Rocket Science 101

[JohnHunt]

If I am going to be able to propose mission designs then I figure that I ought to understand enough about rocket science to do some of my own calculations.

I am not an aerospace engineer. The most that I have had is college physics. I don't specifically remember being taught about the rocket equation or delta-v (although I may well have) but I am certain that I was never taught about IsP.

In this thread I plan to comment on the progress of my own self-education in this field.

First question is this: Can anyone suggest a good source as a primer for Rocket Science 101? Ideally this would be a videotaped lecture series (for free of course), a website, or a book?

[JohnHunt]

QUESTION - How much payload could be delivered to the lunar surface using two Falcon 9 Heavies?

ANSWER - According to this graph (http://en.wikipedia.org/wiki/File:Deltavs.svg), the delta-v from LEO to the Moon's surface is (4.1 + 0.7 + 1.6 = 6.4). Will this result in a stopped landing at the lunar surface or at orbital speed just meters above the surface. I presume the former since, the graph would not be useful if it were the latter.

According to the SpaceX websit (http://www.spacex.com/falcon9_heavy.php)e, an F9H can deliver 32,000 kg to LEO. So two would give 64,000 kg.

The Isp for LOX/H2 is 455 sec (http://www.brighthub.com/science/space/articles/32392.aspx) and so, using this graph (http://www.spacetethers.com/massfraction.gif), the mass fraction would therefore be right at 75%.

Therefore, two F9Hs should be able to deliver 16,000 kg to the lunar surface [64,000 * (1-.75)].

DISCUSSION - Are my calculations correct? I find 16,000 kg to be a higher number than I expected. Perhaps I am not taking something into account. This news report (http://www.flightglobal.com/articles/2008/10/02/316680/spacex-offers-nasa-80-million-lunar-cargo-lander-service.html) indicates that Elon Musk offered to NASA that it could deliver a 1,000 kg package to the lunar surface for $80 million. A single Falcon 9 can deliver 8,560 kg (http://www.spacex.com/falcon9.php) to LEO. So that should be able to deliver 2,580 kg to the lunar surface. Considerably more than what Musk offered.

[joertexas]

If I am going to be able to propose mission designs then I figure that I ought to understand enough about rocket science to do some of my own calculations.

I am not an aerospace engineer. The most that I have had is college physics. I don't specifically remember being taught about the rocket equation or delta-v (although I may well have) but I am certain that I was never taught about IsP.

In this thread I plan to comment on the progress of my own self-education in this field.

First question is this: Can anyone suggest a good source as a primer for Rocket Science 101? Ideally this would be a videotaped lecture series (for free of course), a website, or a book?

This is where I started:

http://www.projectrho.com/rocket/index.html

It deals with science fiction, but the basics are all there, and the website itself deals in hard science. Note the Delta-Vee Nomogram. That's worth a visit to the site all by itself.

Other sites:

http://www2.jpl.nasa.gov/basics/index.php
http://www.niac.usra.edu/files/studies/final_report/341Rice.pdf
http://www.4frontierscorp.com/dev/assets/Braun_Paper_on_Mars_EDL.pdf
http://www.braeunig.us/space/index.htm
http://www.jaqar.com

JR

[joertexas]

QUESTION - How much payload could be delivered to the lunar surface using two Falcon 9 Heavies?

ANSWER - According to this graph (http://en.wikipedia.org/wiki/File:Deltavs.svg), the delta-v from LEO to the Moon's surface is (4.1 + 0.7 + 1.6 = 6.4). Will this result in a stopped landing at the lunar surface or at orbital speed just meters above the surface. I presume the former since, the graph would not be useful if it were the latter.

According to the SpaceX websit (http://www.spacex.com/falcon9_heavy.php)e, an F9H can deliver 32,000 kg to LEO. So two would give 64,000 kg.

The Isp for LOX/H2 is 455 sec (http://www.brighthub.com/science/space/articles/32392.aspx) and so, using this graph (http://www.spacetethers.com/massfraction.gif), the mass fraction would therefore be right at 75%.

