectile to Mach 7 within the friction of Earth's atmosphere. Without one, perhaps it would be capable of faster velocities.

jerf 12y ago
Heh... at this scale, it's not so much "friction" as it is "those damned atoms that are in the way".
politician 12y ago
Haha, that's right, I forgot that Mach is a measurement of speed through a fluid and the local speed of sound. Obviously, Mach 7 on the Moon is greater than Mach 7 on Earth. I guess that means we can fire larger payloads with the same platform.
- jerf 12y ago
  "Obviously, Mach 7 on the Moon is greater than Mach 7 on Earth." Well... Mach 7 through the Moon's atmosphere is probably best described as "undefined" rather than greater than anything....
  - politician 12y ago
    Lunar atmospheric pressure is non-zero [1] (but to your point, it's really really small). [1] http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html
    - zachrose 12y ago
      Would very low pressure mean that Mach 1 on the Moon is be much faster than on Earth?
      - tsm 12y ago
        Yes, exactly
        lutorm 12y ago
        Actually, the speed of sound through a gas only depends on temperature, not pressure. (But the Moon's "atmosphere" can hardly be described as a gas anyway...)
        ecma 12y ago
        At least google stuff like this before posting it (and maybe include a reference) because this is unequivocally wrong. Speed of sound in a gas is calculated with the Newton-Laplace equation [1] which refers to K, the bulk modulus which is derived from the pressure and the adiabatic constant for ideal gases. It also involves rho, the density of the medium which is why gases and non-gaseous fluids have very different results. [1] http://en.m.wikipedia.org/wiki/Speed_of_sound#Equations http://en.m.wikipedia.org/wiki/Speed_of_sound#Equations
fit2rule 12y ago
So we're just a super-cavitating projectile away from having a nice way to launch satellites ..
hollerith 12y ago
The maximum velocity occurs the moment the projectile leaves the gun, so the only time air friction has any chance of reducing the maximum velocity is while the projectile is still in the gun. My guess is that on Earth, air friction's effect on the maximum velocity is negligible. (The projectile will slow down a lot faster in the Earth's atmosphere than it will on the moon, but that fact has no bearing on the maximum velocity.)

Not to mention, this is capable accelerating a projectile to Mach 7 within the friction of Earth's atmosphere. Without one, perhaps it would be capable of faster velocities.

jerf 12y ago

Heh... at this scale, it's not so much "friction" as it is "those damned atoms that are in the way".

politician 12y ago

Haha, that's right, I forgot that Mach is a measurement of speed through a fluid and the local speed of sound. Obviously, Mach 7 on the Moon is greater than Mach 7 on Earth. I guess that means we can fire larger payloads with the same platform.

jerf 12y ago
"Obviously, Mach 7 on the Moon is greater than Mach 7 on Earth." Well... Mach 7 through the Moon's atmosphere is probably best described as "undefined" rather than greater than anything....
- politician 12y ago
  Lunar atmospheric pressure is non-zero [1] (but to your point, it's really really small). [1] http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html
  - zachrose 12y ago
    Would very low pressure mean that Mach 1 on the Moon is be much faster than on Earth?
    - tsm 12y ago
      Yes, exactly
      - lutorm 12y ago
        Actually, the speed of sound through a gas only depends on temperature, not pressure. (But the Moon's "atmosphere" can hardly be described as a gas anyway...)
        ecma 12y ago
        At least google stuff like this before posting it (and maybe include a reference) because this is unequivocally wrong. Speed of sound in a gas is calculated with the Newton-Laplace equation [1] which refers to K, the bulk modulus which is derived from the pressure and the adiabatic constant for ideal gases. It also involves rho, the density of the medium which is why gases and non-gaseous fluids have very different results. [1] http://en.m.wikipedia.org/wiki/Speed_of_sound#Equations http://en.m.wikipedia.org/wiki/Speed_of_sound#Equations

fit2rule 12y ago

So we're just a super-cavitating projectile away from having a nice way to launch satellites ..

hollerith 12y ago

The maximum velocity occurs the moment the projectile leaves the gun, so the only time air friction has any chance of reducing the maximum velocity is while the projectile is still in the gun. My guess is that on Earth, air friction's effect on the maximum velocity is negligible. (The projectile will slow down a lot faster in the Earth's atmosphere than it will on the moon, but that fact has no bearing on the maximum velocity.)