So, who wants to win 1 million Euro?

[Bob B.]

One non-chemical engine with a lot of promise is the nuclear thermal rocket, because reactors can produce a lot of power in a very small volume and produce thrusts comparable to chemical rockets. One was actually developed and tested in the early 1960s but was then cancelled. None have ever been flown, but it is probably an enabling technology for interplanetary human space flight.

The nuclear rocket program, Project Rover,  actually lastest until the end of 1972.  I don't know when the last test firing was, but I know tests ran at least as late as 1968.  The work my father did for NASA during that time was part of the Rover program.

[sts60]

Actually, since Heiwa claims to have all this nautical engineering know-how, maybe he would care to explain why a ship with a constant propulsive force doesn't "slow down to 0 m/s by colliding with atoms?"

This is an exact analogy to a spacecraft undergoing an essentially constant downward force (mg) falling through the atmosphere and experiencing drag force.  At terminal velocity, the aerodynamic drag force is equal in magnitude and opposite in direction to the gravitational force.  The exact same thing applies to a ship when the propulsive force is balanced by hydrodynamic drag.

It's baffling that someone who claims to be an engineer is unaware of such simple concepts.

[Bob B.]

Can you, e.g. explain re-entry. You are aboard the famous International Space Station, ISS, that according NASA is orbiting Earth every 90 minutes at 400 000 m altitude (almost vacuum) at 7 200 m/s velocity and you want to go down to Earth. It means you have to go down 400 000 m and slow down from 7 200 m/s to 0 m/s speed. How to do it?

Do you jump into a little capsule with a little rocket engine to slow you down? Yes, apparently you do that and the result is that you arrive at 120 000 m altitude but that the velocity then has increased to 9 000 m/s as some potential energy of the capsule has become kinetic energy = greater velocity. It is like diving from the 10 m board. It gets faster the closer you get to the water.

Where do you get your numbers, dude?  At an altitude of 400,000 m, orbital velocity is 7,669 m/s, not 7,200.  And you won't be going anywhere near 9,000 m/s at 120,000 m.  The exact velocity will depend on the perigee of your reentry orbit, but your velocity will certainly be less than 7,900 m/s.

[gillianren]

Let me ask a stupid question or two?

I haven't stopped you from doing so before, so why start now?

Oh, that was beautiful!

[peter eldergill]

Again a curious (probably yes or no answer)

Are the speeds used for lunar spacecraft susceptible to relativistic effects? I know that for GPS satellites to work properly you need to take that into account

Pete

[ka9q]

Are the speeds used for lunar spacecraft susceptible to relativistic effects? I know that for GPS satellites to work properly you need to take that into account

Relativistic effects are present at any speed. It's just that at low speeds they're usually too small to notice, and are swamped by various errors.

It's actually somewhat of a misconception that GPS won't work without explicitly taking relativity into account. GPS is a closed-loop system that provides position and time with respect to a set of ground tracking stations and ground reference clocks. The loop is closed when these stations upload orbital elements and clock correction data that each satellite rebroadcasts to the users for use in calculations. This data repeats every 30 seconds and changes every 2 hours.

If relativity were simply ignored, its effects would appear as additional errors to be corrected by the monitoring stations. The system would still provide accurate position and time, but the clock error terms would be considerably larger and take more time to transmit. Since the GPS data rate is only 50 bps, and receiving it is a major cause of receiver first-fix delays, this is important. By including relativity in the model (particularly by biasing the clocks slow before launch to account for the general relativistic blue shift of gravity) only the unmodeled "noise" in the spacecraft clock needs to be transmitted in the ephemeris, and the system is faster and easier to use.

[Chew]

At 120 000 m altitude there is a thin atmosphere with nitrogene and oxygene atoms that you collide with and ... MAGIC ... suddenly you slow down to 100 m/s (at say 5 000 m altitude) and deploy a parachute and land.

So atmospheric drag will work with parachutes but it won't work with anything else?

[ka9q]

So atmospheric drag will work with parachutes but it won't work with anything else?

It works with cats:

http://www.todayifoundout.com/index.php/2010/11/domestic-cats-can-fall-from-any-height-with-a-remarkable-survival-rate/

[Heiwa]

Actually, since Heiwa claims to have all this nautical engineering know-how, maybe he would care to explain why a ship with a constant propulsive force doesn't "slow down to 0 m/s by colliding with atoms?"

A sea going ship, engine of which via the propeller applies a force F to the ship, proceeds at constant speed, say x knots, while the environment (water/air/friction/collisions with atoms) applies a force -F to the ship = there is balance.

If you ran out of fuel and the engine applies force F=0 to the ship, the ship slows down until the speed is 0 knots, when all forces acting on the ship including atom colliding with it add upp to 0.

Evidently if the ship is in a sea current you have to consider that the sea current may modify the ship's speed.

[LunarOrbit]

Your other post was not approved due to it being rather trollish.

[gillianren]

Evidently if the ship is in a sea current you have to consider that the sea current may modify the ship's speed.

