5 ms·

Density is important when comparing two spheres of the same mass. https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/shaped.html https://www.g

Density is important when comparing two spheres of the same mass. https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/shaped.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a... Where frontal volume is determined by mass and density for a sphere. So, two speheres with the same mass, but different densities will have different drag forces.

deleted 5y ago
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dragontamer 5y ago
That's density of air, not of the object. https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/atmos.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...
- cinquemb 5y ago
  I wonder if you could do something like this[0] on the projectile to reduce drag: "It is reported that femtosecond-duration laser pulse can be deployed to reduce drag for blunt-body vehicle in high-Mach flow field of air by generating laser plasma and shockwave. The interaction of plasma shockwave induced by femtosecond-duration laser pulse and bow shock over the head of the blunt-body vehicle is investigated numerically in the flow field of 30 km apart from the surface of the earth at Mach number of 5 and the mechanism of deploying the femtosecond laser plasma to reduce the drag of the vehicle is analyzed. The Navier-Stokes equations are exploited to compute the drag reduction for different femtosecond laser energies. The present numerical experiment proves that the femtosecond laser pulse has a better drag reduction effect than the nanosecond laser pulse under the same condition. When the femtosecond laser energy is 0.06 mJ, the femtosecond laser plasma can reduce the drag by 98%. And the higher the energy of femtosecond laser pulse, the higher the drag reduction ratio and the longer the time of low drag. Deploying three femtosecond laser energy point to reduce the drag of hypersonic vehicle is much more obviously. This energy-deposition mode can improve the optimum drag reduction ratio and save the laser energy." Probably out of the scope for a $200 build, but maybe in the scope of a $2000, $20000, or $200000 build? I also wonder if iterating on the above, combined with a concentrated solar power plant of arbitrary sized solar field that could provide enough energy for a railgun to launch payload with femtosecond laser plasma capabilities out of earths gravity well? Maybe, we could escape from the Goddard age eventually lol [0] https://www.researchgate.net/publication/282709902_Exploring_Femtosecond_Laser_Plasma_to_Reduce_Drag_of_Hypersonic_Vehicle https://www.researchgate.net/publication/282709902_Exploring...
  - bernulli 5y ago
    The laser in the study is moving with the projectile, so I guess even for $2000 you won’t be able to have a 30 km altitude, Mach 6 flying object with perfectly controlled laser launched from a trebuchet.
    - cinquemb 5y ago
      I'm talking about a projectile that has the laser capabilities in itself (i.e the electromagnetic slug is emitting the femtosecond laser pulses after it is launched), not launched via a laser (it could be launched however, but I was assuming later on the comment that both the rail gun and the slug itself will need to be charged up).
      - bernulli 5y ago
        Yes, it's 'moving with the projectile'. How does that make it easier?
        cinquemb 5y ago
        I never said how easy it would be only that there is a difference between a payload that is launched via a laser and a payload that has laser capabilities in it (yes, 'moving with the projectile', like i was talking about from the beginning where you assumed otherwise). But for one, the energy source would have to be on board the payload to power the laser (localized around the payload), and for the other (that I was not talking about) the laser is not on the payload and as the payload travels further away from the launch site, you would use an non linear increasing amount of energy and follow the path of the payload (ionizing air not only around the payload continuously but along the path where the payload no longer is, which would require more energy than needed).
        bernulli 5y ago
        Try to read my comment, I never 'assumed otherwise', the difficulty of having the laser as payload is exactly my point.
- jpollock 5y ago
  The frontal area of two spheres of the same mass will be determined by the sphere's density, and that drives the air resistance. If frontal area didn't matter to drag, wouldn't all spheres would have the same drag coefficient? From that page: Cd = D/(rAV^2/2) where A is frontal area. What am I missing?
  - dragontamer 5y ago
    > Cd = D/(rAV^2/2) where A is frontal area. You're citing a page that uses the word "density" differently than you do. "D" is density-of-the-air. The density you're talking about appears to be the density-of-the-sphere.
    - CRConrad 5y ago
      "Appears to be"? They explicitly said so. Twice.
    - bernulli 5y ago
      It absolutely is, and will then drive differences in reference area, thus affecting the drag of the object.
  - bernulli 5y ago
    You’re missing nothing, you’re correct.

