Thruster May Shorten Mars Trip (from six months to a week!)

Photonics.com News ^ | 9/7/07

[Frank_Discussion]

We live under a constant 9.8 m/s/s load all the time, this thrust would be much gentler than that.

[SunkenCiv]

Wonder how a later generation of this would do blowing ICBMs out of the sky?

[timer]

I’m not a lasar expert but I do know lasars are only about 1% efficient. Achieving a near 100% efficiency seems an impossible dream. We’ll know this fellow has done that when HARD CASH hits the table to develop it.
Past answers : EMSL and the magnetic sail, still viable ideas. So take this with a grain of salt...

[chrisser]

Thank you for that great explanation.

[Jedi Master Pikachu]

100 km/second is around 1/3000 of c.

That's pretty fast considering the Voyagers are set to reach their next solar system (if the universe lasts that long) in around 40,000 years.

[Red Badger]

As long as its not an assault raygun..........or a fully automatic raygun.........

[techcor]

bump for later

[6ppc]

This looks like some kind of scam. A bit of searching finds Young Bae linked to something called the NASA Institute for Advanced Concepts (http://www.niac.usra.edu/) which does not appear to be associated with NASA,

[LibWhacker]

Arghh! Man, I can't even duplicate what I did on that napkin now.

I was using:

d = (1/2)at²

and somehow got a travel distance of only one kilometer after 604,800 seconds (1 week). But now I'm getting 10,584 km, still woefully inadequate to get you to Mars in a reasonable amount of time.

'Course, in reality, the initial velocity isn't zero either. So it's not as bad as I'm making it out to be. But still, he's going to have to scale up the power of his invention by a factor of much, much more than a billion.

More power to him.

[r9etb]

You'd get the delta-V by using v = v₀ + at

Using d = at²/2, an acceleration of 35e-3 m/sec², and t = 604,800 sec, you get a distance travelled of 6.4 million kilometers in one week.

[DuncanWaring]

Does it make sense to you that an object that hasn’t changed its mass becomes more powerful the faster it goes when it is only powered by a constant power source?

It makes splendid sense.

One horsepower is 550 ft-lbs per second.

If your vehicle thruster produces one pound of thrust and the vehicle is not moving, it produces 0 horsepower. If it's moving at a rate of one foot per second, it has 1/550 horsepower. At 550 fps it's producing one horsepower, at 5500 fps it's producing ten horsepower.

Your understanding of the difference between "power" and "thrust" is a little weak.

[LibWhacker]

Oh, that’s right... I wasn’t squaring t. Boy, I’m getting rusty.

[burzum]

Your handwaving about the conservation of energy is unconvincing. Are you honestly trying to tell me that the work done per unit time of a rocket increases with its velocity? The energy for that work has to come from somewhere. This implies that the chemical reactions and fuel use by the rocket increases with its velocity as well (and don’t try to bring up mass lost because if you increase your speed ten fold, you increase your kinetic energy a hundred fold while your mass might only drop by a small amount). Is this what you are trying to say?

[DuncanWaring]

Are you honestly trying to tell me that the work done per unit time of a rocket increases with its velocity?

I most certainly am.

If you had taken high-school physics, you would know that "work" is "force exerted over a distance".

If you push very hard on a very big rock, moving it not at all, you have done no work, no matter how tired you are at the end.

"Power" is the rate of doing work.

If you increase the rate of distance covered (higher speed), you increase the rate of work and hence the power.

Slightly altering the numbers of my previous example, a rocket with 550 pounds of thrust, tethered to remain stationary, does no work. That same rocket moving at one foot per second does 550 foot-pounds of work per second, for one horsepower. That rocket at 36000 feet per second (roughly escape velocity from Earth) has the equivalent of 36000 horsepower.

[DuncanWaring]

P.S. - If you don’t believe me, call down to the local high-school and ask to talk to the physics teacher.

[burzum]

If you had taken high-school physics, you would know that "work" is "force exerted over a distance".

Considering that I am finishing my BS in physics and previously was a reactor operator on a submarine nuclear reactor, I think I have the energy, force, and power ideas covered. Thanks.

I am telling you that you are wrong about the constant force. You are not listening. It is closer to a constant power that is applied and the force applied drops off over time.

Power is the amount of work done per unit time. Let me repeat, this work does not come from nowhere. It isn't magic. You use fuel and chemical reactions. If you are going to exponentially increase the amount of power of your rocket then you are going to have to exponentially increase the size of the support systems in your rocket (i.e. the size of the combustion chamber, the piping for the fuel, the cooling tubes, etc).

Here is a graph of the solid rocket boosters on the Space Shuttle. They separate at 126 seconds (where the graph ends). The initial spikes are caused by mass loss and atmospheric interference. After about 80 seconds (and finally above most atmospheric interference) the profile is clear. Thrust is not even close to constant nor is it trying to be.

[burzum]

P.S. - If you don’t believe me, call down to the local high-school and ask to talk to the physics teacher.

It sounds like you are the one who needs to take this advice. What are your qualifications in physics?

[DuncanWaring]

Let me repeat, this work does not come from nowhere. It isn't magic. You use fuel and chemical reactions.

Of course ... to do otherwise would violate the First Law of Thermodynamics.

If you are going to exponentially increase the amount of power of your rocket then you are going to have to exponentially increase the size of the support systems in your rocket ...

No, you have to increase the size of the support systems if you want to increase the thrust. As the speed of the rocket increases, the power produced by the engine increases because the ft-pounds (or newton-meters) per second increases. But this doesn't increase the acceleration because the F and m of "F=ma" haven't changed. As the rocket accelerates, it's producing more power, which is convenient because it needs to produce more power consistent with the exponential increase in kinetic energy with increases in speed.

Here is a graph of the solid rocket boosters on the Space Shuttle. They separate at 126 seconds (where the graph ends). The initial spikes are caused by mass loss and atmospheric interference. After about 80 seconds (and finally above most atmospheric interference) the profile is clear. Thrust is not even close to constant nor is it trying to be.

The Space Shuttle SRB (or any other solid rocket motor) is not a good example of the point you're trying to make.

Design of the cross-section of the "hole" in the center of the motor is a near-black-art; thrust is proportional to the surface area of the propellant; if the hole starts out circular, the area increases and the thrust increases - sometimes good, and sometimes not. A more typical hole-shape is "star", which gives a more uniform surface area as the propellant is consumed. The variation you show in the SRB thrust is designed in via the propellant configuration - it has nothing to do with speed. Also, the plotted thrust is probably from a test stand at the manufacturer, where the motor isn't moving at all. From the NASA website:

The propellant is an 11-point star- shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure.

The motor described in the original article which precipitated this discussion would produce uniform thrust indefinitely as long as it has power. Liquid-fuel rocket motors are the same - they produce approximately uniform thrust as long as they've got fuel.

[burzum]

The propellant is an 11-point star- shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure.

That's nice, but that doesn't account for a 15-fold drop.

[DuncanWaring]

What 15-fold drop?