Therefore, two F9Hs should be able to deliver 16,000 kg to the lunar surface [64,000 * (1-.75)].

DISCUSSION - Are my calculations correct? I find 16,000 kg to be a higher number than I expected. Perhaps I am not taking something into account. This news report (http://www.flightglobal.com/articles/2008/10/02/316680/spacex-offers-nasa-80-million-lunar-cargo-lander-service.html) indicates that Elon Musk offered to NASA that it could deliver a 1,000 kg package to the lunar surface for $80 million. A single Falcon 9 can deliver 8,560 kg (http://www.spacex.com/falcon9.php) to LEO. So that should be able to deliver 2,580 kg to the lunar surface. Considerably more than what Musk offered.

First, I'd be careful with the 1.6 km/sec figure to land on the moon. The Apollo missions used more than that, and I'm allowing 2.3 km/sec, which is more in line with the actual landing fuel requirements, plus a reserve.

Second, your mass fraction is off, because you don't want to land the entire craft on the moon if you have to lift the fuel to do it from Earth. It's much better to separate the vehicles in lunar orbit and only land the mass you need on the lunar surface.

As an example, the unmanned moon mission I'm planning starts off with a single F9. Jaqar's Lunar Transfer Orbit Calculator is a *huge* help, especially if you plug in the performance numbers for the F9H.

Anyway, the F9 can put 1973kg on a TLI trajectory. With that, I can put a lunar orbiter massing 165kg into a polar LLO, and a lander with a gross mass of 1320kg, and a cargo mass of 330kg, on the surface. That's pretty close to Musk's stated performance, with some variance due to the reserve fuel.

Now, the TLI is done at launch, which you can't do if you have to assemble the craft in LEO first. That by itself is a powerful incentive to not try multiple launches - unless the linkup is done in lunar orbit. You'd have to run the numbers to see how the TLI burns work out.

I decided to go with nitrogen tetroxide and hydrazine for all of the engines, which nets an Isp of 329 sec on average. These systems are simple, lightweight and very reliable, but you'd get better performance from the LH/LOX engines. Just remember to allow for the cryogenic fuel storage and attendant propellant losses on the trip to the moon.

JR

[JohnHunt]

Wow, that detailed response is a big help. I'm going to have to look it over carefully, learn a bit more and see if I can figure out (& replicate) your numbers so that I can learn how to do this myself.

But I wanted to respond briefly.

be careful with the 1.6 km/sec figure to land on the moon. The Apollo missions used more than that, and I'm allowing 2.3 km/sec,

This concerns me. The calculations are only as good as the data that goes into it. Do you think that this is just an odd error (is there a more accurate graph/chart) or is there something fairly significant that the graph doesn't take into account?

Second, your mass fraction is off, because you don't want to land the entire craft on the moon if you have to lift the fuel to do it from Earth. It's much better to separate the vehicles in lunar orbit and only land the mass you need on the lunar surface.

Interesting. I might actually intend to land the whole thing and take off with the whole thing. The reason that I say this is that I am working towards "sustainable space development". If you leave stuff on the lunar surface then (as I see it) that's like throwing it away. So you have to replace it each time. That gets expensive and so threatens sustainability. Besides, if you need to refill the EDS so that it can launch a new payload from LEO then you're going to have to lift quite a bit of fuel from the lunar surface and so you need a fairly large rocket (albeit maybe only 1/6 the size that you need when you depart Earth)

Well, as I'm writing, I realize that I need to think this through some more. But what's your initial reaction?

As an example, the unmanned moon mission I'm planning starts off with a single F9.
I'm sorry. What is this for? Is it for a sample mission? Why not use an F9H? It's not all that much more expensive than an F9. (Of course, don't be coming to me to make up the difference!)

with a gross mass of 1320kg, and a cargo mass of 330kg, on the surface. That's pretty close to Musk's stated performance,
No, Musk is quoted as saying that he could deliver 1,000 kg to the lunar surface for $80 million. Gross mass doesn't count. Payload is what matters and your payload is 1/3 of what he stated. An F9H has 3 times the starting fuel so, perhaps, that is where the figure comes from.

