Breaking the sound barrier

[Thomas_Anderson]

My characters are competing in a race and the vehicles they drive are hover cars that travel at around 300 mph. Anyways, one of them derails from the track, some other stuff happens, and he somehow acquires an upgrade that makes him travel at around 1200 mph. Any similarities to videogames are completely intentional since it all occurs in a simulation.

One of the things I was planning for my derailed character to do was drive through a hunting ground, and the hunters would take potshots at him for fun (it's a simulation, so nobody is in real danger, aside from getting a Game Over). If his vehicle is weak enough to be penetrated by bullets (not military grade, just the kinds of rifles you'd go deer hunting with) would it be strong enough to withstand the force of travelling at Mach 1? Would it be strong enough to hold itself together in a sonic boom, even with two bullet holes punched in it? Or should his vehicle be bulletproof?

Also, what is the effect of a sonic boom on a human body? Say his vehicle passed within a couple feet of somebody while travelling at Mach 1, what would happen to that person? Would they die? If so, would they explode on the spot, or be thrown several feet?

I realize that, since it's a simulation, it doesn't have to be completely accurate to reality, but I'd like to be kind of close to it.

[Pthom]

Hover cars on a track?

The big difference as far as I'm aware between military fire arms and hunting weapons is that the military ones have a greater rate of fire. The bullet in an M-16 assault rifle is relatively small (0.223 calibre). The bullet from an M-14 carbine is 7.62 mm, the same as the AK-47. The velocity of the projectiles is similar to a 30-06 rifle. Either will penetrate the sheet metal of a car, say. A metal-piercing round will, especially. Depending on how far away the weapon is, the projectile might pass completely through the car. Only if that projectile hits (and destroys) something critical, such as the engine, a structural component, the fuel tank...the driver... would there be enough damage to prevent the car from reaching the speed of sound.

There is no effect on the occupant of the vehicle creating the sonic boom.

There is a tremendous effect of the shock wave outside (and behind) the vehicle, however. I have no doubt it would knock over a human being standing close to the path of the vehicle. I doubt it would kill him, but he would be most uncomfortable. I'm pretty sure it would burst his ear drums, knock him unconscious.

Look at this wikipedia article on sonic booms.

[backslashbaby]

The Concord flew faster than the speed of sound. You might look up info about it. Dad said the view out of the window was kind of a blue swirl once they got up to speed. I don't remember him saying anything special about the boom part.

[Thomas_Anderson]

Originally Posted by Pthom

Hover cars on a track?

Sort of. The race track is suspended in the air while other (also simulated) stuff goes on down below. The track is also very long, to compensate for the very high speed of the racers. Normally the racers don't interfere with the ones below, unless one falls, though there are a handful of games that start out on the track and they have to dodge racers.

Only if that projectile hits (and destroys) something critical, such as the engine, a structural component, the fuel tank...the driver... would there be enough damage to prevent the car from reaching the speed of sound.

Hmm, I was under the impression a vehicle had to be very strong in order to withstand the G-Forces of travelling past the speed of sound, otherwise it would tear itself apart, and that bullet holes increase the chance of it being ripped apart. I'm no expert though, so I guess I was mistaken.

EDIT: One more thing, would the grass beneath the hover car be destroyed? Would there be a dirt path wherever he went, or would the grass just stay in the ground for the most part?

[dogfacedboy]

Well, if I remember correctly, the speed of sound (at sea level) is somewhere around 750mph, so you're well beyond that with 1200. And that kind of speed could easily cause problems in your scenario.

As far as the vehicle is concerned, I don't think there would be much structural trouble concerning the speed. I think the biggest issue of durability at those speeds would be your driver/pilot. Travelling at mach 1+ in a straight line puts a lot of pressure on the driver, but maneuvering at that speed increases the g-forces considerably. Unless this is a straight track, your driver will need some kind of flight suit-like protection, as well as the training and conditioning needed to cope with the tremendous strain of these forces.

And speaking of maneuvering, if your track is designed to accomodate vehicles travelling at a certain speed, then any turns will likely be too tight for your new vehicle to handle at it's increased speeds. Picture driving your car around a corner at 30mph. Then try maintaining that same line at 120mph. Sounds like your driver will be running off the track at the first turn.

I suppose a lot of this can be compensated for by saying that it's all a simulation, so the rules could be bent a little. But when you're talking about such an extreme change in speed, most of the physics of your normal race no longer apply.

[dogfacedboy]

Originally Posted by Thomas_Anderson

EDIT: One more thing, would the grass beneath the hover car be destroyed? Would there be a dirt path wherever he went, or would the grass just stay in the ground for the most part?

I think it depends on how your hover cars hover. Is it forced air keeping the vehicle afloat? Magnetic repulsion? Dark matter technology? It sounds like you've got a cool sci-fi scenario going here. It should be easy enough to come up with some kind of future-tech that can have whatever effect you want.

[Shakesbear]

Concord flew at Mach 2 - though the flight I was on it went up to 2.02. We were told that we were flying at twice the speed of sound at sea level - 21 mph. There were only two things that we were told to be prepared for - one was the click of the undercarriage as it came up. And the other was the thrust of three quarters of a million horse power as the plane took off - that was rather like being kicked in the back by an angry mule. I wasn't aware of any sonic boom whilst on the plane - but Concord was not allowed to go supersonic over land. The plane also grew in flight - there was a space between the control panel and fuselage that the pilot could get his hand into and wiggle his fingers about whilst the plane was at Mach 2 - the space contracted when no longer flying supersonic. I seem to remember that one of the problems in the development of Concord was the type of 'stuff' used to make it as it had to withstand the pressure the speed would put on it.

