What factors limit the maximum speed of a ground vehicle?

[|Orion's Thought|]

Whats the fastest you could possibly get a car (any car, current, concept or by own design) to move? Could it reach between mach 2-5?

 

[Averagesupernova]

I believe it is impossible for even planes to fly that fast when in close proximity to the ground.

 

[|Orion's Thought|]

I was just wondering because, if you could get a car up that fast, a scramjet mounted on it might make it go just that much faster (a lot faster).

 

[FredGarvin]

It's definitely not impossible for planes to fly supersonic at close to ground level. It's just a lot harder to do so. Engine thrust decreases with altitude and specific fuel consumption increases as well. In other words it is much less efficient.

Has anyone seen the show that followed the Thrust SST team from England? It was a good show to watch. It gave one a great appreciation for how hard it was for a land based craft to break the sound barrier. Of course, a lot of their issues arose from the definition of what a car was and having to stick within those guidelines. It was a few years ago, but just by looking at the problems they had on that project, I'd say that the state of the art has not approached what is necessary for M2+.

 

[Astronuc]

Two matters - propulsion and aerodynamic stability (fluid structure interaction).

Propulsion is simple - it could be done with enough power - e.g. ramjets or rockets.

The land speed record -

Welcome to the site of the challengers to the world land speed record; currently @ 763 mph, or Mach 1.02. Our goal - 800 MPH - or Mach 1.05!

- http://www.landspeed.com/

After that, the problem becomes the shock waves from the compression associated with hypersonic velocity. Jet aircraft designed for Mach 2 do not do Mach 2 near the ground, and certainly not on the ground (they are not desiged for M2 with landing gear down).

See also - http://www.fluidmech.net/msc/auto_lsr.htm

 

[FredGarvin]

I got to love someone stupid enough to cut the wings off of an F-104 and drive the thing. It really pushes the definition of what a "car" is.

 

[Danger]

From what I remember of 104's, they're probably a lot safer without the wings.

 

[rcgldr]

Mach 8+ is not a problem for a rocket sled, and it's a land based "vehicle".

Rocket sled, 0 to 6000+mph in 6 seconds, the stop is even quicker (test object crashes into barrier. Click on picture to see the video. Acceleration in last stage peaks at 157g's, more than the 100g's of the Sprint interceptor missle.

http://www.46tg.af.mil/world_record.htm

 

[RainmanAero]

Hi Astronuc,

After that, the problem becomes the shock waves from the compression associated with hypersonic velocity. Jet aircraft designed for Mach 2 do not do Mach 2 near the ground, and certainly not on the ground (they are not desiged for M2 with landing gear down).

Exactly, well-put. One could also connect this explanation to the discussion of boundary layer effects we had in another thread.

When an airplane is "up and away" from the earth, it is outside the Earth surface boundary layer of the atmosphere. The result is that shock waves that form around S/S aircraft are fairly "smooth" in their form and their time-varying nature. Certainly there is buffet, but its amplitudes and phase lags with respect to airplane dynamics are small when compared to the terrestrial vehicle on the ground.

Because of the fluid/solid boundary of the earth, the shock waves that form around the vehicle interact with the roughness (irregular surface contour) of the Earth in a highly variable manner. The shocks "dither" with higher amplitudes and more random frequencies. Not only that, but as the vehicle pitches/rolls with respect to the ground, the ground effect aerodynamics will constantly change & induce more asymmetric, time-varying forces. This makes the buffet on the vehicle quite severe, and the dynamic response of the vehicle very sketchy. The bandwidth of the vehicle's steering system will have to be MUCH wider than the cutoff frequencies associated with normal autos.

I'm not sure the rocket sled is a fair comparision, because this is a large accel/decel situation, whereas I think what we are really wishing to focus on is a constant speed vehicle. (If I may suggest)

Rainman