Space Journal Entries

For a while back in 2004 I wrote an almost-daily journal entry about space.

Lava tubes

19th February 2004 - Lava tubes and rilles

If you've been following discussions on sci.space.tech recently, you'll have seen a lot of discussion of the idea of using lava tubes as the basis for a (permanent) Moon base.

Lava tubes are long tunnels formed by flowing molten lava. They are well quite common in volcanic areas of the Earth, and can often be a mile or more in length.

We know that they exist on the Moon - one of the Apollo missions landed near to a feature called Hadley Rille. A rille is a valley created when the ceiling of a lava tube falls in. If you consider that Hadley Rille is visible from the Earth, given a moderately powerful telescope, you gain some idea of the size the Hadley lava tube must have been. You could comfortably put a village down inside it. We don't get lava tubes that size on Earth because the Earth's gravity is too strong.

Some rilles we have observed have gaps in them - sections of lava tube which have not collapsed. So we could postulate that in many places on the moon, there are these lava tubes: massive, naturally formed tunnels. But why are these tunnels so interesting for the purposes of colonisation?

For more information see: http://volcano.und.nodak.edu/vwdocs/planet_volcano/lunar/sin_rilles/lava_tubes.html http://www.asi.org/adb/m/04/02/01/02/

20th February 2004 - Lava tubes and radiation shielding

Yesterday I described lava tubes, and the current evidence for their existence. But why are lava tubes so interesting for engineers investigating the possibility of a manned, long-term lunar base?

The reason is safety - principally with regard to radiation exposure, but other factors come into it as well.

Hazardous radiation in space in the inner solar system divides into two main categories: galactic cosmic rays (GCR) and solar particle events, but for our purposes we can summarize them as high energy particles and low energy particles. On Earth we are protected from the radiation by the Van Allen belts (part of the Earths magnetic field), which trap most of the radiation before it gets to us. We see the interaction between the magnetic field and the particles as the Northern Lights. That's why the Van Allen belts are particularly extensive when there's a solar flare, for instance. But the Moon has no magnetic field, and not even an atmosphere to shield astronauts from radiation. If there was a solar flare that hit the Earth-Moon system, any astronauts on the Moons surface would most likely receive a lethal radiation dose.

Using data from samples brought back from the Moon on the Apollo missions, detailed studies have shown that placing a base in a lava tube with 6 metres of lunar rock and regolith above it would filter out almost all the high energy particles. Less than a metre's depth would shield the base from the effects of a solar flare [1].

So putting the base inside a lava tube would mean that the lunar base would be cheaper to set up, because it would not be necessary to take heavy and bulky radiation shielding to the moon. It would also mean that astronauts on the Moon would be able to continue to work during a solar flare, instead of cowering inside a hardened radiation shelter (although they couldn't go outside, of course).

So what secondary reasons are there for putting a base inside a lava tube?

21st February 2004 - Other lava tube advantages

Yesterday we saw how lava tubes would act as a radiation shelter for astronauts on the moon. There are, however, several other potential advantages in putting a base in a lava tube [2].

Firstly, protection from meteoroids. The moon is constantly pounded by meteors weighing tiny fractions of a gram (we call them micrometeors), as well as meteor showers of larger meteors now and again. Although the micrometeorites are an unfortunate annoyance and can be compensated for by building equipment slightly bulkier than ideal, the larger meteor showers would be a serious danger to an exposed base. Placing a base inside a lava tube would protect it from meteor showers, thus making anti-meteor armour less of a necessity, and the tube would also provide a shelter for delicate equipment that would be easily damaged by meteors.

Second are thermal considerations. The lunar surface varies from ~100 K in the nighttime to ~400 K in the day, which means that thermal regulation of a base in order to keep it comfortable for humans could be a bit of a nightmare. If a base is placed in a lava tube, protection from heat is no longer a problem - all you need to do would is try your utmost to keep heat in, and we have experience of dealing with that kind of problem from polar research bases.

Finally is the problem of lunar dust. Formed from lunar rock by millions of years of bombardment by meteors, lunar dust is a serious headache. It's a very fine powder that gets into everything, clogging up motors, covering solar panels and shorting out electronics. But because lava tube interiors have been protected from meteors, we expect them to be relative dust free. Of course, by the time we've been walking and driving in and out for a couple of weeks it won't be.

