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To The Moon, Alice

One of the nice things about having a blog is that you can self publish. This is the original piece that I submitted to Popular Mechanics, which inspired them to ask for a "revision" which they then edited to what was actually published. I thought that readers here might appreciate it.

Location, location, location.

Those are the proverbial three rules of real estate. They aren't restricted to terrestrial transactions--location matters a lot, sometimes a lot more, in space.

Recently, Michael Benson, a guest columnist at the Washington Post, proposed that the problem with the International Space Station is that it is in the wrong place. He proposes that it be refitted as an interplanetary spaceship.

It's a novel proposal, and he's in good company--a lot of people are thinking about what to do with the ISS after 2015, for which there is currently no official US policy. The foreign partners and other stakeholders recently met to discuss the issue, though if this particular option was discussed, there is no mention of it in the reporting, or the joint statement they provided after the meeting. There's probably a good reason for that.

Mr. Benson is clearly earnest, but the concept is not as well thought out as he seems to think. The ISS is designed for operations in low earth orbit (LEO), but that is a unique environment, and had trips beyond that been its intended use, both the requirements and the design would have looked very different.

What does NASA think?

I called Mike Curie, in the NASA Public Affairs Office for the ISS, to get the official agency response. It was predictable, concise, and (in my opinion) correct: "We welcome and share Mr. Benson's enthusiasm for the space station program, but the proposal is not feasible."

He suggested that I talk to Tom Jones, four-time Shuttle astronaut (and Pop Mechanics space consultant) for further elaboration, so I did.

The idea has several problems," he told me. "If you do it with chemical propellant, the structure won't be able to take those high thrust levels, particularly the fragile solar panels that were designed for zero gee. Also, the Station isn't designed to operate for long periods of time without resupply of things like food, water, and spare parts for maintenance. You'd have to develop a duplicate interplanetary system just to deliver the supplies and rotate the crew."

"Once out in deep space, the ISS doesn't have the radiation shielding it would need for either lunar operations, or even traversing through the Van Allen belts, particularly if you did it slowly with a low-thrust system, as he suggests."

"The Station is also overdesigned for an interplanetary mission in some ways. It's a laboratory facility designed to rely on frequent resupply and contact with Earth. This is not an operational space vehicle. It's more of a technology test bed, to learn how to do things in space, and take advantage of the near-Earth space environment. It's really better and more cost effective to keep it here and use it for what it was designed."

In fairness, Mr. Benson attempted to anticipate these objections:

It's easy to predict what skeptics both inside and outside NASA will say to this idea. They'll point out that the new Constellation program is already supposed to have at least the beginnings of interplanetary ability. They'll say that the ISS needs to be resupplied too frequently for long missions. They'll worry about the amount of propellant needed to push the ISS's 1,040,000 pounds anywhere -- not to mention bringing them all back.

There are good answers to all these objections.

Well, he has answers, but they don't seem to be very good ones. One wonders if he actually ran any numbers.

How much propellant would it take? Well, to leave LEO and go almost anywhere else, you need to have escape velocity. In orbit, that means adding about forty percent to your current speed of twenty-five thousand ft/sec, or about ten thousand ft/sec. The station weighs on the order of a million pounds. Assuming that you could provide the necessary thrust without snapping off the solar arrays, using liquid oxygen/hydrogen (the most efficient practical propellant combination we have today at a generous specific impulse (Isp) of 480 seconds (not far from theoretical), it would take almost as much propellant as the payload (over 900,000 lbs).

Now that's not necessarily a lot--it would be a couple dozen launches of, say, a Delta IV, which might cost a few billion dollars. But the problem is that all that does is get the ISS out of earth orbit. It doesn't have any way to park in orbit when it gets to the moon or Mars, or even an asteroid encounter. To do that it needs (in Mr. Benson's words) a "drive system and steerage module" (whatever that means) which he hand waves off as "technicalities."

You also need propellant. A lot of it.

That means that we not only have to accelerate the ISS itself out of LEO, but also all of the propellant that it will need at its destination as well, which would likely be many hundreds of thousands of more pounds. So we have to recalculate our escape, and now we need, say, a million pounds of propellant to send with the station to its destination, and another two million to blast the whole lot out of earth orbit. So now we're up to many billions of dollars for the propellant delivery to LEO, even ignoring the "technicalities."