[Stanmiller]

Whatever kind of unobtanium drive it has, the laws of aerodynamics still apply.

Structural damage can occur in the transonic region, from Mach .9 to about 1.1. The vehicle has to be designed to stay in one piece while undergoing aerodynamic buffeting. Aircraft not properly designed for this flight regime have broken apart in mid-air.

See this link for a discussion of the Bloodhound SSC supersonic world land speed record car. Go to vehicle dynamics for some good info on the design criteria.

http://www.bloodhoundssc.com/car/vehicle_dynamics.cfm

--Stan

[benbradley]

Originally Posted by backslashbaby

The Concord flew faster than the speed of sound. You might look up info about it. Dad said the view out of the window was kind of a blue swirl once they got up to speed. I don't remember him saying anything special about the boom part.

You won't hear the "boom" inside, though it could be a rough ride during the transition around the speed of sound. As I recall, the Concord only flew supersonic over the ocean, as even at high altitude going supersonic causes too much noise on the ground.

Originally Posted by Thomas_Anderson

Hmm, I was under the impression a vehicle had to be very strong in order to withstand the G-Forces of travelling past the speed of sound, otherwise it would tear itself apart, and that bullet holes increase the chance of it being ripped apart. I'm no expert though, so I guess I was mistaken.

It's not really "G-forces," it's the air pressure on the plane's surface. Many of those fancy military aircraft have surfaces made of titanium for its high strength, low weight, or whatever it is.

But yes, if a bullet hits, it may penetrate the surface, especially if it hits the thing head-on so the speed of the craft and the bullet add. And yes, I would guess a bullet hole in a craft going faster than sound could be a problem - it may add drag where the bullet hole is, and eventually rip the metal away from the surface past the bullet hole, adding even more drag.

Also, to have a decent chance at hitting it, the hunters would have to be looking around and/or know which direction it's coming from and fire at it as it comes toward them. They won't hear it until after it's passed them, and even if they aim ahead and a bullet catches up it won't be going as fast relative to the craft as if they fire head-on.

High g-forces will result from making sharp turns (or accelerating very quickly to the speed of sound, as does a rocket), but to do that (or even to be able get up to that speed) the thing is going to look thin and sleek, more like a rocket, military fighter jet or stealth bomber than anything I envision as a "hover car."

A little big of googling (fastest land vehicle) brings up this video of a (apparently the ONLY) faster-than-sound land vehicle, basically two turbine jet engines on wheels:
http://www.youtube.com/watch?v=LKQ-xj5C2m8&NR=1

Even at 300mph, I wouldn't think of these things as "hovercraft." Maybe they need the fan to hover when starting off and at low speed, but at those higher speeds it won't need much of a wing to stay in the air. I'm thinking control surfaces on the front and back edges would be enough to "fly" the thing, though it would surely be fly-by-wire (computer-controlled, as micro-adjustments would need to be made too quick for human response times). Cars in NASCAR races have gone airborne, so "flying" is a problem at well under 300MPH.

Originally Posted by backslashbaby

EDIT: One more thing, would the grass beneath the hover car be destroyed? Would there be a dirt path wherever he went, or would the grass just stay in the ground for the most part?

The air grass might be flattened for a short instant, but I think unharmed. Larger plants and animals might not fare as well.

[Lhun]

Originally Posted by dogfacedboy

Travelling at mach 1+ in a straight line puts a lot of pressure on the driver, but manoeuvring at that speed increases the g-forces considerably.

No, speed doesn't matter at all. The only thing exerting a force would be friction or acceleration. Even a vehicle travelling at 300 ought to have a wind screen, so friction is unlikely to affect the driver. Acceleration is different question, but that depends on how hard the vehicle can actually accelerate, which is independent from it's top speed. Zero to Mach 1 in a second would squash the driver, flight suit or no, over 30 seconds, it's just about one g.

[Nivarion]

my two cents.

at human reaction time, by the time they saw the hover craft coming the hunters would have lost all opportunity to shoot it. It'd be gone already.

but for humor, if they did see it coming, the muzzle velocity of a hunting round is 2800fps just double of your hover craft. That speed is a shot in the dark, I don't feel like looking up the actually speeds, but I know it's somewhere around there. (pun not intended)

To actually shoot it they would need to see it coming and acquire their targets in less than 1/8th a second (it's gone 200 feet by this point) and shoot at where it will be in 1/2 a second (another 600, for a grand total of 800 feet from where the shooter saw it) assuming the distance between the shooter and the target spot, and the target and the target spot are equal.

I would kiss the feet of the sharpshooter able to make this shot.

If bullet holes you need, then passing through an active range should (might) get you your results.

And the only things a military grade bullet would have that a civilian bullet wont is either a solid bronze bullet or a steel core.

[Dommo]

http://www.youtube.com/watch?v=q9S0z1ofcIc

Sound breaking awesomeness that is visible to the naked eye. Gotta love those shockwaves.