So we've established that putting a lunar base inside a lava tube would be a good idea. So how do we go about finding a good site?

22nd February 2004 - Lava tube sizes

Yesterday I brought you to the conclusion that lava tubes look like good locations for manned lunar bases. But before I tackle the thorny problem of finding lava tubes in the first place, I feel I ought to give some idea of scale of these lava tubes. I said that you could put a village down inside the largest ones. This was probably an exaggeration, although there's at least one rille 3 km wide. Here's some example data [3]:

A small lava tube might be 20 m under ground, 20 m across and 15 m high.

A large tube might be 500 m under ground, 300 m across and 100 m high. So pretty large.

A good rule of thumb is that lunar lava tubes grow to about 10 times the size of their Terran equivalents.

Another aside: in Latin, the plural of rille is rima, and lava tubes are ducta. So if I start babbling about rima and ducta, you know what I'm on about.

Anyway, back to the point: how do we go about finding intact lava tubes?

23rd February 2004 - Lava tubes and satellite photography

So, after yesterday's interlude, I will address the problem of how to find lava tubes on the Moon.

There are several techniques available to use. The first one I will consider is satellite photography.

Initially this may seem more than a little silly: how can (visible spectrum) photographs show lava tubes, when they're underground? Fortunately, we can use indirect methods to find lava tubes. If you recall, last Thursday I explained that rima are collapsed lava tubes, and this fact can be used to find intact lava tubes: all you need to do is look for gaps, or more precisely discontinuities, in a visible rille.

A detailed study was done by Coombs and Hawke [4] of imagery from the Apollo missions and the Lunar Orbiter spacecraft, which turned up more than ninety possible lava tubes. An analysis of the data from the 1994 Clementine mission has yet to be carried out [5]. Which brings us neatly to another problem with visible spectrum photography.

The easiest way to pick out rima is by the shadows of their walls - nice dark lines on a light background. Unfortunately, the Clementine mission was geared toward providing good data for geochemical data, and so they arranged to have all its photos taken at local noon - when the shadows are at their least significant. This is a pretty annoying obstacle to using Clementine data to find lava tubes. That and the rather low resolution.

But there are of course ways to look for lava tubes other than satellite photos.

Thanks to T.L. Billings, Oregon L-5 Society Portland Chapter.

24th February 2004 - Lava tubes and surface-penetrating radar

Yesterday I discussed satellite photography. As I mentioned, there are problems with this technique, but there are others that might do the job just as well.

Consider a very simplified radar: you send out a pulse of radio waves, and you wait for reflections to come back. The longer the reflection takes, the farther away the thing that's reflecting.

During the Cold War, the US satellite reconnaisance researchers developed radar into a way to find concealed underground missile silos, and then as a demonstration published images of the network of underground rivers beneath the Nile delta, if I remember correctly. This worked by getting reflections back off features progressively deeper into the ground.

So this could work for looking for lava tubes. Mount a big radar on a Moon-orbiting satellite, and then set it looking at the surface of the Moon. Most of the time it'll get one reflection back, from the surface, but when it encounters a lava tube it'll get another reflection back from the tube roof, and then a third from the tube floor. This proposed mission would in fact provide two useful sets of data: a three-dimensional map of the Moons surface, which could be useful in mission-planning or geological studies, and then lava tube positions as a by-product.

There are naturally a couple of problems to overcome with this technique: on a basic level, resolution, penetration, and signal to noise ratio [#billingsspr]. Resolution is very important, as to find the sorts of lava tubes we're interested in we need a 10 m resolution, or even finer. Penetration is also important, as we want to find lava tubes even quite deep down, and a low signal to noise ration is vital if we want to extract any useful data at all! To get a high resolution, you need a high frequency, but increasing the frequency of the radar signal reduced the penetrating power. It's a thorny issue. Additionally, to get a 10 m resolution you'd need to know the position of the spacecraft to within 10 m.

Even with those problems, this seems to me to be the most economical way of getting the sort of data that would be useful for finding the sort of lava tubes suitable for locating a Moon base in.