Ah, you say, but he suggested using low-thrust high-Isp ion-propulsion systems, which will require much less propellant.

So he did, but he didn't consider the radiation problem, as Tom Jones noted. You'd fry the crew and the electronics, including solar panels, in short order, even if you're lucky enough not to be hit by a solar flare in all that time.

Considering all the other factors he explained, clearly, the ISS is built for LEO, and it should stay in LEO.

But that raises another question. Is it in the right LEO?

The ISS is in a 52-degree inclination orbit. This location was chosen in 1991, when it was decided to bring the Russians into the program, using some of their modules as the core of the station. At the time (and now) their primary launch site was Baikonur, and that was the lowest inclination to which they could launch from that location. The Shuttle pays a high payload penalty to reach that orbit (the original space station plan was to have it at 28.5 degrees, the same as the Cape's latitude, so they could get there with a due-east launch and maximum payload). In fact, every vehicle that goes to the ISS would deliver more payload if it were in a lower inclination. With Russian plans to start launching Soyuz out of the Arianespace launch site in Kourou, near the equator, they will have the capability to get to almost any inclination, so the old Baikonur constraint will be gone.

It might be worth doing a trade study to see if its inclination could be lowered, using ion propulsion, over a period of months or years, as I suggested several years ago. This would avoid the radiation problems of sending it out of LEO by this technique, because the whole trip would remain in LEO, and in fact the radiation reduces with the inclination. This would not only save money on resupply costs (or rather, provide more payload for the same amount of money, because the cost of the flights is fixed, while their payload can vary), but also perhaps put it in a more desirable location to serve as a way station to beyond LEO. It would also put it to use as the test bed that Tom Jones pointed out that it truly is, proving out long-duration ion thrusters that might allow future vehicles to operate more effectively.

So it might be time to consider a move to a better neighborhood--just not one quite as out of this world as Mr. Benson suggests.


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Dennis Wingo wrote:

The station is in a fine orbit where it is. Lowering the inclination does not have that many benefits. The payload penalty is 6.3% according to all of the existing launch vehicle docs. It is in a better orbit for lunar polar departures and for ion propulsion systems for the spin out and spin back. Geek points if you can figure out why.

narciso wrote:

We'll need another station, at a libration point; probably L-3 or L-4; but this station is no way to get there. Reminds of the MIR in Armageddon, or that Russian satellite in Space Coyboys.

Mike Puckett wrote:

"Dennis Wingo wrote:
The station is in a fine orbit where it is. Lowering the inclination does not have that many benefits. The payload penalty is 6.3% according to all of the existing launch vehicle docs. It is in a better orbit for lunar polar departures and for ion propulsion systems for the spin out and spin back. Geek points if you can figure out why"

Orientation re the Earth's magnetic field and the Van Allen belts.

B.Brewer wrote:

Changing the station's orbit is easy. Changing the politics that put it there is not.

ken anthony wrote:

The ISS isn't a space ship, but neither is anything else on the drawing boards (to my limited knowledge and humble opinion.) We need different vehicles for different phases of the journey. They should stay in place and be reused. For example, a lunar lander should be based in lunar orbit at a refueling facility. It's only job is ferry from orbit to lunar surface and back again. Every lunar mission should not bring it's own lander. It's ridiculous. Make it robust and keep it on station.

Capsules so far have been used to take people to and from earth orbit. Fine. But a space ship, once in orbit, should remain in space. We take the mass to orbit once! Then we use it to transfer to other orbits where a landing vehicle is already waiting.

Specialize each vehicle and you'd get a lot more bang for the buck. Use each vehicle once (with all modules together) guarantees the cost stays way above what it could be.

I know it's preliminary, but is anybody designing a ship for SPACE? That is, with enough radiation shielding and the ability to be refueled with room for say a year or more worth of consumables and recycling facilities; Able to go anywhere in the inner solar system with up to perhaps a dozen crew? Others with less crew for faster transit times perhaps. I realize there is a trade-off for time and consumables along with other variables. A real space ship.