deleted 5y ago

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dragontamer 5y ago

That's density of air, not of the object. https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/atmos.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a...

cinquemb 5y ago
I wonder if you could do something like this[0] on the projectile to reduce drag: "It is reported that femtosecond-duration laser pulse can be deployed to reduce drag for blunt-body vehicle in high-Mach flow field of air by generating laser plasma and shockwave. The interaction of plasma shockwave induced by femtosecond-duration laser pulse and bow shock over the head of the blunt-body vehicle is investigated numerically in the flow field of 30 km apart from the surface of the earth at Mach number of 5 and the mechanism of deploying the femtosecond laser plasma to reduce the drag of the vehicle is analyzed. The Navier-Stokes equations are exploited to compute the drag reduction for different femtosecond laser energies. The present numerical experiment proves that the femtosecond laser pulse has a better drag reduction effect than the nanosecond laser pulse under the same condition. When the femtosecond laser energy is 0.06 mJ, the femtosecond laser plasma can reduce the drag by 98%. And the higher the energy of femtosecond laser pulse, the higher the drag reduction ratio and the longer the time of low drag. Deploying three femtosecond laser energy point to reduce the drag of hypersonic vehicle is much more obviously. This energy-deposition mode can improve the optimum drag reduction ratio and save the laser energy." Probably out of the scope for a $200 build, but maybe in the scope of a $2000, $20000, or $200000 build? I also wonder if iterating on the above, combined with a concentrated solar power plant of arbitrary sized solar field that could provide enough energy for a railgun to launch payload with femtosecond laser plasma capabilities out of earths gravity well? Maybe, we could escape from the Goddard age eventually lol [0] https://www.researchgate.net/publication/282709902_Exploring_Femtosecond_Laser_Plasma_to_Reduce_Drag_of_Hypersonic_Vehicle https://www.researchgate.net/publication/282709902_Exploring...
- bernulli 5y ago
  The laser in the study is moving with the projectile, so I guess even for $2000 you won’t be able to have a 30 km altitude, Mach 6 flying object with perfectly controlled laser launched from a trebuchet.
  - cinquemb 5y ago
    I'm talking about a projectile that has the laser capabilities in itself (i.e the electromagnetic slug is emitting the femtosecond laser pulses after it is launched), not launched via a laser (it could be launched however, but I was assuming later on the comment that both the rail gun and the slug itself will need to be charged up).
    - bernulli 5y ago
      Yes, it's 'moving with the projectile'. How does that make it easier?
      - cinquemb 5y ago
        I never said how easy it would be only that there is a difference between a payload that is launched via a laser and a payload that has laser capabilities in it (yes, 'moving with the projectile', like i was talking about from the beginning where you assumed otherwise). But for one, the energy source would have to be on board the payload to power the laser (localized around the payload), and for the other (that I was not talking about) the laser is not on the payload and as the payload travels further away from the launch site, you would use an non linear increasing amount of energy and follow the path of the payload (ionizing air not only around the payload continuously but along the path where the payload no longer is, which would require more energy than needed).
        bernulli 5y ago
        Try to read my comment, I never 'assumed otherwise', the difficulty of having the laser as payload is exactly my point.
jpollock 5y ago
The frontal area of two spheres of the same mass will be determined by the sphere's density, and that drives the air resistance. If frontal area didn't matter to drag, wouldn't all spheres would have the same drag coefficient? From that page: Cd = D/(rAV^2/2) where A is frontal area. What am I missing?
- dragontamer 5y ago
  > Cd = D/(rAV^2/2) where A is frontal area. You're citing a page that uses the word "density" differently than you do. "D" is density-of-the-air. The density you're talking about appears to be the density-of-the-sphere.
  - CRConrad 5y ago
    "Appears to be"? They explicitly said so. Twice.
  - bernulli 5y ago
    It absolutely is, and will then drive differences in reference area, thus affecting the drag of the object.
- bernulli 5y ago
  You’re missing nothing, you’re correct.