So what are the last two methods?

25th February 2004 - Lava tubes, seismic survey and EMR

Yesterday I outlined the ground-penetrating radar technique for finding lava tubes on the Moon. Another two methods have been suggested [#stephenson], and I will briefly describe them.

Firstly, seismic survey. This involves deploying a network of seismic sensors (geophones) in the area you wish to search for lava tubes; essentially, microphones embedded in the ground. Then you send shock waves through the ground, like a miniature earthquake. You can generate the vibrations either by explosive charges, or by impacts (i.e. crashing something into the ground from orbit).

Although seismic survey provides excellent resolution and good penetration of the surface, there are problems: you have to non-destructively land your sensors and then embed them. That limits you to a small area. If you're using explosives, you need to set them up as well, or if you're chucking stuff from orbit you need to have some sort of accurate delivery system. To generate good data, you need to have lots of sensors and lots of shock waves from different places. I think that this technique would be good for mapping an area you've already identified as interesting.

The final method is still theoretical, and combines the ideas of GPR and seismic survey. You lob devices from orbit - devices that, when they hit the Moon's surface, behave a bit like EMP bombs, converting their kinetic energy into a very powerful radar pulse. The radar pulse bounces off features in the Moon - such as lava tubes - and the reflections are detected by a very long baseline array of receivers, and converted into 3D geographical data.

26th February 2004 - Lava tubes: summary

So, to summarise what I've been writing over the last week:

  1. Lava tubes are natural tunnels formed by flowing lava.
  2. In some places, they have collapsed to form rilles.
  3. On the Moon they are roughly an order of magnitude larger than their equivalents on Earth.
  4. They offer considerable protection for a manned base from a variety of environmental dangers.
  5. Lava tubes would provide an ideal location for a the base for a mission lasting longer than a couple of days.
  6. Satellite photography can be used to look for intact lava tubes indirectly by looking for discontinuities in rilles.
  7. GPR can see lava tubes directly, but has serious problems associated.
  8. Geophones can generate precise data on lava tubes, but require a network of sensors and a number of shock waves in different places.
  9. "EMP thumpers" combine the advantages of geophones and GPR, but is still under development as a technique.

Tomorrow I will begin to discuss problems (and their possible solutions) associated with staying on the Moon for any length of time.

By the way, a disclaimer: writing these articles is part of a learning process for me. I don't suggest that they're even factually correct (although I do my best). If you find them useful, then great! And if you find any problems with them please let me know.

17th March 2004 - Lava tube feasibility

On Friday, the British Library finally sent me the paper on lava tubes [4] I'd been referred to by Tom Billings (getting hold of it was a non-trivial exercise). It was definitely worth the effort, however: it's very interesting indeed.

As I suggested on 2nd March, the problem with designing a lava tube-based outpost is that, at the moment, we've no way of knowing whether there actually is a lava tube where we think there is one. I was pleasantly suprised to find my speculation confirmed by Coombs & Hawke; they say, "There is one major problem to consider... the difficulty in confirming, absolutely, that a tube does in fact exist... and determining what its exact proportions are."

The best thing about this paper, however, is that it provides a table of very strong lava tube candidates, some more than a kilometre in width. Some of these lava tubes could be ideal sites, except for the unfortunate fact that none of the tubes assessed as 'prime candidates' are close to the equator. That's a shame, because an equatorial position would be the most fuel-efficient to get to.

Even so, it's possible that strategists might decide it better to go with a lava tube we know rather than spend a lot of money on techniques such as SPR in the hope of finding a more conveniently located site.

Moon base design

27th February 2004 - Powering a moon base

Yesterday I said I'd start talking about problems with lunar colonisation, and their possible solutions. To start at a completely arbitrary point, how do we power our hypothetical moonbase? Without power, your life support system goes down and you're dead. So quite an important problem, then.

Interestingly, despite quite a lot of Googling last night, I couldn't really find any good suggestions of how to solve this particular issue - lots of complaining about the problem, but no coherent thought about possible solutions.

