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Thread: Maximum speed in space(ish)

  1. #1 Maximum speed in space(ish) 
    Forum Freshman remfan's Avatar
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    Hey Guys

    I hope I can explain myself properly...

    Due to these limiters. 1. Engine capacity. 2. Friction. 3. Gravity.

    (Theoretically).... If a car, on earth, has a maximum speed of 100mph. What speed would/could it reach in space?

    Would the only limiter be 1. Engine capacity.?

    This has been an ongoing topic between my friend & I, and you thoughts on it would gratefully appreciated.

    Thanks in advance

    Rob


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  3. #2  
    Quagma SpeedFreek's Avatar
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    No sure how to begin to answer that.

    A car is designed to work on the surface of a planet, using its engine to turn its wheels and gravity holding it against the road, so the wheels grip due to friction and drive the car forwards. There is air resistance.

    In space, there is no surface of a planet for the car to drive on.

    Or do you mean on a planet but with no atmosphere?


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  4. #3  
    Moderator Moderator Janus's Avatar
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    Well, that depends on a couple of things, If you were to elimate air friction, you'd still be left with mechanical friction. But for the sake of argument, we'll ignore that as well. Then the top speed of the car would be limited by the RPMs the engine can sustain and the gear ratio.

    Example: the top speed rating for a 2004 Corvette is 170 mph. The car red-lines at 6500 rpm, and in 6th gear, the gear ratio is 1: 0.56 ( the wheels make 1 revolution per every 0.56 revolutions of the engine). This means at top rpm in 6th gear, the wheels are turning at 11607 rpm. If the wheels have a diameter of ~2.2 ft, this gives a circumference of 7ft ( the car will travel 7 ft for every wheel rotation. This works out to 81437 ft/min or 15.4 miles/min or ~925 mph.

    That would be the theoretical limit for this car. Other factors, like the above mentioned mechanical friction and simply the limitations of the tire will make it lower.

    It may seem weird that air resistance can cause that large a difference, but consider this:

    The Corvette has a 425 hp engine and is rated at 170 mph

    The Bugatti Veyron has a hp of 1000 and a top speed of 253 mph. That's 2.35 times more HP than the Corvette for just a 50% increase in speed
    The Super sport version has a hp 1200 and a top speed of ~270 mph. That's 200 hp more for only an increase of 17 mph.

    Basically air resistance goes up by the square of the speed.
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  5. #4  
    Forum Freshman remfan's Avatar
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    Thanks for the response... I used the "car" as an example. So, forget the car scenario. But, I understand. I basically wanted to know the difference in possible speed from an engine in space with no physical constraints other than its own limitations as an engine. This is frustrating me lol. I cant explain in words what im thinking on this matter (must be my age).
    e.g.: if an engine reached a certain speed in earths atmosphere, what speed could it reach in space (vacuum)? Surely it would reach a higher speed (because of lack of gravity, friction etc...). Is there a calculation to show this? My friend & I disagree on this. One opts for it will keep increasing its speed gradually (up to a point). But, this speed will exceed its earthly limitations. Another says it will increse upto just under the speed of light (this will take millions of years- just a bit longer than my computer booting up!).

    Thanks
    Rob
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  6. #5  
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    In space there is no road for the car's wheels to push against. If there were something solid to push against, then one could imagine that the car could be shifted to an ever higher gear. With each higher gear, the torque transmitted to the wheels and accelerating force would get smaller and smaller. However, no matter how small the force, as long as there is some acceleration, the speed will increase. Then if you had an unlimited fuel supply, you could get it up close to the speed of light.

    Of course, there is no such thing as a road in space, and there are no frictionless gears, the transmission and wheels will fly apart if you spin them fast enough, and you don't have an unlimited fuel supply.
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  7. #6  
    Administrator KALSTER's Avatar
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    Quote Originally Posted by Janus View Post
    Well, that depends on a couple of things, If you were to elimate air friction, you'd still be left with mechanical friction. But for the sake of argument, we'll ignore that as well. Then the top speed of the car would be limited by the RPMs the engine can sustain and the gear ratio.

    Example: the top speed rating for a 2004 Corvette is 170 mph. The car red-lines at 6500 rpm, and in 6th gear, the gear ratio is 1: 0.56 ( the wheels make 1 revolution per every 0.56 revolutions of the engine). This means at top rpm in 6th gear, the wheels are turning at 11607 rpm. If the wheels have a diameter of ~2.2 ft, this gives a circumference of 7ft ( the car will travel 7 ft for every wheel rotation. This works out to 81437 ft/min or 15.4 miles/min or ~925 mph.

    That would be the theoretical limit for this car. Other factors, like the above mentioned mechanical friction and simply the limitations of the tire will make it lower.

    It may seem weird that air resistance can cause that large a difference, but consider this:

    The Corvette has a 425 hp engine and is rated at 170 mph

    The Bugatti Veyron has a hp of 1000 and a top speed of 253 mph. That's 2.35 times more HP than the Corvette for just a 50% increase in speed
    The Super sport version has a hp 1200 and a top speed of ~270 mph. That's 200 hp more for only an increase of 17 mph.

    Basically air resistance goes up by the square of the speed.
    I think you have forgotten about the differential ratio. From this link: Gear ratio - Wikipedia, the free encyclopedia, the Corvette will travel 40.6 mph for every 1000rpm in 6th gear, or a maximum of 284.2 mph at 7000rpm in 6th gear.
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  8. #7  
    Forum Freshman remfan's Avatar
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    Thank you Guys. I was sure you would know!
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