Saturday, January 24, 2015

Export industry for Mars

A couple of posts below, Ken lays out a good argument for developing industry on Mars.

The only point I disagree with is the assumption that there's no market for exporting products from Mars.

An export industry is an industry, and there is, or soon will be, a very viable market; consumables.

Let's look at fuel depots in LEO for just one example; they're going to happen, and fairly soon. So, how does one get fuel to the fuel depot? Currently, plans are to launch it from Earth. This seems logical, because low earth orbit is only around 200 miles from Earth's surface. However, when it comes to spaceflight, the number that matters most is propulsive delta-V, not distance. To get to LEO, you have to accelerate your payload by around 9.2 kilometers per second (8 kps for orbital speed, and about 1.1 to 1.2 for gravity and aerodynamic losses).

Therefor, if you have a fuel depot for, say, methane and oxygen, or hydrogen and oxygen, getting it to LEO from Earth is a hard hill to climb. So, why not get it from Mars? That sounds preposterous because Mars is a long way from Leo, but that's in distance, not in Delta-V, and Delta-V is what counts.

Launching from the surface of Mars to low Martian orbit  takes 4.1 kps, a lot less than the 9.2 from Earth to Leo. But, how do you get from low Mars orbit to LEO? Part of that is propulsive Delta/v - 2.3 kps to get you on an earth-transfer trajectory. So, your total propulsive delta-V? 6.4 kps. From Earth transfer, you can aerocapture (no shielding required) into LEO. So, your fuel arrives in LEO having needed 6.4 kps propulsive Delta-V compared to 9.2 from Earth. That's quite a savings, but it's only a small part of the benefits. The rest is in launch architecture.

Take the F9 upper stage as an example. If you had it on the surface of Mars, not only could it do single stage to orbit with what, on earth, is its max payload, it could do Mars surface to earth transfer. (Falcon 9 stage sep occurs around mach 6, so 1.7 kps, and though the first stage has dealt with about half the gravity losses, the second stage has to supply around 6.8kps)

What this means in practice is that your Mars launch vehicle can be single stage to orbit - far, far cheaper and easier than staging, because it eliminates the need for the 90% of the vehicle you discard at staging (and even with reusability, this cost dynamic applies. a 747 and a Lear Jet are both reusable, but the 747 costs one hell of a lot more to buy and operate.)

With ISRU on Mars, you could easily produce methane and oxygen, launch them to low Mars orbit, where you use them to fill up a supercold transfer tank. (you avoid boil off losses by keeping the tank very, very cold, easy to do in space; you have a layer of aluminum foil a few feet from the tank, between it and the sun). Once the tanker is filled, a small rocket motor using some of that fuel boosts it through the trans-earth burn.

Hydrogen/oxygen is a bit trickier due to the temperature demands for Hydrogen, plus the tankage size needed. It's probably be more efficient to ship it as water, and electrolyze it as needed in earth orbit.

And speaking of water, that, too, is a commodity that'll be needed as space infrastructure grows. And that gets us to food, which is already a significant amount of upmass for ISS, and the demand for it will rise as we see more human occupation of LEO as well as further out.

So, given all of the above, we have an economic model for a major export industry on Mars. However, there's an even bigger economic factor, one that harks back to the birth of Transatlantic trade. The early colonies in North America were founded with people and supplies from Europe, and initially, the only value returned was that of the land in the New World. However, the ships delivering supplies were going back empty, and that's free cargo capacity, then as now. Back then, cash crops such as tobacco began to fill those empty holds. The same dynamic applies to a Mars colony; the vehicles delivering colonists and supplies to Mars will be going back to earth empty at first, so that's basically free cargo capacity. For a colony on Mars, you'll already have ISRU production of water, fuel, etc, and for vehicles like MCT, which is supposed to be reusable (and thus must return to earth), you'll have 100 tons cargo capacity to LEO. Whether MCT is able to land on Mars (I personally doubt it) of is a space-only craft, you'll have some kind of Mars-ascent system. The capability is inherent in the design concept of the raptor-powered heavy lifter; an upper stage of that reusable launch vehicle would have the inherent ability to land on Mars, refuel, and do either single stage to Mars orbit, or, single stage from Mars surface to Leo. So, to use the early transatlantic trade analogy, you'll already have the empty holds returning to Earth, so why not use them for an export industry?

