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Cheap Satellites Follow Up

Rocketman has a post on the X-Prize and related subjects that's worth reading, but there are a couple problems with it.

This is the most egregious:

The difference in energy required for a vehicle to reach the 100 mile altitude necessary to achieve orbit is ~25 times greater than the energy necessary to reach an altitude of 50 miles (I leave it as an exercise for the readers to figure out the difference in energy necessary between 62.5 and 100 miles).

This makes no sense at all. The difference in altitude between 50 and a hundred miles is, well, fifty miles. It's merely doubled, so it makes no sense that it would be twenty five times the energy.

The problem of course, is that there are two components to energy--the specific potential energy as represented by the altitude (approximated as gravity times the altitude), and the kinetic energy, corresponding to the velocity (half the velocity squared). By ignoring the latter, this statement comes out completely garbled (and the exercise left for the readers is utterly meaningless, and would be frustrating to any who attempted it). Energy is a combination of both altitude and velocity, and the big problem in getting into orbit isn't the former, but the latter.

Orbit is harder because it has go faster, not because it has to go higher. X-Prize is probably achievable at Mach three or four (say, a couple thousand miles an hour), and getting to a hundred miles wouldn't require much more energy. Orbit requires seventeen thousand miles an hour--that's the real killer.

He makes another point that's more arguable (as opposed to physical nonsense), and I'll argue it, as I did in last night's post and today's Fox column.

The statement that the "'harsh environment' of space was less harsh than that imposed by the ocean on the submersible" is just silly. Deep Rover operates in the ocean at a maximum depth of 1000 meters (3280 ft). At that depth, you are surrounded by water that is at ~40 degrees F and ~120 PSI. In space you are in a vacuum and your vehicle is exposed to direct solar energy that heats up one side of the vehicle and the vacuum of space that cools off the other.

The temperature extremes that exist in space create some difficult engineering problems because of the differences in thermal contraction and expansion that occurs between dissimilar materials. I have had to deal with these problems in my designs, and it is not trivial to engineer effective solutions.

Unlike vehicles that operate in salt water, the choice of materials that can be used in space is extremely limited. Most common materials get brittle at cold temperatures, and they also outgas in a vacuum, which changes their material properties. Some materials have problems with salt water, but there are many common materials that can be used under the conditions Deep Rover operates at.

But the biggest difference between a submersible and a spacecraft is that submersibles do not have to fly. You can afford to have relatively large factors of safety and, if necessary, redundant components in a submersible because weight is not a big issue. Also, spacecraft are subjected to tremendous dynamic and acoustic vibrations during launch, vibrations submersible never see. Designing and testing components to handle the vibrations of launch is again not a trivial problem (I speak from experience on this as well).

No matter what Maryniak would like to believe, space is an extremely harsh environment to design for. It also is not cheap to test components to determine how they will handle that environment. You cannot just sail out to deep water and drop your vehicle in the ocean to test it like you can with a submersible. Environmental chambers with liquid nitrogen ?cold walls,? large halogen lamps and huge vacuum pumps are needed to conduct these tests. And even the largest of these chambers is incapable of testing a complete launch vehicle, so components have to be tested individually.

They're both harsh environments--but they're different kinds of harsh. The marine environment is extremely corrosive, and it's much more difficult, from a structural standpoint, to deal with many atmospheres of positive pressure (the deep sea) than a single atmosphere of negative pressure (the vacuum of space). Yes, space has radiation and temperature extremes that the ocean doesn't, but both environments are harsh. For example, the choice of materials that can operate in salt water are limited as well.

Many of the implications of expensive launch are subtle, but they validate Gregg's (and my) point.

Every objection that he has would be obviated by cheap launch, a point with which even he agrees at the end. If launch were cheap, you could afford heavier satellites, because the additional mass wouldn't be so expensive. If launch were cheap, you could afford more redundancy. Cheap launch systems will have relatively smooth rides (because they'll have to in order to be reliable and affordably reusable) so the launch environment won't be an issue. Cheap launch implies affordable test facilities on orbit, so the components can be tested more easily.

So I'm not sure what his point is in arguing with Gregg on this issue.

Posted by Rand Simberg at September 18, 2003 04:24 PM

Comments

Rocketman is quoted as saying:  "Deep Rover operates in the ocean at a maximum depth of 1000 meters (3280 ft).  At that depth, you are surrounded by water that is at ~40 degrees F and ~120 PSI."  That statement's pretty egregious in its own right.  The pressure of water at 1000 meters deep is more than an order of magnitude greater than 120 PSI.