The primary power source envisaged for a Moon base is solar energy from photovoltaic cells. The Moon doesn't have an atmosphere to get in the way of the Sun's light, so the intensity of the light is pretty much constant for the whole of the lunar day, and more intense than even direct sunlight on Earth. Big advantages for solar power. But then you have to take into account the length of the lunar day.

A lunar day is the same length as a Earth month: 14 Earth days of light, followed by 14 Earth days of darkness. If your base relies solely on solar power, you're in big trouble. This means that if you want to use solar power, you need to couple it with some form of energy storage.

Enough batteries to heat and light a base for a fortnight will weigh tonnes. So batteries aren't much good unless you have a vast launch budget. Neither are flywheels a good prospect: over the course of two weeks, a lot of energy will be lost to friction. Not to mention that a flywheel big enough to store that kind of energy will also weigh tonnes. The most promising technique at the moment seems to be fuel cells: devices that generate electricity directly from a chemical reaction. These would be especially good if they could be made to run in reverse, because they could then be used in a similar fashion to batteries. Even better would be if the fuel from the base crews landing/ascent module was the same as the fuel for the fuel cells: it would considerably simplify storing the fuel. Only one set of tanks needed!

You can tell that my favourite solution for solar power is PV cells with fuel cells. But the ideal solution would be a small nuclear reactor. Although it would weigh a lot, it would be able to power a much larger base than the equivalent mass of PV cells and fuel cells, and would therefore be a much more efficient use of launch capacity. Unfortunately, the political climate is such that you'd never get permission to launch a nuclear reactor for fear of what would happen if something went wrong.

So the base has power, but what form would the base take?

1st March 2004 - Beachhead Moonbase design constraints

On Friday I suggested methods for supplying power to a Moonbase. But what would a Moonbase look like? To do this, I must first define what the requirements are, and what restricts my options.

Firstly, I am considering a "beachhead" Moonbase; that is, a first landing base with no current infrastructure. I am considering the minimum requirement.

Secondly, recall that we are going to site the base inside a lava tube. Because an orbital drop is never going to be able to land inside a lava tube, there's a requirement already: the base must be portable without too much inconvenience.

Thirdly, astronauts will need to get into and out of the base to carry out activities on the Moon's surface. So an airlock will be required.

Although power generation will be farmed out to a separate fuel cell generator (probably a sensible idea for safety's sake) there is still a lot of heavy equipment needed. Atmosphere regeneration, of course, and water management: what happens to water from showering or going to the toilet?

Additionally, a lot of other hardware is required: communications equipment, cooking equipment, furniture, medical kit; the list goes on and on.

According to my scheme, all of the features above must be available in my beachhead Moonbase. Tomorrow I will discuss the problems with some current proposed Moonbase designs.

2nd March 2004 - Missing the obvious

Yesterday I outlined some design constraints for a beachhead Moonbase. Unfortunately, I've been missing a crucial fact over the last couple of weeks; this is a good example of why these journal entries are a learning experience.

While looking at some data on lava tube sites that have already been identified, something struck me: all the lava tubes that have been located so far have been rille discontinuities, i.e. the lava tube has collapsed at each end of the possible lava tube, probably leaving a lot of debris that would hinder access to the tube. I then also finally realised that entirely uncollapsed tubes would have no way in - they would be completely underground. The fact is that even if a lava tube was found from orbit that looked absolutely perfect for inhabitation, it is most probable that the tube would be inaccessible.

So, in order to gain access to a lava tube some heavy plant would very likely be required: bulldozers, excavators and possibly tunnelling equipment, none of which is going to be on the cards for a beachhead Moonbase (although I have some ideas about how some of this stuff could be made available).

So, having established that a beachhead Moonbase cannot be assumed to be located in a lava tube, I'm back to square one: how to protect astronauts from solar particle events and cosmic rays while they clear access to a lava tube. Well, one way would be with a metre or so of regolith. But heavy equipment is needed to move the regolith on top of the base. I really don't like the thought of astronauts being unprotected so far from home: if a big solar flare occurred, they would be to far away to return before it hit, and could easily suffer a fatal dose of radiation.

Which came first, chicken or egg? I will need to ponder this further.

