So, who wants to win 1 million Euro?

[Noldi400]

Heiwa:

Linear momentum (p) is directly proportional to kinetic energy, being p=mv

Not quite - it's a square relationship as KE = p²/2m

Your point about the Rocket Equation still stands, though.

Well, yes. I phrased it badly, forgetting that you engineers are SO DAMN LITERAL! _[SNICKER] (My daughter has her M.Sc. in Engineering Physics - I should know better.)

The point I was trying to get across  (hopelessly, I know) to Heiwa is that there is a specific relationship between LM and KE of a given rocket in a given circumstance; that relationship is, as you quite correctly pointed out, a square.  I temporarily forgot that "directly proportional" means "multiplied by a non-zero number" in this context.

I was just really wondering why H. is so focused on an energy equation that he clearly doesn't understand when the Rocket Equation is so straightforward and difficult to screw up, even for a non-engineer non-mathhead like me.

Then I saw this:

Mag40:
Read it and weep -

https://sites.google.com/site/wtc7lies/home/anders-bjorkman-s-world

http://forums.randi.org/showpost.php?p=4280640&postcount=130
Weight (kg) or load (kg) = mass (kg). Yes, I am an engineer. What are you?

I THOUGHT "Anders Bjorkman" sounded familiar - I've read some of his babbling on JREF. He's got more 'roos loose in the top paddock than the Victoria Zoos.

[frenat]

The plot thickens when you realize that France has some of the most stringent laws in the world regarding the practice of engineering.  Criminal penalties apply to the practice of engineering without a degree or license, and to the malpractice of engineering even by licensed practitioners.  I wonder if the Beausoleil magistrates are aware of what is happening on their doorstep.

Aha, you are now an expert of French law? Are you suggesting I am breaking the law? That is serious! Off topic, of course. Anyway, the Tribunals d'Instance and de Grande Instance are located at Nice but you can always alert our local police municipale or nationale here at Beausoleil, if you are concerned. I think you are just upset not beating my Challenge. Yes, it is hard not to solve my Challenges.

Have you still not understand that it is IMPOSSIBLE to win my Challenges. Physical principles do not allow it.

Not a single person here is upset that they aren't winning your challenges.  Not a single person here believes they are serious challenges or that you have any of the money you say you do.  You have refused to provide proof and you have refused to acknowledge your many errors.

It is not impossible to win your challenges because physical principles do not allow it.  That has been shown multiple times already in this thread and IGNORED by YOU.  It is impossible to win because YOUR EGO will not allow you to admit you are wrong. 

For the record, I'm not upset either.  I'm amused by you.

[ka9q]

it's a square relationship as KE = p²/2m

And that's the "dismal equation" of rocketry, expressed more clearly than I did.

It shows that the energy you need to get a certain impulse (momentum) p from a reaction mass m increases as the square of the impulse p. E.g., you can get 1 newton-second (N-s) of impulse from a reaction mass of 1 kg by giving it an energy of 1/2 joule. Getting 2 N-s from that same 1 kg of ejected mass requires 2 joules of energy or 1 joule per N-s, twice the 1/2 J/N-s of the first example.

A given total impulse is therefore a tradeoff between energy and reaction mass. If you want to carry less reaction mass, you'll have to eject it with more total energy to get that same total impulse. If you can afford to carry more reaction mass, then you won't need as much energy to eject it.

It's even better if you don't have to carry any reaction mass at all because you're surrounded by it, as in the case of land, sea and air transportation.  A car only needs a source of energy; it doesn't need to carry any reaction mass because it can simply grab the road with a set of tires and use the earth as its reaction mass.

Space is (mostly) a vacuum, so you generally have to carry your reaction mass with you.

But space isn't really quite so bleak. In the inner solar system, at least, you are surrounded by fairly abundant energy from the sun, so you don't necessarily have to carry your energy with you; that's why most spacecraft are solar powered. And those same solar photons carry momentum in the form of radiation pressure, and it's possible to make use of them as reaction "mass" with a solar sail. Although I don't think a true solar sail has yet been successfully flown, the phenomenon of radiation pressure is very real and visible in the operation of many spacecraft, usually as a wanted or unwanted source of torque on a spacecraft body.

