So, who wants to win 1 million Euro?

[Sus_pilot]

Heiwa, I've given you a great source to read that reduces everything to layman's terms.  I suggest you read it, since you clearly are a layman.

Also, you really do need to grow up:  stop lecturing everyone on politeness and stop using perjorative terms.

[ka9q]

I'm sure it's calculated in a launch, but due to air resistance and so forth, does the rocket get shut down at a certain speed or is it all precalculated and shut down after a certain time?

Depends on the stage. Lower stages usually cut off at propellant depletion. Solids simply burn out, while liquid rocket engines are usually shut down gracefully when sensors detect that their propellants are below a specified level. During Apollo launches you'll hear the call "level sense arm" and a time during S-II flight; that call lets the crew know when the propellant level sensors in the stage will be allowed to shut down the five J-2 engines. I'm not sure why they were armed, perhaps there was concern about propellant sloshing causing a premature shutdown.

First stage steering is almost always pre-programmed to minimize aerodynamic drag, heating and mechanical stresses. Upper stages actively steer to a specific "target state", i.e., the desired orbit. Their guidance systems shut down on their own when they reach the specified velocity at the specified position in space. The last stage will burn longer or shorter if necessary to compensate for poorer or better than expected overall engine performance. This usually leaves unburned propellant in the tanks that has to be vented after shutdown to keep them from eventually exploding.

For example, on the recent SpaceX Falcon9 launch the first stage burned longer than normal to compensate for the failed engine, and the second stage also had to burn longer because the first stage was less efficient with only 8 engines. This left insufficient fuel for a restart to put the secondary payload in its proper orbit with high probability, so the second burn was inhibited.

[cjameshuff]

This is certainly true for ion engines because they burn for such a long time, but chemical rockets burn so quickly that the change in gravitational acceleration during a burn is very small. Their burns can usually be modeled as instantaneous impulses with little error.

It is a significant factor during initial launch, because about 1 g of acceleration is lost just keeping the rocket from falling back to Earth. A rocket that produces just 1 g will hover without climbing and burn all its propellant without going anywhere. The higher the acceleration, the lower these losses during the initial climb and acceleration to orbital velocity.

Once you're in orbit, this isn't so...typical maneuvers don't use any thrust to directly counter gravity, and the Oberth effect actually makes it preferable to make maneuvers that change the specific energy of the orbit deeper in the gravity well.

[ka9q]

It is a significant factor during initial launch, because about 1 g of acceleration is lost just keeping the rocket from falling back to Earth. A rocket that produces just 1 g will hover without climbing and burn all its propellant without going anywhere. The higher the acceleration, the lower these losses during the initial climb and acceleration to orbital velocity.

Ah, I thought the question was about the change in local gravitational acceleration during a burn, which is minimal for nearly all chemical engines. You are quite right about the large gravity losses during first stage flight, and that's why the S-IC stage had such enormous acceleration at burnout. Gravity losses are maximum during first stage flight when the rocket is pointed mostly upwards to get out of the atmosphere more quickly, and are made even worse by having to overcome the high weight of the as-yet unburned propellants. They gradually decrease as the rocket pitches over to horizontal. (Gravity loss is proportional to the sine of the thrust vector from horizontal.)

One rocket with minimal gravity losses is the Orbital Sciences' Pegasus, which is dropped from its carrier airplane in a horizontal attitude. It does pitch up and climb after ignition, but at a much lower angle than a surface-launched vehicle.

[gillianren]

Also, you really do need to grow up:  stop lecturing everyone on politeness and stop using perjorative terms.

At the very least, pick one.  Either use stupid, childish terms to describe men braver than pretty much any of us here or else stop trying to get us to be more polite to you.  For the record, we're being polite.  Believe me, we could be much ruder if we wanted to be.  Picking on your errors, stating that you aren't an engineer, and saying that we don't believe your money exists is not rude.

[cjameshuff]

One rocket with minimal gravity losses is the Orbital Sciences' Pegasus, which is dropped from its carrier airplane in a horizontal attitude. It does pitch up and climb after ignition, but at a much lower angle than a surface-launched vehicle.

I'm pretty sure that's a rather different matter...I don't think it's dropped at nearly high enough altitude or velocity to make a major difference in gravity drag, it doesn't take long at all for a ground launched rocket to gain enough vertical velocity to reach 12000 m (which it will do well before it actually reaches that altitude).

I think the main gain is in reduction of aerodynamic drag which would otherwise be a problem for such a small launcher. It's a tiny rocket, only massing 18500-23130 kg and only delivering 443 kg to low orbit, and there's about 13000 kg of atmosphere in its way from the ground that it has to plow through (again, at rather high speed by the time it'd hit 12000 m). Larger rockets don't care as much about aerodynamic drag.

[nomuse]

Thanks Glom (edit: and Andromeda, posting while I typed )

Oops, I also forgot about the changing gravitational field. Is that a large or small effect on a launch?

I'm sure it's calculated in a launch, but due to air resistance and so forth, does the rocket get shut down at a certain speed or is it all precalculated and shut down after a certain time?