Posted by Michael E. McNeil at September 18, 2003 05:59 PM

Thank you for pointing out my error Michael. I accidently entered 3280 in of water instead of 3280 ft of water when I did my calculation.

Posted by Rocket Man at September 18, 2003 10:13 PM

Rand Simberg is pretty critical of my post. I'm sorry he doesn't like it, because I largely agree with his post. Building satellites would be a lot easier and cheaper if launch costs were reduced. But I was not talking about satellites in my post, I was talking about launch vehicles. Building a launch vehicle capable of reaching orbit is much more difficult than building satellites.

His first comment is -

This is the most egregious:

The difference in energy required for a vehicle to reach the 100 mile altitude necessary to achieve orbit is ~25 times greater than the energy necessary to reach an altitude of 50 miles (I leave it as an exercise for the readers to figure out the difference in energy necessary between 62.5 and 100 miles).

This makes no sense at all. The difference in altitude between 50 and a hundred miles is, well, fifty miles. It's merely doubled, so it makes no sense that it would be twenty five times the energy.

I probably could have been clearer in the post, but I was comparing the difference between just reaching an altitude and attaining orbit. If you follow the link in the post you will learn that -

Basic Primer: Getting to Space vs. Getting to Orbit

There is a big difference between the two, which I?ve found the vast majority of people don?t appreciate. Comparatively speaking, getting to the ?boundary? of the atmosphere or the beginning of space, which NASA typically considers to be about 50 miles up, and falling right back down is easy, but getting to orbit and staying there is hard.

The best way to appreciate this is to look at the amount of energy required. To do a straight up and down trip to the 50 mile point only requires reaching a speed of mach 5, but staying in orbit requires reaching mach 25. However, the energy required to reach a particular velocity increases as the square of the velocity. So, you actually need 25 times more energy to reach orbit than to reach the edge of the atmosphere.

Rand is also critical of my comments about the difference between a submersible and a spacecraft -

They're both harsh environments--but they're different kinds of harsh. The marine environment is extremely corrosive, and it's much more difficult, from a structural standpoint, to deal with many atmospheres of positive pressure (the deep sea) than a single atmosphere of negative pressure (the vacuum of space). Yes, space has radiation and temperature extremes that the ocean doesn't, but both environments are harsh. For example, the choices of materials that can operate in salt water are limited as well.

Many of the implications of expensive launch are subtle, but they validate Gregg's (and my) point.

Every objection that he has would be obviated by cheap launch, a point with which even he agrees at the end. If launch were cheap, you could afford heavier satellites, because the additional mass wouldn't be so expensive. If launch were cheap, you could afford more redundancy. Cheap launch systems will have relatively smooth rides (because they'll have to in order to be reliable and affordably reusable) so the launch environment won't be an issue. Cheap launch implies affordable test facilities on orbit, so the components can be tested more easily.

First of all, salt water is only corrosive over time. For something that is not continuously immersed in salt water, a good coat of paint and a good wash down with clean water afterwards will keep the majority of the corrosion at bay. And while your choice of materials for a submersible is limited (as I acknowledged in the post), it is not as limited as it is for space vehicles.

Second, as I stated before, I agree that satellites would be a lot cheaper if launch costs were reduced. But again, I am not talking about satellites; I am talking about launch vehicles. The purpose of the post was to discuss how the X Prize can help to bring launch costs down, which in turn would bring down the cost of launching and building satellites.

Finally, I agree with Rand that building satellites is not inherently more difficult than building submersibles, but I disagree with Gregg Maryniak that building launch vehicles is not inherently more difficult than building submersibles.

Posted by Rocket Man at September 18, 2003 10:45 PM

I didn't say I didn't like you post. In fact, I said, "Rocketman has a post on the X-Prize and related subjects that's worth reading."

And when you say that, "I disagree with Gregg Maryniak that building launch vehicles is not inherently more difficult than building submersibles..." you apparently disagree with a strawman, since I doubt that Gregg has ever, or would ever, say that. He certainly doesn't say it in the excerpt you cite.

Posted by Rand Simberg at September 19, 2003 07:28 AM

Shameless plug alert: I've got more ammo at http://avoyagetoarcturus.blogspot.com/2003_09_01_avoyagetoarcturus_archive.html#106398671610645647. Enjoy!

Posted by Jay Manifold at September 19, 2003 08:57 AM

Sorry Rand, I was not trying to put words in your mouth. I am an engineer, not a writer, and one of the reasons I started my blog was to improve my writing skills. What I probably should have said was something like -

I'm sorry he disagrees with some of the points in my post, because I largely agree with his post.

After rereading my post and Gregg's article, you are right that Gregg never specifically said "that building launch vehicles is not inherently more difficult than building submersibles," and the excerpt of his article I posted does not make it clear that he is even talking about launch vehicles.