[Donnie B.]

The Planetary Society attempted to launch a true solar sail test vehicle a couple years ago, but the launcher failed.  They have an improved model ready to go and are looking for a ride to orbit.

[ka9q]

The Apollo SPS engine, which is an AJ10-137 engine with 91,000 lb thrust

91 kN thrust, actually.

It's only that large because it was specified before the choice of lunar orbit rendezvous, when it was thought that the entire Apollo spacecraft would land on the moon and take off again.

[cjameshuff]

And those same solar photons carry momentum in the form of radiation pressure, and it's possible to make use of them as reaction "mass" with a solar sail. Although I don't think a true solar sail has yet been successfully flown, the phenomenon of radiation pressure is very real and visible in the operation of many spacecraft, usually as a wanted or unwanted source of torque on a spacecraft body.

Solar sails have had bad luck just getting to the point where they can begin deployment, but IKAROS flew a while back. A small sailcraft, but a real sailcraft, designed to maneuver using nothing but photon pressure.

MESSENGER used solar sailing to adjust its trajectory for flybys, using its solar panels as sails. It wasn't designed for this initially, but once tried as a way to avoid a risky burn just before the first Mercury pass that could give useful science data, it apparently became a standard way to fine tune MESSENGER's trajectory:
http://www.jhuapl.edu/messenger/the_mission/publications/OShaughnessy.LCPMC.2011.pdf

[LunarOrbit]

MESSENGER used solar sailing to adjust its trajectory for flybys, using its solar panels as sails.

Does that mean they have to worry about unintended course changes just because light from the Sun just happened to be hitting the solar panels?

[Sus_pilot]

What was the Arthur C. Clarke short story about a race involving solar yachts?  I just re-read it a week ago, but I don't have that collection handy right now... 

[frenat]

What was the Arthur C. Clarke short story about a race involving solar yachts?  I just re-read it a week ago, but I don't have that collection handy right now... 

Sunjammer
http://en.wikipedia.org/wiki/Sunjammer

[cjameshuff]

Does that mean they have to worry about unintended course changes just because light from the Sun just happened to be hitting the solar panels?

They have to model photon pressure on the panels to accurately control the orbit, but that's the case for Earth satellites as well. The difference is that they were able to replace small but significant adjustment maneuvers (on the order of m/s) by taking advantage of the pressure on the panels.

[grmcdorman]

I was trying something out; I thought I'd take Heiwa at his word, specifically that the final KE of the spacecraft must equal the initial KE plus (or minus) the energy of the fuel burned.

That is, he states this formula for a reduction in velocity:
 0.5 M_i * V_i² - 0.5 M_f * V_f² = M_fuel*enthalpy*efficiency
or, for an increase:
 0.5 M_f * V_f² - 0.5 M_i * V_i² = M_fuel*enthalpy*efficiency
is true (am I using the correct term for the fuel energy, J/kg, here?).

So I solved for V_f for an acceleration, with fixed values for all but efficiency:
   V_f = sqrt( ( 0.5 M_i * V_i² + M_fuel*enthalpy*efficiency ) / (  0.5 M_f ) )
or step wise:
   initial kinetic energy, KE_i: 0.5 M_i * V_i²
   fuel energy used in velocity change, E_f: M_fuel*enthalpy*efficiency
   final KE, according to Heiwa, KE_f: KE_i + E_f
   final mass, M_f: M_i - M_fuel
   thus, final velocity: sqrt( 2 * KE_f / M_f )
The results were surprising, especially for lower fuel energy values (i.e. E_fuel << KE_i).

Before I post this, can some one else do this independently so I can be sure I haven't messed up my calculations? Did this in a spreadsheet (Open Document, i.e. Libre Office/Open Office) so I can provide that as well if one sends me a PM.

[ka9q]

Does that mean they have to worry about unintended course changes just because light from the Sun just happened to be hitting the solar panels?