3rd March 2004 - Lava tube initial steps

I've thought further about the problem of actually gaining access to a lava tube, and assuming it's the plan to build a long-term base in a lava tube, mission planners have two options:

  1. Send a surface scouting mission - probably a rover - to a potential site, to ascertain whether the chosen lava tube is accessible. If not, a different site could be investigated, or a hardened shelter and excavation equipment could be sent with a crew to dig their way into the tube.

    Once an accessible site is found or access to a tube is cleared, send kit for setting up a base inside.

    Advantages: You don't go to the expense of sending a hardened shelter and heavy excavation equipment unless you have to.

    Disadvantages: For each site, an extra launch is necessary for a rover just to scout out the local area. Also, the project will take longer to get off the ground because of having to wait for the results of the rover's survey.

  2. Assume lava tube access will be blocked, and send a hardened shelter, excavation equipment, and stuff for populating a tube immediately. If access is blocked, the crew will clear it, and set up a base.

    Advantages: doesn't require a surface scouting mission in advance, so quicker to implement.

    Disadvantages: the extra mass of a hardened shelter and heavy plant has to be sent every time.

Neither of these options are ideal, especially from a financial point of view. However, there are lava tubes on Earth which are directly accessible (the Oregon L5 Society uses such a lava tube as a moonbase simulation facility), so it's not unreasonable to assume that there are such lava tubes on the Moon. The problem is, how to locate them?

I expect that, at least initially, financial constraints will dictate that a beachhead will be a hardened shelter near to a lava tube, and that equipment for either gaining access to the lava tube or for establishing an inflatable-modules-covered-with-regolith base would come later, if the lava tube is inaccessible. Of course, if the lava tube is accessible immediately, then that's an added bonus.

4th March 2004 - Inflatable habitats

When thinking about the practical design of a Moonbase, the thing that always bothers me is the need for space. Unlike the ISS, a Moonbase would be under the influence of gravity, and so the usage of volume would be very different.

Reasons for needing the extra space apart (having provided it, it's certain that activities would expand to fill it) inflatable modules seem the best way to go about creating it. Why?

The shuttle has a limited volume in its main bay - hence the relatively small size of the ISS modules, but inflatables could provide a much more efficient use of the volume. Consider a balloon and a matchbox. Uninflated, you can fit the balloon inside the matchbox, but when inflated, the volume of the balloon is much, much larger than that of the matchbox. This isn't the best analogy - the anticipated inflation ratio is smaller, and the inflatable module would unfold rather than stretch to its inflated size, but it provides the necessary example.

The original proponents of inflatables for a Moonbase were Kokh et al. [8], who suggested several designs. One of their most important points is that pure inflatable designs are impractical because they require a lot of outfitting - they can have no built in furnishings, and because pure inflatables only come in spherical, cylindrical and toroidical configurations, there is always an inconvenient curved surface underfoot which needs to be decked over for comfort.

Kokh et al. suggest a 'hybrid inflatable', which consists of some hard elements which have inflatable volume expanding between them or out of them. Their favourite design was dubbed the 'Moonbagel': a hard cylinder containing equipment and inflatable walls that expanded out into a torus, the original cylinder filling the 'hole' of the torus.

As meritous as their work was, I personally disagree with the main purpose of their paper, and I will discuss that at a later date.

NASA later took an interest in the Moonbagel design with respect to providing living space for astronauts on the way to and from Mars. Their adaptation, called TransHab, incorporated foot-thick walls for radiation shielding and a central structure fabricated from a lightweight honeycomb material [9].

The Moonbagel concept seems very promising to me, with a few adaptations. I will discuss these tomorrow.

5th March 2004 - Applying the Moonbagel

Yesterday I said that the Moonbagel hybrid-inflatable [8] looked like a good design for basing a Moonbase design around. However, as I said, there are a few specifics that I feel should be addressed. Please refer to Kokh et al. so that you know what I'm talking about.