I decided to go with nitrogen tetroxide and hydrazine for all of the engines, which nets an Isp of 329 sec on average. These systems are simple, lightweight and very reliable,
Well, I appreciate that. But what I am working on it is sustainability which counts. And you can't replenish either because you can't get nitrogen. But you can get oxygen and so I figure that I need to go with that regardless if it is more difficult. However, since we're on the topic, might there be a useful way of chemically combining the oxygen with something so as to ease the storage loss? Especially as a fuelled up EDS sits in LEO waiting to be used, a cryogenic tank might loose a lot of its propellant. Or, would shades and a solar-powered condenser (what is a cryogenic condenser called) be adequate to permanently maintain the liquid oxygen? Thx,

[joertexas]

Quote:
be careful with the 1.6 km/sec figure to land on the moon. The Apollo missions used more than that, and I'm allowing 2.3 km/sec,

This concerns me. The calculations are only as good as the data that goes into it. Do you think that this is just an odd error (is there a more accurate graph/chart) or is there something fairly significant that the graph doesn't take into account?

I think the graph is the minimum, not landing plus reserve fuel. The Apollo landings used something like 1.9 km/sec on average.

Quote:
Second, your mass fraction is off, because you don't want to land the entire craft on the moon if you have to lift the fuel to do it from Earth. It's much better to separate the vehicles in lunar orbit and only land the mass you need on the lunar surface.

Interesting. I might actually intend to land the whole thing and take off with the whole thing. The reason that I say this is that I am working towards "sustainable space development". If you leave stuff on the lunar surface then (as I see it) that's like throwing it away. So you have to replace it each time. That gets expensive and so threatens sustainability. Besides, if you need to refill the EDS so that it can launch a new payload from LEO then you're going to have to lift quite a bit of fuel from the lunar surface and so you need a fairly large rocket (albeit maybe only 1/6 the size that you need when you depart Earth)

Well, as I'm writing, I realize that I need to think this through some more. But what's your initial reaction?

Since this is the first mission, I'm not worried about sustaining these vehicles and spacecraft for a long time. They will only need to operate long enough to assay the lunar ice in part of Peary Crater.

Quote:
As an example, the unmanned moon mission I'm planning starts off with a single F9.

I'm sorry. What is this for? Is it for a sample mission? Why not use an F9H? It's not all that much more expensive than an F9. (Of course, don't be coming to me to make up the difference!)

A F9H may become feasible - if it flies. Until then, I won't count on it being available.


Quote:
with a gross mass of 1320kg, and a cargo mass of 330kg, on the surface. That's pretty close to Musk's stated performance,

No, Musk is quoted as saying that he could deliver 1,000 kg to the lunar surface for $80 million. Gross mass doesn't count. Payload is what matters and your payload is 1/3 of what he stated. An F9H has 3 times the starting fuel so, perhaps, that is where the figure comes from.

You're probably correct.


Quote:
I decided to go with nitrogen tetroxide and hydrazine for all of the engines, which nets an Isp of 329 sec on average. These systems are simple, lightweight and very reliable,

Well, I appreciate that. But what I am working on it is sustainability which counts. And you can't replenish either because you can't get nitrogen. But you can get oxygen and so I figure that I need to go with that regardless if it is more difficult. However, since we're on the topic, might there be a useful way of chemically combining the oxygen with something so as to ease the storage loss? Especially as a fuelled up EDS sits in LEO waiting to be used, a cryogenic tank might loose a lot of its propellant. Or, would shades and a solar-powered condenser (what is a cryogenic condenser called) be adequate to permanently maintain the liquid oxygen? Thx,

From what I've read, it's possible to store LH and LOX with sustainable losses. However, I'd as soon transport ice or water, and crack it when it's needed versus trying to store the LH and LOX.

