HP vs Torque vs Fuel Efficiency

[niky]

Originally Posted by sendler

True. My point exactly. In all gears those lines will all show the same power at the wheel as the engine is sending out even though the torque is much higher in the lower gears, the wheel speed is much lower. It's a wash. The power at the wheel is the same in any gear.

No. The force at the wheels is different due to the gearing. Power to the wheels is force applied over time, which is shown in the second graph. Perhaps it's better understood as acceleration in "gravities"... which is always greatest in the lowest gear (barring traction issues), which indicates that more work is being done.

Yes. the acceleration inlower gears is always much greater than the higher gears because the speed is much lower. Hence, the energy of the vehicle is much lower. So any new energy that is added from the engine power is a big change. At faster speeds the energy of the vehicle can be 20 times, ect, what it was at the slower speed. But you are still only adding more energy from the same engine output 1 unit at a time. Now each time you look at it, you have increased the energy of the vehicle only 5%.

Speed is inconsequential. If you can exert the same amount of accelerative force at high speeds as at low speeds, you can give the same acceleration rate. See the example, again, of rockets, which have linear acceleration because they don't rely on gearboxes to translate engine rotation into motive force. It is only because we use driven wheels to move cars that acceleration tapers off.

Look at the dyno of the trucks again. They are modern turbo diesels and they do have a very weird, ultra flat power band. At any identical wheel speed where the gears have an rpm that is on the power band, say 3rd is 3,500, 4th is 3,000 and 5th is 2,500, The acceleration would be exactly the same. The gearing can't transform the power it is given. It only transforms the wheel speed to the engine speed.

Again, the dyno is meaningless in this context, because you're using it incorrectly. It's an extrapolation based on wheel torque. And altering the ratio of wheel speed to engine speed DOES alter the acceleration potential of the engine. Which means it DOES have an effect on work done. Even on cars with a totally flat torque curve, acceleration is always fastest in the lowest gear.

I do get what you're trying to say. If a motor makes x hp at y speed, it should have z acceleration, regardless of gearing or rpm. However, the dyno you cite doesn't show that. And only electric motors can make constant power over wide spreads, which is why most electric cars can do without a gearbox.

I'm not using horsepower per second. I am using horsepower times seconds. Which is energy. The same as a Watt hour or a Joule. Momentum is just one of many forms of energy.

You're describing momentum. You have to use the units for momentum. Have you ever tried multiplying horsepower by time? I made that mistake when building the Excel calculator that generated the first sheet. It spat out gibberish. Pro-tip, don't ever try to calculate wheel-horsepower by running engine output through a gear-ratio conversion.

Horsepower is torque/time. Horsepower x time is equal to (torque x time)/time. Which, by process of cancellation, is torque.

Originally Posted by mort

This is nonsense. It takes power to accelerate in space just like on earth. If your engine produces 100 hp and your rocket mass is 600 lb your acceleration at 100 mph will be .625 g. At 1000 mph acceleration will be .0625 g

-mort

Your acceleration rate would stay the same until relativistic effects increase the mass enough to blunt accelerative force. There is no drag in space, so acceleration will not taper off.

Actually, that's slightly wrong. As the rocket accelerates, it's using up propulsion mass... which makes it lighter... which makes it accelerate faster.

As seen here:

The acceleration change for the first stage is more dramatic because it's leaving the gravity well. The final stage is more indicative of how a rocket will accelerate in flat space, with acceleration increasing slightly every second that reaction mass is consumed.

A rocket with infinite fuel or, even better, a ramscoop, will show up on that graph as a flat line. It accelerates at a constant rate given constant thrust output.

It doesn't matter what speed you're going. Applying force x in one direction causes x acceleration in the other, along any vector, no matter what your velocity is on the other vectors.

This is because a rocket doesn't use propellant to push against the space behind it. It pushes on the propellant itself. Which makes the actual speed of the rocket inconsequential, much like a hypothetical infinite treadmill under the wheels of an airplane at take-off.

Besides, as the entire Milky Way is moving at 135,000 miles per hour towards the great attractor, a difference of just 900 mph in the rocket's relative velocity is just a drop in the bucket.

[mort]

Originally Posted by niky

Your acceleration rate would stay the same until relativistic effects increase the mass enough to blunt accelerative force. There is no drag in space, so acceleration will not taper off.

Hi niky,
I am using the word power to mean power, in the exact specific physics sense: P = F x V

None of NASA's graphs show power they show acceleration. In the case of big rockets the power generated at lift-off is lower, because the nozzle is poorly coupled to the atmosphere for low speed - it's low efficiency. By design they are built to deliver maximum efficiency near the expected burn-out altitude and speed. Each rocket nozzle is designed to get good efficiency over a specific speed range. Faster and you lose a lot of thrust. Since all the propellant will be used, and for various engineering legacy reasons, rocket designers never talk about power, it is irrelevant to the design. Specific impulse and thrust are what is tossed around.
But just for fun consider: The main engines on the Shuttle: near the point of normal shut off, that is over 100 miles up, and at about 17,000 mph with the fuel almost used up, the mass of the Shuttle was about 250,000 lbs. Because of mechanical and human limits, acceleration was generally below 3 g, the engines were throttled back to about 750,000 lbs of thrust. So 750,000 lbs x 17,000 mph/375 = 34 million horsepower. Full power (or 109% throttle - the allowed maximum) is where the maximum amount of fuel that can be pumped is being burned and the nozzle is near 100% efficiency. Fuel cannot be pumped faster, the engine is at peak power.
If you had more (weightless) fuel and continued to run the engines, your acceleration would fall off. So at around 140,000 mph they would just be able to make about 1 g. That's because the engine cannot produce ever increasing power. As you go faster engine-nozzle efficiency falls off too. Of course, as you mention fuel isn't weightless and the effect of throwing off mass is important.