First, dimensions. Consider the floor to ceiling distance of the room you are (probably) seated in now. I expect that you will observe that the ceiling is 8' to 10' above the floor, i.e. 2.5 m to 3 m. This is because that is the amount of vertical space people feel comfortable in, so it would make sense to have that much headroom in a Moonbagel. But remember that the floor of an unmodified Moonbagel would be curved, like the inside of half-pipe. Unless the Moonbagel is decked, it would be difficult to make efficient and comfortable use of the space inside. If the Moonbagel was decked, it would be good place the decking about a third of the way up, so as to get the best trade-off between floorspace and headroom. The lower third of the space available could then be used, for example, for stowage.

But this brings me neatly on to the problem of radiation shielding. In order to get the most efficient possible usage of cargo mass as well as to improve the expansion ratio it would be sensible to have the walls as lightweight as possible. But there is no way that such lightweight walls - for the sake of argument 5 cm thick - would provide sufficient radiation shielding for the Moonbagel's inhabitants. Shielding would have to be provided by a lava tube or by regolith heaped over the inflatable. Assuming there is no accessible lava tube, and that the Moonbagel is 1.5 * 3 = 4.5 m tall, that means digging a hole 2.25 m deep, putting the Moonbagel in and inflating it, and then piling the regolith back over the top. There have been builders at my school recently. It took quite a long time for them to dig a hole 2 m deep, and required some heavy equipment [*].

Back to dimensions. For maximum strength and best unexpanded to expanded volume ratio, a 'slice' of a Moonbagel should be as close to circular as possible. However, the container fills the 'hole' of the torus.

I'm going to need a diagram to be able to elucidate my ideas further. A task for the weekend, maybe.

[*]It's starting to seem inevitable that heavy earthmoving equipment will be needed for the construction of any size of permanent base, so I'm not going to dwell on it.

8th March 2004 - Applying the Moonbagel 2

On Friday I began a discussion of how to apply the Moonbase hybrid-inflatable design.

I took the opportunity to draw a quick diagram to visualise what I was trying to describe.

images/moonbagel1.png

As I mentioned on Friday, placing a floor inside a Moonbagel a third of the way up its diameter maximises usable floorspace, and this diagram illustrates the concept quite well. To me, the positioning looks like the optimal configuration for such a small module; in a larger module, I would put two floors in, at a third and at two thirds of the way up inside.

By now you've no doubt noticed the caption: "Moonbagel/Space Shuttle". That's because I chose the dimensions for this example quite carefully to fit into the Space Shuttle's main bay: the module as shown would fit into exactly a quarter of the bay, uninflated (the dashed outline).

I tackled another problem with this sketch. The bulkiest part of a module is the central core, which must contain hardware (wet & environmental systems) and the envelope, when uninflated. The original sketches show the core being held above a vacuum space by the pressure in the inflatable section. I don't like that configuration, because it would impose unnecessary stresses on the points where the ring meets the core. As shown, I've chosen a configuration where the base of the core is on a level with the base of the ring, and rests on the same surface.

Those are the main points I tried to fit into the diagram. Once I'd drawn it problems with it started appearing to me: I'm forever self-analysing. For instance, where should an airlock go? In this design, I can't find a suitable place to fit one. And where could the flooring be stored while in transit? Where could power cables connect to the module? The expansion coefficient isn't as big as it could be, either. However, I can't see a module much larger than this in its uninflated form being practical to move around on the Moon.

9th March 2004 - The Lunar Hostel scheme

Over the last few days I've been referring frequently to Kokh et al. [8] in their paper on the Lunar Hostel concept. But on Friday I stated that I disagreed with the main thrust of the paper. Why?

In essence, what they propose is to provide a "big dumb volume" on the Moon's surface (a "hostel") with absolutely minimal amenities, and have all the necessary life support systems etc. provided by visiting spacecraft.

I believe this to economically foolish, and I don't think any agency would fund this scheme. The problem is mass. Landing on the Moon takes a certain mass of propellant, and it's necessary to take the propellant needed to get back into orbit again with you. It's desirable to keep the amount of propellant you need to take onto the surface with you to a minimum, so that you can take more supplies, equipment etc. with you instead.

So if you're visiting a base, you want to leave as much as possible of the stuff you take with you behind. You don't want to land a huge piece of hardware for recycling the air inside the hostel, and lug it back into orbit again. The same goes for water systems. It'd be much more desirable to land a big tankful of water and then take an empty tank back to orbit than to land a toilet, a shower, a water recycling system, and so on, and then have to launch it all back.