Rockets are cool

Pete

I know this has probably already been answered in depth already (I'm too lazy to skip through to the end of the thread first, plus if I did, I'd find there was no reason for me to ever post.)

My memory is that part of the Shuttle profile was indeed throttling down the SSME -- I believe right after the solid rocket boosters separated -- just so it wouldn't be moving too quickly through the lower atmosphere.  There's a bit of a pause there in the acceleration profile until the lighter spacecraft also gets higher, then the engines kick on again.

[Bob B.]

Just out of curiosity:

Since the mass of the rocket is changing, does the thrust also change to compensate and keep acceleration constant or does the acceleration increase?

This question has already been answered quite adequately by others, but I just want to add one comment.  There are at least a couple examples of which I'm aware in which a launch vehicle's engines are throttled to reduce aerodynamic stresses at a time when those stresses are greatest. 

One example is the Space Shuttle.  The orbiter's main engines where throttled down to 65% from about 35 s to 65 s during ascent.  I assume you've seen the video of the Challenger explosion?  Just before the explosion you here the Capcom say "Challenger, go at throttle up", which refers to the time when the engines throttle back up to their normal 104%.  It's my understanding that the SSME could vary their thrust through a range of 65-109%.

The other example is the Delta IV-Heavy, which throttles its RS-68 engines down to 60% for some time.  I don't know if the RS-68 has variable thrust or simply two thrust settings of 60% and 100%.

There may be other examples that I'm either unaware of or can't think of right now.  As others have already said, large rocket engines are typically not throttleable.  Their thrust, however, does change as the rockets rise to higher altitude because the ambient air pressure decreases.

[nomuse]

I remind you that topic is So, who wants to win 1 million Euro? 
...

No, the topic is pointing out the errors Anders made in his physics.

Since your method is wrong and your starting assumptions are wrong, no matter how many times anyone does the math the answers are still going to be wrong.  It would be pointless, stupid, and also a form of lying about science for anyone to pretend they can get the right answer using your wrong method.

[Heiwa]

This is certainly true for ion engines because they burn for such a long time, but chemical rockets burn so quickly that the change in gravitational acceleration during a burn is very small. Their burns can usually be modeled as instantaneous impulses with little error.

It is a significant factor during initial launch, because about 1 g of acceleration is lost just keeping the rocket from falling back to Earth. A rocket that produces just 1 g will hover without climbing and burn all its propellant without going anywhere. The higher the acceleration, the lower these losses during the initial climb and acceleration to orbital velocity.

Once you're in orbit, this isn't so...typical maneuvers don't use any thrust to directly counter gravity, and the Oberth effect actually makes it preferable to make maneuvers that change the specific energy of the orbit deeper in the gravity well.

Try to keep to topic, i.e. So, who wants to win 1 million Euro?

As I am offering the €1M award, you have to listen to me and ... be polite. Do not post nonsens posts that I am uneducated, blah, blah. Only uneducated idiots do that, so please avoid it.

One hurdle seems to be how to slow down, brake, the space ship on arrival Moon to get into orbit around Moon as described in previous posts. The space ship is pretty heavy, 43 000 kg, and has just one big rocket engine that can apply a 97 400 N force on it by clicking a switch. The fuel consumption seems to be 30 kg/s. You are in 3-D. To apply the strong force, 97 400 N, it must be applied in the right direction all the time and the direction changes all the time as you turn into orbit. In this case you also go backwards as you are braking - slowing down - and you are pressed into your seat while braking ... looking aft. It is quite complicated and I wonder how the NASA pilots did it.

Apollo 11 apparently managed to slow down from 2400 to 1500 m/s speed by braking at full blast for 6 minutes wasting 10 000 kg fuel in 1969 with the pilots looking in the wrong direction and to win 1 million Euro you have to repeat it.

Navigation at sea is also complicated - http://heiwaco.tripod.com/news8.htm . Do not ever blame the Master if anything gets wrong. 

[Bob B.]

My memory is that part of the Shuttle profile was indeed throttling down the SSME -- I believe right after the solid rocket boosters separated -- just so it wouldn't be moving too quickly through the lower atmosphere.  There's a bit of a pause there in the acceleration profile until the lighter spacecraft also gets higher, then the engines kick on again.

You beat me to it, however the throttle down occurs before SRB separation.  The SRBs separate at about 124 s, well after throttle up.  It's my recollection that the throttle down is to help relieve aerodynamic stresses.

[Heiwa]

One example is the Space Shuttle. 

Is it? Space Shuttle trying to get into Moon orbit? You are trolling off topic and should be warned.

[dwight]

Yeah Bob B. YOU HAVE BEEN WARNED.

[Heiwa]

Bye, bye!

[Bob B.]

To apply the strong force, 97 400 N, it must be applied in the right direction all the time and the direction changes all the time as you turn into orbit.

It is not necessary to keep the thrust vector precisely aligned with the velocity vector, though it is most efficient to do so.  Such maneuvers can be accomplished by maintaining a fixed attitude.