Quick question about rocket engines

[Echnaton]

I heard one ex-USN person claim it was a set up by Iran - a plane loaded with dead people on a suicide mission.

They used that idea in an episode of Sherlock.

And Star Trek DS9.

[Allan F]

What episode?

[Echnaton]

Rules of Engagement

[ka9q]

Multiple launches don't save mass, the actually increase it overall. They do make the mass of each individual launch less.

I'd like to see an analysis.

Flying to the Moon via a liberation point also saves little, if any propellant.

I can cite several spacecraft that did this specifically to save propellant. E.g., the two GRAIL spacecraft went into lunar orbit via the earth-sun L1 point. The cost was >3 months of transit time vs 3 days for Apollo. This kind of trajectory could be used to transfer bulk cargo to a lunar base while crews would use the more costly direct transfer. This is what I meant by reducing total launch mass.

[Allan F]

Rules of Engagement

Tnx. Didn't remember that one.

[Luther]

Wouldn't that have been Iran Air 655, shot down by USS Vincennes?

Correct, I was a guest in his house, so I did not tell him he was an idiot.  I just rolled my eyes.

Well if it had happened more recently, he could just stand up and be proud, and declare that the United States has the right to kill anyone it deems a threat to national security, any time, under any circumstances.

[Dalhousie]

Multiple launches don't save mass, the actually increase it overall. They do make the mass of each individual launch less.

I'd like to see an analysis.

All other things being equal dry mass tends to go up linearly whereas wet mass geometrically.  For the same final payload the overall mission mass will be greater.

Flying to the Moon via a liberation point also saves little, if any propellant.

I can cite several spacecraft that did this specifically to save propellant. E.g., the two GRAIL spacecraft went into lunar orbit via the earth-sun L1 point. The cost was >3 months of transit time vs 3 days for Apollo. This kind of trajectory could be used to transfer bulk cargo to a lunar base while crews would use the more costly direct transfer. This is what I meant by reducing total launch mass.
[/quote]

Good luck with sending crews on trajectories to the Moon that take months rather than days.  The only advantage of lagrange points in crewed missions is they offer the possibility of leaving communications relays for the lunar far side.

As you say, unmanned missions are different.

[ka9q]

All other things being equal dry mass tends to go up linearly whereas wet mass geometrically.  For the same final payload the overall mission mass will be greater.

I don't understand. The Tsiolkovsky equation only deals with mass ratios, not absolute masses. I can see how some overhead mass might not scale linearly with size, e.g., electrical power, guidance and command systems, but these are now so light compared to structure and even rocket engines that I doubt they make much difference unless the launch vehicle is very small.

I'd think a bigger factor would be the I_sp of the rocket engines themselves, with larger ones traditionally having better performance. But I don't know the scaling laws for the current state of the art.

Good luck with sending crews on trajectories to the Moon that take months rather than days.  The only advantage of lagrange points in crewed missions is they offer the possibility of leaving communications relays for the lunar far side.

My whole point was that not all of the mass to support a crewed mission has to be sent with the crew. If you send the support equipment and fuel in advance, it can take its sweet time getting there by an energy-efficient trajectory and be waiting when you launch the crew on a fast, higher-energy path.

[Dalhousie]

All other things being equal dry mass tends to go up linearly whereas wet mass geometrically.  For the same final payload the overall mission mass will be greater.

I don't understand. The Tsiolkovsky equation only deals with mass ratios, not absolute masses. I can see how some overhead mass might not scale linearly with size, e.g., electrical power, guidance and command systems, but these are now so light compared to structure and even rocket engines that I doubt they make much difference unless the launch vehicle is very small.

Simplifying somewhat you can consider mass budgets consist of three categories, fixed mass, linear mass, and geometrical mass items. Fixed mass items - comms, guidance, etc. are basically the same regardless of overall mass.  As you say, these are very small in this day and age.  However it is still more efficient (all things being equal) to provide just one set of these rather than two or more.  Linear mass items are things like engines and tankage, the bigger they are more massive.  Lastly geometric mass items are propellant.  Double the dimensions of the tanks and you cube the volume.  However the mass of the engines and tankage has not cubed, because larger volumes store more efficiently with fewer losses (even allowing for the need for stronger tanks.

I'd think a bigger factor would be the I_sp of the rocket engines themselves, with larger ones traditionally having better performance. But I don't know the scaling laws for the current state of the art.

Indeed, which is why I said all things being equal.

Good luck with sending crews on trajectories to the Moon that take months rather than days.  The only advantage of lagrange points in crewed missions is they offer the possibility of leaving communications relays for the lunar far side.

My whole point was that not all of the mass to support a crewed mission has to be sent with the crew. If you send the support equipment and fuel in advance, it can take its sweet time getting there by an energy-efficient trajectory and be waiting when you launch the crew on a fast, higher-energy path.

You can do that if you want, but if you do, it is better to send them direct to the lunar surface to wait for the crew to land.  No station keeping propellant needed, no rendezvous or docking in orbit, even if contact is lost, they won'r be going anywhere.

IMHO of course!

[ka9q]

You can do that if you want, but if you do, it is better to send them direct to the lunar surface to wait for the crew to land.  No station keeping propellant needed, no rendezvous or docking in orbit, even if contact is lost, they won'r be going anywhere.

Well yeah, you sent the bulk supplies and equipment to wherever you intend to send the crew.

I don't know if a direct ascent would be more efficient for landing on the moon. That would require overcoming lunar escape velocity (in reverse) as well as whatever orbital velocity you have left after leaving earth and climbing to the moon's orbit. If you take a slow, low energy route to lunar orbit via a libration point, then to land you only need get rid of lunar orbital velocity, which is roughly sqrt(2) that of lunar escape. The question is which route has the lower total delta V requirement; I don't know.