In complete opposition to the LRS's idea, I think the way development of the Moon should proceed is with most of the equipment and supplies being taken to the Moon in big, slow containers, while crew exchange happens in lightweight, fast spacecraft with minimal functionality (a la Soyuz or Apollo).

Lunar Mission Planning & Propulsion

10th March 2004 - Big and slow or small and fast

Yesterday I mentioned that I thought "development of the Moon should proceed... with most of the equipment and supplies being taken to the Moon in big, slow containers, while crew exchange happens in lightweight, fast spacecraft." What did I mean?

The ESA have a current project named SMART-1, a very small lunar orbiter mission. It was launched on the back of a big commercial satellite, into an orbit much closer in than that of the Moon. What's interesting is that it is currently using an ion drive to climb out of the Earth's gravity well to the moon, while hardly using any propellant at all (ion drives have a very high specific impulse). The downside is, of course, that it takes a very long time: SMART-1 will take eighteen months to move from the orbit it was inserted into at launch to its final lunar orbit.

I find this very interesting, because it suggests an interesting scheme for getting kit to the Moon. Big containers of non-perishable equipment and supplies could be thrown into a relatively low orbit, and then could engage ion drives to move themselves into lunar orbit (probably taking a couple of years or so), before using conventional methods for landing on the Moon. That would make most efficient use of the launcher's lifting capacity by minimizing the amount of propellant needed in the cargo. On the downside, it means two different types of engine and two different sets of propellant tanks are needed, as well some way of producing quite a lot of power.

Fortunately, power isn't too much of a problem: solar panels couild be designed to first be used to power an ion drive, and then detached as part of the unloading process and used at a lunar base.

Unfortunately, perishable goods (like astronauts) couldn't use that transit method, so an Apollo-style spacecraft would be needed to transfer personnel from Earth to Moon and back again. Ideally, the L/AV would be left at the Moon and refueled by visiting astronauts, to save on Earth launch mass. My slight worry is that we currently do not have a suitable launcher for this type of mission, unless the Russian Energia launcher is available or the Saturn V could be resurrected.

This scheme is an example of Lunar Surface Rendezvous (LSR) for freight, and Lunar Orbit Rendezvous (LOR) for personnel/perishable goods.

11th March 2004 - Heavy lift (sort of)

I've been trying to work out which single-shot launcher provides best performance to LEO, by looking at the Ariane [10] and Boeing Delta-IV [11] launchers (no particular reason; these are just the launchers I thought of as being capable). I got the latest versions of the documents I referred to available, but I suspect they may be a little out of date (four years old).

Another thing to note is that the Delta-IV is only available for US government launches now, because Boeing was finding the commercial market insufficiently lucrative.

I'm interested in the maximum lift capability to LEO, pretty obviously, and for my purposes I'll consider a 45-degree inclination orbit at an altitude of 1000 km.

The Ariane 5 ES costs about $150,000,000 a shot (probably more). It consists of two solid fuel boosters and a cryogenic (liquid hydrogen/oxygen) main stage, with a solid fuel upper stage. Its performance to a LEO as described is just over 18,000 kg.

The heavy variant of the Delta-IV (Delta-IVH) costs in excess of $120,000,000. This is the only figure I could find, and it's even more outdated than my documentation for the launcher; I suspect the true price is closer to $200,000,000. The launcher consists of three Delta-IV CBCs (cryogenic main stages) strapped together, with a cryogenic second stage. It's a pretty big launcher. Performance to the aforementioned orbit is about 22,500 kg, in a payload bay of comparable size to the Space Shuttle's.

So for sheer mass capability, the Delta-IVH beats the most capable current member of the Ariane family hands down, and has a comparable price tag.

12th March 2004 - Using ion propulsion

So supposing I've potentially got 20 tonnes into LEO, but I actually want to get something to the Moon, what's the fraction of that 20 tonnes that has to be propellant?