JR

[joertexas]

By the way, here is the article about storing cryogenic propellants in space:

http://www.dunnspace.com/cryogen_space_storage.htm

JR

[joertexas]

This website is a good resource, and it includes the article I posted above.

http://www.dunnspace.com/index.htm

JR

[JohnHunt]

Could any of you guys explain a few things for me?

I understand that EML1 is along the line of sight between the Earth and the Moon. It is mentioned as a good place for depot storage and for a manned base. Why is this? Does one have to spend fuel at all in order to stop at this location? If so, what advantage would there be in doing that?

Also, charts show delta-v's between certain locations. Are there reasonable assumptions as to whether one is stopped at that place or whether one is in an orbit there? For example, I would guess that the delta-v's listed for the Moon, L-points, Mars, and Mars' moons presume that one is stopped at those locations. But I would guess that the delta-v's listed for LEO, GTO, GEO, TLI, and LLO presume a velocity and direction needed in order to maintain those orbits. Is this correct?

Is TLI just a Homann Transfer to the Moon or is it a unique path and not really an orbit?

Is there an fuel savings by going directly from the Earth's surface to the Moon as opposed to launching into a "parking orbit"? such as LEO and then firing one's EDS from there? If there is a difference in the amount of fuel to do these different things, then how can one explain that when looking at a chart of delta-v's?

[sgeos]

The first thing to remember is that you do not ever "stop" as such. Everying is always in motion. An orbit is essentially a velocity and a direction. Barring things like atmospheric friction or collisions, your orbit will not change. You can spend fuel to change the characteristics of your motion. Delta-v (change in velocity) is this change. As various celestial bodies move about, they will exert gravitational pulls on each other to greater or lesser extents. You do not need to spend fuel to stay in orbit, but you may need to spend fuel to correct your obit if it is changed by something in the environment (passing planet, atmospheric friction, collision, etc). You also need fuel to proactively change your orbit.

Think of EML1 as the top of a hill, with the earth on one side and the moon on the other. If you are perfectly balanced on top of the hill, you will stay put. If anything throws you off balance (ie, a passing planet), you will roll toward the earth or the moon depending on the direction of the push. To the extent you are more or less already on top of the hill, it doesn't take a whole lot get back there if you roll a little one way or the other. EML1 is a great place to be, because you can easily roll things down to either the earth or the moon from this location.

In certain cases, I think you might be able to save fuel by skipping the parking orbit, but you'd need enough fuel to get to your destination in one shot. I think that this is the appeal of HLVs.

[RaresH]

It's funny you mention Project Rho's Atomic Rocket Joe. That's where I'm learning my 'very basic' rocket science. As an artist I think it's a great resource for creating conceivable ideas. Even if it is for entertainment.

[joertexas]

It's funny you mention Project Rho's Atomic Rocket Joe. That's where I'm learning my 'very basic' rocket science. As an artist I think it's a great resource for creating conceivable ideas. Even if it is for entertainment.

It's where I started...

JR

[RaresH]

Is there an fuel savings by going directly from the Earth's surface to the Moon as opposed to launching into a "parking orbit"? such as LEO and then firing one's EDS from there? If there is a difference in the amount of fuel to do these different things, then how can one explain that when looking at a chart of delta-v's?

Yes there is. You can get it by plugging in your delta V to orbit and delta V to your destination in the equation total delta V = squareroot(deltaVtoOrbit^2 + deltaVtoDestination^2). so you add the square root of each deltaV and calculate the square root of that to get your total deltaV. It does save a lot of fuel but you have to have a smaller launch window in that you have to wait till the earths orbit aligns closely with the injection window of your destination.

[joertexas]

Yes there is. You can get it by plugging in your delta V to orbit and delta V to your destination in the equation total delta V = squareroot(deltaVtoOrbit^2 + deltaVtoDestination^2). so you add the square root of each deltaV and calculate the square root of that to get your total deltaV. It does save a lot of fuel but you have to have a smaller launch window in that you have to wait till the earths orbit aligns closely with the injection window of your destination.

Which, by the way, the first two Moon Shot missions would do. The OTV launches would not be able to take advantage of this phenomenon, because the OTVs have to be assembled in LEO.

JR