5 ms·

Smaller projectiles lose more energy to air drag.

ambicapter 5y ago
You mean objects with lower surface area to mass ratio, AKA objects with lower density. If he wants some high-density objects to throw I think there are stores in the US that sell small, dense metal objects he could use.
- dragontamer 5y ago
  Mass is not a factor in drag equations. https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/vel.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a... Velocity-squared is the factor in drag.
  - bernulli 5y ago
    But very much in inertia, which is acting against the drag, trying to sustain its initial velocity. https://en.wikipedia.org/wiki/Ballistic_coefficient https://en.wikipedia.org/wiki/Ballistic_coefficient
  - jpollock 5y ago
    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
      [deleted]
    - 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).
dragontamer 5y ago
Doubly so when you consider the supersonic barrier. Air drag is proportional to the drag-coefficient * velocity^2. So doubling your speed quadruples the drag. Except... the drag-coefficient itself is a complex curve over velocity. It barely changes from 0mph through 300mph, but once you reach transonic and supersonic speeds, the drag-coefficient skyrockets. As a result: 250mph is more than 4x more efficient than 500mph from a drag perspective (4x predicted from Drag equation, but in practice might be 8x or more). This is mostly a problem for airplane efficiency and fuel consumption, but I'm sure it applies to war-machines and projectiles too. -------- But modern airplanes fly at very high heights, so the air is thinner, reducing drag, but requiring a pressurized cabin lest everyone gets hypoxia.
- fho 5y ago
  > It barely changes from 0mph through 300mph Tell that to cyclists :-) You can barely go above 25 km/h (~15 mph) without feeling like you are just pedaling to overcome the drag. Reducing the Cd on a bike, ie going recumbent and putting a aerodynamic shell on it, ie switching to a velomobile allows you to go 40 km/h (~25 mph) with relative ease.
  - Toutouxc 5y ago
    Just in case you misunderstood the parent post -- your Cd as a cyclist is pretty much constant around your normal cycling speeds. Your total air drag goes up with the square of velocity, that's for sure, but if you went transsonic on your bike, your Cd would ALSO increase dramatically on top of the already brutal velocity-squared multiplier.
    - rich_sasha 5y ago
      It’s almost worse, because you’re working against this quadratic force, at a rate proportional to speed. So power output, the usual limiting factor for a cyclist, scales like 3rd power of speed.
    - fho 5y ago
      Fair enough ... I really did not get that :-)
OneTimePetes 5y ago
Only when they are flying to normal atmosphere.. One can use a laser + charge to pre-boil the flight-path into a plasma channel. Sort of like cavitation, but with gasses.

You mean objects with lower surface area to mass ratio, AKA objects with lower density. If he wants some high-density objects to throw I think there are stores in the US that sell small, dense metal objects he could use.

dragontamer 5y ago
Mass is not a factor in drag equations. https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/vel.html https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/a... Velocity-squared is the factor in drag.
- bernulli 5y ago
  But very much in inertia, which is acting against the drag, trying to sustain its initial velocity. https://en.wikipedia.org/wiki/Ballistic_coefficient https://en.wikipedia.org/wiki/Ballistic_coefficient
- jpollock 5y ago
  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
    [deleted]
  - 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).

dragontamer 5y ago

Doubly so when you consider the supersonic barrier. Air drag is proportional to the drag-coefficient * velocity^2. So doubling your speed quadruples the drag. Except... the drag-coefficient itself is a complex curve over velocity. It barely changes from 0mph through 300mph, but once you reach transonic and supersonic speeds, the drag-coefficient skyrockets. As a result: 250mph is more than 4x more efficient than 500mph from a drag perspective (4x predicted from Drag equation, but in practice might be 8x or more). This is mostly a problem for airplane efficiency and fuel consumption, but I'm sure it applies to war-machines and projectiles too. -------- But modern airplanes fly at very high heights, so the air is thinner, reducing drag, but requiring a pressurized cabin lest everyone gets hypoxia.

fho 5y ago
> It barely changes from 0mph through 300mph Tell that to cyclists :-) You can barely go above 25 km/h (~15 mph) without feeling like you are just pedaling to overcome the drag. Reducing the Cd on a bike, ie going recumbent and putting a aerodynamic shell on it, ie switching to a velomobile allows you to go 40 km/h (~25 mph) with relative ease.
- Toutouxc 5y ago
  Just in case you misunderstood the parent post -- your Cd as a cyclist is pretty much constant around your normal cycling speeds. Your total air drag goes up with the square of velocity, that's for sure, but if you went transsonic on your bike, your Cd would ALSO increase dramatically on top of the already brutal velocity-squared multiplier.
  - rich_sasha 5y ago
    It’s almost worse, because you’re working against this quadratic force, at a rate proportional to speed. So power output, the usual limiting factor for a cyclist, scales like 3rd power of speed.
  - fho 5y ago
    Fair enough ... I really did not get that :-)

OneTimePetes 5y ago

Only when they are flying to normal atmosphere.. One can use a laser + charge to pre-boil the flight-path into a plasma channel. Sort of like cavitation, but with gasses.