It moves the Space Shuttle to the launch pad. It has over 5,000hp. This would have impressed me, except I had just watched another History Channel show on drag racing-some funny cars pump out 7,000hp.
This got me thinking about engine design (a HUGE interest of mine). Why are the funny car engines more powerful than the shuttle crawler’s, but they’re much much smaller? What is the difference in how you design a funny car engine around 5000 hp and how you design a 5000hp crawler engine designed to haul a space shuttle?
I imagine that a funny car engine wouldn’t work well driving the crawler…but why not?
The crawler’s engines are probably running fairly steadily at something like 1000 RPM. Also, they’re not driving the machine directly. They drive generators, which in turn, power a set of DC traction motors, adding up to 6,000 HP.
Basic engine specs:
Two Alco, 16 Cylinder, Diesel engines, 2,750 HP (2,050 Kilowatts) each. Powers 41,000 KW DC Generators for traction motors.
Two White Superior, 8 Cylinder Diesel Engines, 1,065 HP ea, Powers two 750KW AC Generators for on-board power.
Funny cars, on the other hand, drink nitromethane instead of Diesel.
When you get down to it, I believe the key difference is torque. The crawler’s engines (or rather, its DC traction motors) produce humongous gobs of low-speed torque. Funny cars, on the other hand, are insanely high-revving engines - loads of RPM, but no grunt behind it.
I think stupendous amounts of torque would be the primary design goal for the crawler’s powerplant (or powerplants, perhaps?) rather than HP.
I’m sure there is someone who can explain the relationship between horsepower and torque better than I, so I’ll let them confuse me rather than me confusing you :), but when it comes to overcoming inertia, torque is the prime mover IIRC.
A drag racing motor is around 7 liters in displacement, runs at 8000rpms, has a massive supercharger attached to it, and burns nitromethane at an incredible rate. Not surprisingly, this is hard work for a motor, and they require a complete rebuild after every run.
I don’t know anything about the shuttle crawler, but I would imagine it uses a very low revving diesel engine of much higher displacement. Perhaps more than one. The low revs and immense torque of a diesel make them ideal for large equiptment that needs to run for long periods of time while producing a high level of power.
I was talking with someone at work who is friends with a guy who races a top fueler (the class above funny cars). He told me that these dragsters are the fastest accelerating objects on earth.
The driver set an NHRA record covering the quarter mile in 4.428 seconds.
d=0.5at^2
a = 2d/t^2 = 20.255280/4.428^2 = 4.2G
(Yes, that’s an average acceleration and I dunno the real acceleration curve of a dragster)
There’re plenty of things that accelerate at far more than 4.2G. Let’s try a smokin’ fastball - a pitcher with 3-foot long arms and the ball leaving at 100mph (about 150fps). The ball has about 6 feet (full windup) to accelerate from 0fps to 150fps, resulting in an average acceleration of…
t = 6f/75fps = 0.08sec
a = 150fps/0.08sec = 1875f/sec^2 = over 58G (!)
I recently took a course on Combustion Engine design, so maybe I can offer some insight into this one.
Horsepower is a function of both torque and the engine rpm,
HP = Torque * RPM / 5252
All that horsepower really is a way for gearheads to make their engines sound powerful. The thing that causes acceleration is torque, and only torque.
Really large engines (shuttle mover, ship engines, construction equipment) operate a much lower RPM’s than smaller ones, as there is much more mass to sling around (some pistons can get up to 30" diameter!, but operate at 100 rpm. The larger the combustion chamber volume, the more power an engine will make. Just imagine 1 cu ft of fuel/air exploding vs. .1 or less cu-ft. This is why larger=more power.
So a large engine operating at 1000 rpm, and making 800 ft-lb of torque has a hp of 152 HP.
A smaller engine operating at ~5000 rpm at the same HP would be putting out only 159 ft-lb of torque.
So the end result, is that a funny car engine would not work well becuase it does only puts out a fraction of the torque of the larger engine, whereas the HP is irrelevant.
Sure, but can’t you run the smaller engine through a 5:1 reduction gearing and get exactly the same 800 ft-lb at 1000 RPM as the larger engine produces? (I’m assuming zero gear losses - a little optimistic in the real world, I’ll admit. But still.)
I think Johnathan D hit the nail on the head with the longevity thing. Big industrial, marine, and aviation engines are designed to run at full power for hours or days on end - try that with a dragster motor, or even an ordinary street-legal automotive motor, and something will break pretty quickly.