Yes, all spacecraft do. And it's not just when sunlight hits the solar panels but when it hits any part of the spacecraft.

In fact, the effect is greater when sunlight strikes a reflective surface than an absorptive one like a solar panel. When a photon is absorbed, the target picks up its momentum. But when it reflects, the target picks the photon's momentum, and then additional momentum from launching the reflected photon -- and this can be in a different direction. That's how you "tack" with a solar sail.

Because of changing cross sectional areas, absorptivities and reflection angles, solar photon pressure on spacecraft is notoriously hard to model and predict. It's one of the major residual perturbations in the orbits of the GPS satellites that requires frequent ephemeris updates.

Radiation pressure also appears to be the cause of the famous "Pioneer Anomaly". In this case the radiation was in the far infrared from the RTG generators rather than reflected visible/near-IR from the sun, and the unbalanced force was due to reflection of RTG heat from the back of the antenna. Because the antenna is pointed close to the sun, this resulted in a steady sunward acceleration of the spacecraft, opposite to the direction of solar radiation pressure which at that distance is very small anyway.

I once did a back of the envelope calculation and found that the magnitude of the Pioneer "anomaly" was close to the acceleration one would expect from its thermal radiation. So I assumed that this was the most likely reason, but mine was hardly a scientific, rigorous analysis. The Portuguese team that conclusively proved the effect used a very elaborate ray tracing model similar to that now used in rendering computer graphics: http://arxiv.org/pdf/1103.5222v2.pdf

Radiation pressure can have very significant effects on astronomical objects given enough time; see Poynting–Robertson effect and Yarkovsky effect.
 

[ka9q]

While solar radiation pressure can affect the orbit of a spacecraft, usually the most important effect is the creation of external torques that complicate attitude control. This is usually unwanted, but it can also be taken advantage of. Many small AMSAT spacecraft have used solar radiation pressure to maintain a stabilizing spin. These spacecraft generally have "turnstile" VHF/UHF antennas consisting of metal blades (often hardware store measuring tape) projecting out from the corners of one side.

By painting one side of each blade white and the other black, the differential solar radiation pressure creates a torque that causes the satellite to spin away from the white side. (Note this is opposite to the spin in a science museum gift shop radiometer, which works on a different principle.) The satellite is kept from spinning ever faster by the damping effect of eddy currents induced in its conductive metal structure by rotating in the earth's magnetic field.

Often the solar radiation torque is unwanted. The Voyager spacecraft carried a large amount of attitude control hydrazine to counteract the expected solar radiation torque on the long magnetometer boom, but somebody realized after launch that an even easier way to do this was to simply spin the spacecraft slowly around its antenna axis (which is pointed at the earth, and thereby close to the sun) to cancel out the net torque on the boom. As a result both Voyagers have considerably more hydrazine than had been planned, enough to last until their RTGs no longer produce enough power to run them in another decade or so.

[Heiwa]

Do you deny the existence of aerodynamic drag? 

No, evidently not, pls refer to my presentation - link in post #1. IMO the drag and lift forces acting on, e.g. the Apollo 11 command module, when velocity was say 6 000 m/s  will rotate the module and kill the people inside. The module was not stable ... like a Ford Edsel.
But I fear the module will burn up already at 7 000 m/s speed in the atmosphere.

[Heiwa]

Heiwa:

Can you tell us why you are so wedded to your attempt to draw conclusions from the kinetic energy of a spacecraft?  The Tsiolkovsky rocket equation, which is derived from the linear momentum of a rocket when acted upon by outside forces, will give you an answer that accounts for the energy changes.

I am only interested in the kinetic energy of the space ship itself before/after burning fuel. Masses/velocities are known. The difference is the kinetic energy of the fuel burnt that produce the force that change the velocity of the space ship. The Tsiolkovsky formula is not required for that - why use it? And it seems that too little fuel was available to produce the velocity changes of Apollo 11 and that the numbers produced by NASA do not add up as shown in my simple presentation.