It doesn't matter what speed you're going. Applying force x in one direction causes x acceleration in the other, along any vector, no matter what your velocity is on the other vectors.

This makes me think you are confusing force and power.
Every engine has a maximum power output. For a rocket that happens at the maximum fuel use rate and maximum nozzle efficiency. When it is at that output the force (thrust) the engine makes is exactly the power divided by the speed. As the speed increases the thrust decreases. If you want to continue accelerating at a constant rate, you need to add more power (which can't happen) or throw off more mass.
-mort

[niky]

Acceleration is a direct product of power.

Here you describe thrust as power divided by speed.

Speed of what? Of the exhaust gases, correct? What do you measure the speed of the exhaust gases against out in deep space?

The spacecraft, of course. In free space, there are no other inertial frames to measure exhaust velocity by. Exhaust leaving the rocket at 100 mph relative to the nozzle will leave it at 100 mph relative to the nozzle regardless of the absolute velocity of the spacecraft at the time.

Again... we're talking about a craft launched from a planet embedded in a galaxy that is moving at over 100,000 miles per hour towards the Great Attractor. If absolute velocity actually affected acceleration, a rocket fired towards the Great Attractor would accelerate measurably slower (compared to the Earth) than one fired away from it. Which doesn't happen.

[sendler]

Originally Posted by niky

...

You are bringing some great info to the thread. The acceleration vs gear charts are something I haven't seen and did help me clarify some fuzziness I had on force vs power vs energy. I could make a long post to discuss some of the errors I see in your long post and then you could make another longer post to show me where I misspoke. It seems to me we are both over our heads a bit. I hope no one will really take my advice on how to build a rocket ship.
.
The other thing that is happening is that the discussion is branching out a few different ways. We all know that air resistance becomes a sizable force the faster you go and actually becomes significant to the the complex analysis at speeds greater than 30 or 40 mph. While I on the other hand am just trying to examine the pure physics of mass, velocity, force, power and energy and torque multiplication. For which a rocket in space that doesn't use fuel ( ie lose mass, or gain mass due to relativity or is not flying toward or directly away from a black hole.) is a good example for some things I want to look at up to the point where we talk about torque multiplication and power which rocket scientists don't often car about.

[mort]

Originally Posted by niky

In free space, there are no other inertial frames to measure exhaust velocity by.

Inertial Frame of Reference
-mort

[sendler]

"the centrifugal acceleration of the Earth because of its rotation about the Sun"
.
Should that be edited to say revolution?

[niky]

Originally Posted by mort

Inertial Frame of Reference
-mort

So what are you measuring the velocity of exhaust flow by? How fast the exhaust is flowing compared to a planet hundreds of thousands of kilometers away, or how fast it is flowing compared to the nozzle it is coming out of?

Because that exhaust will always be moving thousands of miles per hour relative to something. And it won't matter. What matters is how fast it is moving relative to the spacecraft. Period.

Let's make this simple. You're inside a bullet train moving at 200 mph. Does it take more effort to run towards the front of the train or the back? Because that's what you're doing firing a rocket while moving through space. There's no wind resistance, no drag, no nothing to make you use more force to accelerate along any vector.

Or, if you want a long-winded explanation:
HowStuffWorks "What would happen if you fired a gun on a train moving as fast as a bullet?"

[mort]

Originally Posted by niky

Because that exhaust will always be moving thousands of miles per hour relative to something. And it won't matter. What matters is how fast it is moving relative to the spacecraft. Period.

Hi niky,
Oh, I have an idea, try reading this.
You know there are other ways to accelerate a body in space besides throwing mass out the tail pipe. If you use a microwave engine what would you be measuring your speed against then?
-mort

[niky]

I did. Twice now. I am still puzzled why you think that it actually disproves my point. Unless you're suggesting that a rocket and its exhaust form a non-inertial frame of reference... because they don't. I wish you would explain how a rocket travelling at one arbitrary speed in relation to a distant object is any different from another rocket travelling at another arbitrary speed in relation to the same distant object.

This is basically a two-body problem. The rocket is one body, the exhaust is another. Nothing else really matters until you get up to a significant portion of the speed of light.

-

If the EM Drive is real, and it actually works, it's a photon drive, just like a light sail, so, yes, you are throwing out mass behind you in the form of radiation.

Of course, the inventor claims that the radiation never leaves the device, and that it can produce electricity on deceleration (in spaaaaaace), which means that he has just invented perpetual motion. Since deceleration is merely acceleration in the opposite direction.

-

http://www.youtube.com/watch?v=57q3_aRiUXs

Find it funny that the Chinese test rig isn't isolated from the atmosphere. A metal microwave resonator would make a very nice, toasty surface. In which case, it's producing thrust by heating the air around it... as the world's most expensive and over-rated electric fan.

I'd test the whole thing in a vacuum chamber with a Faraday cage to isolate it. A rotating rig sitting on the lab floor is just so... drama queen-ish.

[sendler]

So P=M*V*A, P is persistence or momentum. Not Power.