I suspect this correction will do as much good as any of the others, but "evidently" doesn't mean the same thing as "obviously."  It is closer to "apparently."  What you basically said there is that you guess you'd have to consider the current, if the ship is in a current, but you are sure there are arguments against it.  Though of course, if you did mean that, it would be about equal to the rest of the knowledge you've shown about physics.

[Sus_pilot]

Actually, since Heiwa claims to have all this nautical engineering know-how, maybe he would care to explain why a ship with a constant propulsive force doesn't "slow down to 0 m/s by colliding with atoms?"

A sea going ship, engine of which via the propeller applies a force F to the ship, proceeds at constant speed, say x knots, while the environment (water/air/friction/collisions with atoms) applies a force -F to the ship = there is balance.

If you ran out of fuel and the engine applies force F=0 to the ship, the ship slows down until the speed is 0 knots, when all forces acting on the ship including atom colliding with it add upp to 0.

Evidently if the ship is in a sea current you have to consider that the sea current may modify the ship's speed.

Ok, good. 

Now, take an object, say, a re-entry vehicle using a blunt body, or a glider, like the Shuttle, or an airplane with the engine shut down and imagine that the force to move it is gravity pulling it towards Earth.  Since said object encounters aerodynamic drag, the forces will, at some point balance out and a certain velocity will be reached, depending on the amount of drag said shape will encounter.

"But wait!" I hear you cry.  "Why does an airliner need engines?  Just like my ocean liner, it needs them to move forward.  Your whole analogy is wrong!"

No - in level flight, an airplane needs enough thrust to maintain the same speed at a given angle of attack.  Else, to stay level, it would have to increase the wing's angle of attack, increasing drag, and slowing down. At some point, the wings' critical angle of attack will be exceeded and it will cease to fly, and the aircraft will simply fall (unless the pilot recovers from the stalked condition by lowering the angle of attack).  Or, without engines, if it maintains the same angle of attack (and same drag), it will descend, converting potential energy to kinetic, to overcome said drag.

"Ah!  I still have you!  What about the fact that the airplane climbs?"

Well, the if you have more thrust than is needed to maintain level flight for a given angle of attack, that energy has to go somewhere.  You can either lower the angle of attack, reducing induced drag, and go faster (until induced, parasite, and form drag all equal thrust, so you stabilize at your new speed).  Or you can keep the same angle of attack and climb. 

Oh, one more thing.  A vessel does not "know" about current.  It just knows about the mass of fluid, be it water for a ship, or air for an aircraft, blunt body, whatever.  The only time currents or wind come into play is at boundary of two masses moving at different velocities and/or directions (else, how can I have flown a Cessna 172 at a ground speed of over two hundred knots without damaging the airplane when it's never-exceed speed is 163 knots?).

[ka9q]

Are the speeds used for lunar spacecraft susceptible to relativistic effects?

It really depends on the spacecraft and the mission as to whether the effects are noticeable. For Apollo, probably not; there were enough other sources of error, such as the incompletely modeled lunar gravity field, to swamp out the effects of relativity.

But for a mission like the recently concluded GRAIL spacecraft pair that measured that lunar gravity field, the instrumentation was probably sensitive enough to detect relativistic effects, so they had to be accounted for in the data processing.

[Glom]

OK Heiwa, you want to do a simple energy difference calculation? Fine. Forget burning the fuel. Just imagine that the spacecraft dumps all that fuel overboard in a non-propulsive way. Its mass decreases, it's velocity remains unchanged. Its kinetic energy therefore has decreased. Where did that energy go? How did the kinetic energy of the spacecraft change? The answer to that might help you with the answer to your original issue.

Where did that energy go? It was dumped!

So kinetic energy of spacecraft before = kinetic energy of spacecraft after + kinetic energy of dumped propellant?

That's energy balance. What you've been doing isn't energy balance because you're missing out terms. That's why you keep getting the wrong answer.

[Zakalwe]

Hint 1: Fuel != kinetic energy.
<snipped for space>
When you dive from a 10 m board you do not need a parachute.

Let me start off by also reinforcing that I am most certainly not an engineer. I am nothing more than a bloke with a keen interest in the Apollo program.
I can recognise when people do know what they are talking about, and when people don't. And I have learned an absolute load about Apollo from reading this forum, including learning enough to correct parts of my understanding.


Heiwa,
This thread is now 44 pages long with nearly 700 posts in it. During it's course you have repeatedly been shown to have fundamental errors in your understanding and knowledge. You have been given copious examples that demonstrate, sometimes in painful detail, where you have been in error. The post above shows just how thin your understanding is, or alternatively how desperate you appear to be to be wilfully ignorant and a troll.

Do you you accept this?

Do you acknowledge where your understanding has been incorrect and can you state that you have corrected your thinking?

Instead of throwing up another spurious example of your lack of understanding, can you address these issues? Doing so would go a long to showing that you are not deliberately being obtuse and trolling for reaction, but that you are able to learn new things and correct errors in your understanding.