Thrust powerd car; can it be done??

[Jared_80]

I have been studying drivetrain configurations and I have come to the conclusion that they all have serous drawbacks. They either don't put power to enough of the wheels, or they add to much weight and kill your efficency. Why can't we use a thrust powerd car? Racers have been trying to use thrust for years, but the sanctioning organazations will not allow it, but I have not heard of any such law on the street. What potential drawbacks would this have, and what advatages. I know that you would reduce alot of weight by replacing a conventional transmition, driveshaft, and axle; with a variable pitch fan blade, and this would also elemanate traction limits during accelaration, as well as filling in the normaly negative pressure area behind the car thus reducing aerodynamic drag dramaticly. Could anyone else give me some more pros and cons?

[KC Ron Carter]

Go by the airport and then visualize the road with 3 minutes between cars.

My guess is a qualified one, FAA 1958549.

Later,

[Jared_80]

Originally Posted by KC Ron Carter

Go by the airport and then visualize the road with 3 minutes between cars.

My guess is a qualified one, FAA 1958549.

Later,

I see your point, but you are probably not considering several things.
1 This is fan driven thrust, not jet propulsion, meaning no excess heat.
2 The amount of thrust required to push a car along at highway speed is only about 15hp-20hp worth. Not exactly gale force winds in fact I would bet that you could comfortably drive 50' behind one of these cars, because with a fan for propulsinn you are using a lower velosity higher thrust area comonation.
3 I am not talking about making a mass produced car of the future that will be the only thing on the road. I am talking about making myself the fastest street car on the planet.
So other than your airport theory can you think of anything else?

[ivymike1031]

Originally Posted by Jared_80

2 The amount of thrust required to push a car along at highway speed is only about 15hp-20hp worth. Not exactly gale force winds in fact I would bet that you could comfortably drive 50' behind one of these cars, because with a fan for propulsinn you are using a lower velosity higher thrust area comonation.
3 I am not talking about making a mass produced car of the future that will be the only thing on the road. I am talking about making myself the fastest street car on the planet.
So other than your airport theory can you think of anything else?

The fastest street car on the planet would require a bit more than 15-20hp at top speed, assuming it was proportioned similarly to other supercars.

Let's assume that by some dumb luck you were able to get the same TSFC as an aircraft engine. Take the cruise TSFC of a CFM56-5C, for example, 0.545 lbm/hr/lbf. Let's assume you're right about using 15hp to travel at 80mph. You'll be burning 70.3 lb/hr of gasoline, or about 6.66 gal/hr. You'd be getting 12mpg - not exactly efficient. That's probably the BEST you could expect to do, too, in a perfect world.

Mechanical traction is far more efficient than thrust as a means of propulsion.

[Jared_80]

Hold on a minute, are you trying to compere hp to thrust? The two do not exactly coralate, and as far as trying to compare the efficency of a jet aircraft to a car that is crazy. You cruse at what 500mph, assuming equal CD and surface area you would be dealing with 64 thimes more aerodynamic drag at crusing speed! (assuming 70mph vs 480mph) How much energy does it take to overcome that drag? Well the avrage street car uses aprox 50% of it's crusing energy to overcome the air at a mere 70mph, I don't calm to be an expert on jet aircraft but I would guss that you use more like 98%. So I don't think that you are making a fair compareson there. Also you said that mechanical energy is more effective at propelling an object???? I would beg to differ. Why then do thrust propelled cars go faster than their equaly powerful turbine counterparts?? Just look at the land speed records.

[ivymike1031]

the "thrust" required to push a car down the road at 80mph is 20hp, according to you. ("The amount of thrust required to push a car along at highway speed is only about 15hp-20hp worth.")

The thrust-specific fuel consumption of an engine, TSFC, is the rate of fuel consumption divided by the thrust output. I picked a "cruise" power setting instead of a takeoff power setting for the comparison, to give you a bit more efficiency. TSFC for a car can be calculated from the information you provided. 20hp @ 80mph means a drag of 93.75lbf (thrust = drag at steady state). If you got the same TSFC out of your fan that the aforementioned turbofan got, then you'd be burning fuel (at 20hp, 80mph) at the rate that I mentioned above.

I don't care to do internet searches all afternoon to try to figure out what vehicles you're referring to. Please state some figures. What is the fuel consumption of the mechanically-driven vehicle at top speed? What is the fuel consumption of the thrust-propelled vehicle at the same speed? What are the engine power outputs?