BTW, I much enjoyed both this and the edited version, with the details here much appreciated.

Ralphe wrote:

I read Mr. Benson's article and thought---Interesting, for about five seconds. I was more charitable than you Rand, I just rule of thumbed 6000 fps to get it to a place where the moon might capture the thing and didn't worry about what kind of orbit might result.

I do those kind of ballpark calculations to help me get to sleep. Like you just worrying about the propellant mass and little else, and even though there is no booster we can just hook up to the station to give it a lift, was enough to make it a no go venture.

I dismissed a low thrust system like an ion drive only because we don't have one.

I read your Article yesterday and even as edited it was interesting enough but your post above is better. I am happy to see what you really wrote.

Even if we could, or in the future can, send the thing somewhere else, it makes no sense to do it till we know where we want it and when we want it there, even assuming we will be able to use it somehow. It ought to cost less to haul fuel in the future than now. If it doesn't, then moving the station anywhere makes no sense, but it is good to Mr. Benson cares and got published. I would say at a minimum put it in a higher orbit and don't just send it downwards.

It might be part of a hotel someday, or at least a tourist attraction.

Josh Reiter wrote:

You know, I thought that part of the original article didn't quite have your voice, Rand. I imagined then, that it was clipped for brevity.

Regarding the solar panels, couldn't they just be rolled up for the departure burn? Once escape velocity was achieved they could be unfurled again, correct? Also, isn't the ISS about as well shielded against radiation as the Apollo SM/LM?

Godzilla wrote:

I have a simple solution for the radiation breakage problem. Send a giant cardboard box to LEO, with lotsa styrofoam balls for conditioning and radiation shielding (its hydrocarbon based, so it has lots of hydrogen for shielding). Tie it with a giant piece of string and pull it with a tug.

*sarcasm mode off*

Brock wrote:

Ken, before anyone is going to spend the $$$ to design Lunar infrastructure as you describe, there's got to be a clear need for it. The business case for regular lunar traffic (at this time) is speculative (to say the least). Someone's going to need to go the Moon "fully packed for the trip back" and prove a case while they're there before it happens.

I would love to see a space ship as you describe though.

ken anthony wrote:

there's got to be a clear need for it

We landed six lunar modules on the moon. Imagine if we only sent a single more capable one. Your right that the need may not be clear, but if we're spending my money (and yours) I want to do it with vehicles that have a life greater than a few days and a single crew.

Jim Oberg wrote:

I'm already usig the 'location' mantra in another article -- great minds occasionally parallel...

Frank Glover wrote:

A better argument could've been made with Skylab in this respect, in that, one-Saturn launch as it was, it could at least take high thrusts on its long axis. Now, given a gas-core nuclear thermal rocket (where'd I put that magic wand...?), you could do at least a complete Mars orbit mission in 90 days, a time we know Skylab could handle without re-supply.

(Yeah, I know I'm ignoring solar panels and its off-axis solar telescope and thermal management matters too [even assuming the original thermal reflective material didn't fall off], but radiation issues might be tolerable for just three months. Indeed, providing adequate shielding from the engine itself, would give you exceptional protection in that direction...)

But for this purpose, ISS is gossamer.

KeithK wrote:

You couldn't roll up the solar panels for a departure burn. They weren't designed to be repackaged - the mechanisms are pretty much one way. It would probably be a challenging task even if they had originally planned to do it. The flex arrays are pretty fragile (if they weren't we might not care about the G-forces anyway).

Josh Reiter wrote:

Hmmm, I thought they rolled up one of the arrays when they moved the truss.

From spaceflight now: "During two recent shuttle flights, astronauts retracted the two wings of the P6 array and disconnected it from the station's power system. Spacewalkers had problems retracting the 4B panel, however, encountering a frayed guidewire that repeatedly hung up on grommets during the retraction process. "

I understand that in principle one could say that the arrays are a one way install when taking all the problems with retracting and redeploying the arrays. However, in practice, the arrays can and have been retracted and redeployed.

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This page contains a single entry by Rand Simberg published on July 30, 2008 2:31 PM.

Speaking Of General Zod was the previous entry in this blog.

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