Later, as the Mars colony's capabilities expand, if they're able to build pressurized habitats for Mars (they'll have to be able to do that) they can just as easily build them for space, and put them where they are needed for far, far less cost than launching them from earth. It might also be worth their while to build fuel depots (which are largely just large tanks), fill them in Mars orbit, and send them earthward - because one of the many advantages of launching from Mars is you're not limited by the diameter of the launch vehicle or payload shroud (restrictions imposed by earth's atmosphere). This of course applies to habitat design as well; if somebody wants, say, a 100 foot diameter sphere, or a segment of a large wheel, that's easy to launch from Mars but impossible from Earth, due to the atmosphere. 




      

4 comments:

ken_anthony said...

CJ, this is what I need a brilliant guy like you for. I play it too safe, assuming no exports (to get on base) and you come along and knock it out of the park. That's two more points for our team. Well done. This info is going to be used on my trip.

You and I are going to make it happen while dragging others kicking and whining across the goal. I've got four weeks before I leave to put some pressure on my local attorney to give me some satifying referrals.

C J said...

Thanks!

Not brilliant though... I was just pulling together a few details I hadn't seen together anywhere else.

I'm planning a post on MCT soon, outlining my reasons why I doubt it will be what so many hope; a spacecraft that lands on both Earth and Mars. I'll need to get more heavily into the math for that, but... for a colony, this has major ramifications. If it's entirely space-based (basically a habitat) you'll need a dedicated system for descent/ascent, and fortunately, the reusable upper stage of the BFR will have to have that capability anyway.

In fact, I'll do a post on the whole system, including SpaceX's mention of fuel depots being needed. I've got a strong hunch I know what they have in mind for the system architecture, and it's not what the speculation is envisioning. The good news; I think what they have in mind will reduce the per-mission cost by a heck of a lot.

Here's a teaser for part of it; I think MCT's design is already on the internet, hidden in plain sight. It's something you're quit familiar with, too. :)

Doug Plata said...

What is the delta-v for water ice at the lunar poles to LEO using aerocapture?

C J said...

Doug, first, let me try to explain some of the issues.

A lunar polar launch, unlike an earthbound polar launch, is largely unaffected by rotational aspects that give low latitude launch sights an edge on earth. However, "largely"does not equal "zero". The earth rotates once a day, giving an equatorial launch site an eastbound speed of 1070 miles per hour. For, say, Kennedy, it's at 28 degrees north, so rotational speed (and thus speed the LV does not have to produce) is reduced to 915mph (assuming a 28 degree eastbound launch inclination). The speed at the north or south pole would of course be zero.

So this gets us to a lunar polar launch. The moon rotates once per orbit of the earth (27.3 days), so it's equatorial ground speed is the circumference (6783 miles) divided by the time (655 hours) giving us an equatorial ground speed of 10.35 mph. That's all you lose by launching from the lunar poles, BUT... that's not including what kind of LEO orbit you're looking for. LEO could be low or high inclination (ISS is an example of high inclination). Getting from the moon's equatorial zones to a high inclination LEO requires some additional Delta-V, which is why the Russian Zond sample probes returning from the moon used skip reentry in order to land in Russia (the alternative was a significant delta/V burn at the earth-moon libation point). However, from a lunar polar region, you're basically going into lunar polar orbit first, so a high inclination LEO target would be easier, and a low inclination one harder.

To be honest, I don't know how to calculate the various possible burn profiles and orbits exactly, so I can't give a precise number. What I can do is base the figures on a launch from the moon's low latitudes (such as the Apollo landing sites)to a low inclination LEO (approx 2.54 kps, or 1.57 mps). To go from the lunar poles you lose a negligible rotational advantage of 10mph (.0027 MPS). But, if you're aiming at a low inclination LEO, you're going to at need two mid-course burns, and I don't know how to calculate the needed delta/V for that, though it's probably under .3kps
The above figures assume airocapture into LEO.

What this shows is that if water (and thus hydrogen/O2) is the desired commodity, and it's easily obtainable at the lunar poles, it'd be a lot less delta/V to get it from the moon (ballpark it at 2.8 kps) than from Mars (ballpark it at 4.1 kps)

On the other hand, the moon is believed to be carbon-poor, so methane would be problematic, as would food production (and the even lower lunar grav might be an issue there as well).

My own personal take on all this; resource acquisition in space is not much different from on Earth; what you get, and from where, varies according to what it is you're after. So, for some things, the Moon is a better case, for others, Mars, and for others, asteroids.