Gregg's full article is 16 pages long, so I did not post the whole thing. But the thrust of his article is the "parallels between aviation and spaceflight and specifically human spaceflight. When reading Gregg's comment about submersibles considering the context of the entire article, I was left with the impression that he was comparing submersibles to launch vehicles. If you consider Gregg's comments about submersibles without refrence to the rest of the article, I would actually agree with him (and you), if launch costs were cheaper. As you said -

The real reason comes back to transportation costs. Simply put, satellites are expensive because getting them into orbit is expensive.

If I got the wrong impression from Gregg's article, I have no problem admitting to it.

Posted by Rocket Man at September 19, 2003 09:05 AM

Frankly the idea of putting the primary propulsion system for attaining orbit ON the vehicle itself is demented. I think far better results would be gained by devising a ground based launching system based on a superconductive coil gun or rail gun technology. Supplementing this initial velocity gain with rocket boosters would probably make it viable. Power it with a couple nuclear reactors.

All in all this sort of system would probably cost far far less per launch than the ridiculous system we have now. In addition the lower acceleration rates for, let's say, 10 mile long launch system would actually make orbit practical for even invalids.

Frankly the government should just fire NASA and start over. Begin with a crew that doesn't want to do the same damn thing like some sort of mobius strip. Ever since chemical rocket scientists took over NASA it's been little more than a joke. What exactly was the point of going to the moon in a launch vehicle that cost as much as a skyscraper?

*shrug* go figure.

ed

Posted by ed at September 19, 2003 06:24 PM

I think far better results would be gained by devising a ground based launching system based on a superconductive coil gun or rail gun technology... the lower acceleration rates for, let's say, 10 mile long launch system would actually make orbit practical for even invalids.

Uh... not quite. Work out the numbers for constant acceleration:

distance = (acceleration/2)*(time)^2

and

velocity = acceleration*time

imply that

acceleration = (velocity^2)/(2*distance)

Picking a practically-achievable maximum velocity for a rail gun of 4 km/s (which is still far from orbital velocity) and using your ten-mile rail, the acceleration turns out to be over 50 gees... and since it's reciprocal with length, that means that reducing it to a more-reasonable 2.5 gees would require a track over 200 miles long. Since acceleration goes like the square of final velocity, reducing acceleration by reducing muzzle velocity is less of a losing game, but it still means much lower velocities (under 0.9 km/s in this case).

Rail-guns (or more practically, coil guns) might serve as a first-stage replacement, but no more.

Posted by Troy at September 19, 2003 09:24 PM

Ah, 2 areas I know something about: Submarines (I've been involved with the development of Submarine Combat Systems) and Spacecraft (I've been involved with the development of Data Handling Systems for a Satellite).

Submarines are hard. But very easy to test, you can replicate the environment quite easily. Also, most submarine systems are only expected to work in a relatively benign environment, ie inside the pressure hull. Sure, you get drastic pressure changes (like when the snort suddenly cuts, and those diesels suck out the air so you go from 1 atm to 0.8 in about 5 secs), you get salt water spray, you get all sorts of chemicals in the atmosphere, etc. In military subs, you also have to withstand shock, vibration etc from enemy action. But it's all testable, and quite cheaply.

Compared with the Spacecraft torture chambers that are Thermal-Vacc testers, where you have to wait days to get the chamber pumped out to a sufficient hardness... testing submarine stuff is easy. Simulating the whole space environment with free electrons, atomic oxygen, X-rays etc etc is damn near impossible. Just the vibration testing for the ride up requires specialised, expensive vibration tables. Failure at that point often means explosive disassembly.

Actually making the stuff, rather than testing it, is probably comparably difficult. Most submarine electronics are serviceable on-board though, most space electronics are on RPVs, autonomous platforms, or just plain not accessible without a spacewalk.

Remember I'm a Software engineer who only knows enough about hardware to be able to design systems. But even with that caveat, Space is much harder than Submarine gear, just as Submarine gear is much harder than Surface gear.

Posted by Alan E Brain at September 21, 2003 05:23 PM

The Russians had a nice (but heavy) workaround to the hardware-in-a-vacuum problem: they put all the critical stuff inside a pressure shell, and let it run in a much more normal environment (this even for unmanned vehicles).

I think much of what's perceived as "harder about space" is an artifact of the U.S.'s particular path into space: we had a high-tech mentality, coupled with low-throw-weight boosters. So we've made it harder than necessary to do... more money for the aerospace biz, though, donchaknow?

Posted by Troy at September 21, 2003 08:50 PM


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