--- edit
my earlier calc was for 15hp at 80mph, not 20hp at 80mph. The thrust or drag magnitude corresponding to 15hp@80mph is 70.31 lbf. 70.31*lbf * TSFC -> fuel mass consumption rate
fuel mass consumption rate over fuel density -> fuel volume consumption rate
rate of travel over fuel volume consumption rate -> mpg

[454Casull]

What's a thrust? Is that a type of engine?

:confused;

[ivymike1031]

huh? Thrust, as in m.dot * deltaV? You know, the force that is exerted by the air upon an airplane engine, or the force with which the road pushes a car forward? The force that overcomes drag and moves the vehicle?

[454Casull]

Well what is he trying to say? He says "racers have been trying to use thrust for years".

"Why can't we use a thrust powered car?"

[ivymike1031]

He is under the impression that a car propelled by a ducted fan would accelerate more rapidly, for the same engine power output, than a car pushed forward by mechanical traction.

[454Casull]

Originally Posted by ivymike1031

He is under the impression that a car propelled by a ducted fan would accelerate more rapidly, for the same engine power output, than a car pushed forward by mechanical traction.

I wonder what kind of fan could provide that much thrust?

[ivymike1031]

a REALLY big one... check out http://www.moller.com/skycar/m400/

(not a ground vehicle)

[Moppie]

There are variety of practicle problems to go with a "thrust" powered car that havnt even been bought up yet, the idea is clearly unpracticle (its aleady been tried though) but a vechile traveling on the ground suffers from considerably more drag than one that is flying, tyres and axle assemblys create a surprising amount of drag. Having done several hours flying I can tell you there is a noticable increase in accleration when the planes leaves the ground, and loses a lot of friction.
In a vechile large enough, and heach enough to carry people comfterbly the level of friction only increases.

And of course you get massive problems with cornering. Unless the thrust is very carefully vectored it can literaly push the vechile off the road. Its well known that a driven wheel has the most traction, while one that is free wheeling has a lot less (hence the popularity of AWD).

Then of course there are problems with higher center of gravity, major safety problems with having a propeler spinning at very high speeds in a crash, the noise far to much for use in urban areas, and then simple things like how to reverse.



But if you still don't belive the hard maths in the posts above, or the more praticle reasons immediatly above, why not try it and see.

Simple build a rubber band powered wheel driven car, and rubber band driven thrust powered car.
Use the same rubber band on both, and I can garuntee the wheel driven car will signifacntly out perform the trust driven one.

[SaabJohan]

The thrust needed for a car to cruse at 80 km/h won't be much, around 500 N.

A Volvo RM6 engine have a specific fuel consumption of 0.91 kg/kp. This is an old jet engine from the fifties, but it should get us some idea of what the specific fuel consumption might be. If we would use an turboprop engine on constant rpm we would get a much better fuel consumption.
0.91 kg/kph is equal to 0.093 kg/Nh, this means 46.5 kg/h at 80 km/h or 8.3 liter per 10 km. Even if we could cut the consumtion in half by a more efficient engine we would have a consumption of around 5 times higher than of a normal automobile.

Let us now consider the amount of thrust needed for acceleration, lets say that we have a 1500 kg car that we want to do 0-100 km/h in 6 seconds. For this we would need an engine which can deliver around 8000 N of thrust. 6 seconds to 100 km/h is what most sportier family cars can do, but their top speed is usually only around 250 km/h, our car will be able to do around 420 km/h.

The specific fuel consumption isn't equal in all situations, this means that an engine which can give 8000 N of thust will have a poor fuel consumption giving only 500 N. In short terms thrust powered cars aren't suitables for automobiles except for high speed records.

Also, imagine what will happend to pedestrians or even a car that comes in the way when giving full throttle; a pedestrian will easily fly away of that thrust.

[JaSeMoney999]

random notes...
in terms of drivability in front other cars.. you can stand not to far behind a 737 and only notice a slight wind.
the problem is jet engines perform horribly at slow speeds and low altitudes. because at faster speeds more air moves through the jet without having it have to work to move the air. in modern high bypass jet engines.. only 2-3% of the air going through it is entering the burner can. this air is expanded by the fuel burning and it pushes a turbine which drives the front turbine to push most of the air back. at high speeds the jet doesnt have to work hard. because it is moving through the air. at highway speeds the performance would be terrible.

also remeber when they raced the f-166 vs the crazy viper. the viper won in accell, bacause where as mechanical drive is putting all the force to the wheels and creating rotational blah thrust is only blowing air
thats my 6 cents donno if it can help much or not