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I was thinking about an extremely narrow object and considering that I think in the ideal case it could only deal with the force of collision with air particles at the front of the projectile (although I assume it would need fins to be stable).

At that point I assume if somehow methods of generating higher and higher levels of propulsion were found, this infinitely narrow projectile could go faster and faster except that at some point I wonder if it would start generating such a high level of heat that the projectile would be destroyed.

So I suppose the limits to the speed of a projectile on Earth are:

  1. Aerodynamics: is there a mathematically calculable maximum level of aerodynamicism? Is smaller or thinner always better? Would a hypothetical nano-jet have the lowest aerodynamics? Or is it less important since the counterforce of aerodynamics is met with the momentum of the projectile, meaning of an object is very small, it will need to be more dense, or have an extremely high amount of energy; so basically, being tiny is not actually particularly optimizing? But I think narrow objects are in fact faster; arrows from a bow and arrow for example.
  2. Energy. Perhaps fuel is currently the practical best known method but in the future we could find some better method, maybe fusion.
  3. The material of the projectile; ideally, something very durable that is somehow as close to infinitely heat-resistant as possible.

So, even considering a future where we can amazing materials, amazing engineering capabilities to make projectiles any size or shape, and enormous amounts of energy, are there still certain limits we would be approaching in the Earth’s atmosphere? I was thinking that if you could start going extremely fast, I am not sure what the current known theoretical or scientific maximum is in terms of some special material that can withstand high levels of heat, but I wonder if the air itself would take on the energy and at a certain point this projectile would be generating so much collision energy against the atmosphere that even if it could withstand it the air itself might get turned into a plasma, like the sun? So basically eventually we could imagine even if the projectile could survive at some point it might actually become destructive to the Earth?

What are the practical and theoretical limits, in the near and deep future?

John Smith
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  • One limit is power. – Solar Mike Jan 15 '23 at 09:18
  • Why do you need a material that is heat resistant? Why not have a barrier to stop the heat getting to the material? – Solar Mike Jan 15 '23 at 09:43
  • @SolarMike I fail to see the difference in a world without force fields? – DKNguyen Jan 15 '23 at 19:43
  • @DKNguyen so in your opinion force fields are a scientific impossibility? Just look back on what has been written in sci-fi that is now possible... – Solar Mike Jan 15 '23 at 19:45
  • @SolarMike But if that's the perspective you are going to take then you might as well just say the limit is infinitesmally close to c. Because why not? OP is already talking about infinitely small projectiles carrying technology and infinitely heat resistant materials. The OP's thoughts are muddled between a technological device with propulsion and the actual limit of any object such as meteorite. OP also does not state how long the thing must survive. That's not to say I'm expecting a specification but it illustrates the muddiness of the thoughts. – DKNguyen Jan 15 '23 at 19:52
  • To the OP: A very hot fire will ionize the air producing plasma and we already know air gets very hot, to the point of auto-ignition from friction in things such as rail guns and meteors so it's verified that it would turn to plasma. Also you should know things, particularily those involving fluid dynamics such as jets have scale effects and are tend not to be favourable as you get smaller (as is the case with jets and nearly all current technological approaches). Mostly only biology has been able to exploit small scale aerodynamics advantageously, but those do not involve speed. – DKNguyen Jan 15 '23 at 20:13

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For very high speeds in air (~3000 MPH), aerodynamic heating is severe and the limiting factor on speed is the temperature of the body at which it begins to lose strength. This is why ordinary rockets during high-speed re-entry require very good heat shields to keep from burning up, and why the SR-71 was speed-limited to somewhere around mach 3.

for high speeds in air (~550 MPH), airspeed is limited by the available power from the engine that's doing the pushing. More power means more speed, but the fuel consumption rate will become uneconomical for anything except military planes. Passenger planes use the smallest engines they can, to maximize economy, and operate them at their peak efficiency point. This is why almost no matter who made the plane or how big it is, if it is running high-bypass turbojet engines it will cruise at 500 MPH and do it near 40,000 feet.

For high speeds in small aircraft that are not operated by the military, the limiting factor is money. speed costs money, and more speed costs more money- and creates severe hazards that most people refuse to take (think: Reno Air races).

niels nielsen
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