I'm going to assume that I can have my 20 tonnes in an orbit parallel with that of the Moon, at an altitude of 1000 km. I'm also going to assume that the only problem is getting out of the Earth's gravitational field, to a target orbit the same as the Moon's (i.e. an orbital radius of 384000 km). So from the equations for a circular orbit (see for instance an A-level Physics textbook) initial velocity:

v0 = 7.35 kms-1

Final velocity:

vf = 1.02 kms-1

Giving

Δv = 6.33x103 ms-1

I'm going to make the blatant assumption that as much electrical power is available as I require, and use a high-powered ion thruster. Now, the example I looked up on the Internet [12] had a quoted specific impulse Isp of 3800 s. I can use the fact that exhaust velocity is equal to specific impulse divided by the gravitational field strength on the Earth's surface to find exhaust velocity

ve = 37240 ms-1

Then from the rocket equation in the form

e-Δv/ve = mf / m0

The ratio of final mass to initial mass is 0.843.

This is interesting, because it implies that out of 20 tonnes in LEO 16 tonnes will make it to lunar orbit: a much better ratio than for a conventional booster! Assuming, however, the payload is destined for the Moon's surface, that sixteen tonnes must include: the ion engines themselves and fuel tankage; the solar panels required to power the ion engines; the landing retrorockets and enough fuel to land the payload; and the landing gear. So it's probable that less than half of the original 20 tonnes would consist of non-propulsion payload. But the setup would probably still be an improvement on a conventional chemical-rocket-only system.

20th March 2004 - Power for ion propulsion

There's a few problems with ion propulsion from LEO to the Moon that I hadn't considered. The first problem is in providing the electrical power required.

Last week I suggested using solar panels. The problem with that idea is that the spacecraft will be travelling through the Van Allen belts, where there is a high level of ionising radiation. Ionising radiation destroys solar panels, so it's best to minimise the amount of time spent in the Van Allen belts.

Here's a rough approximation. The ion thruster I described last week (Isp of 3800 s) had a thrust of 0.2 N [12]. Suppose the spacecraft has an initial mass of 20,000 kg (20 tonnes). That means the acceleration from this 5 kW thruster will be of the order of

1x10-5 ms-2

At that rate of acceleration, it'd take a long, long time (~ 20 years) to make a velocity change of

6.33x103 ms-1

20 years, a large number of which would be spent in the Van Allen belts.

So what about increasing the number of thrusters? Let's say 2 years for a transfer is reasonable. Then 10 thrusters are needed, corresponding to 50 kW of power needed. If a solar array is degraded by 10-50% on a single transit through the Van Allen belt - and my spacecraft is spending quite a while there - I would need an initial capability of 100 kW. Unfortunately, the technology doesn't currently exist to make a deployable array that big. Additionally, the spacecraft would only be able to make one trip from Earth to the Moon.

Assuming that somehow there's been such a deployable array constructed and used, and since these (very heavy) arrays have been taken all the way to the Moon, it wouldn't be unreasonable to want to use them again. It would make sense to land them and use them to power a base. Only problem is, they'd need to be undeployed first. A good demonstration of the reason for that is as follows: take a strip of card (a bookmark, perhaps). Make a fist, and grip the middle of the strip between your thumb and the side of your index finger. With the strip horizontal, hold your fist a couple of inches above a table and allow it to drop. Watch what happens to the strip as your fist hits the table. Now imagine what would happen to the deployed solar array under an even larger shock. No-one's ever built a multiply-deployable array either: there's another challenge.

There is one possible solution to the Van Allen problem: run the solar panels at a high temperature (~ 400 K), when, in theory, they may be able to self-heal.

Next issue is mass. Once again, assuming

mf / m0 = 0.843

and

m0 = 20 tonnes

there is a requirement 3.14 tonnes of fuel. I'll round that to 3.5 tonnes to allow for the tankage factor. Typical specific masses for solar electric propulsion (SEP) systems are of the order of 20 kg/kW (I really think this is seriously underestimated), so a 100 kW system would have a hardware mass of 2 tonnes. That leaves 14.5 tonnes for payload and subsystems for lunar landing.

Now, what happens when the system scales up? A Saturn V was capable of sending 47 tonnes to the moon, and a hundred tonnes to LEO. What happens then?

23rd March 2004 - Big ion propulsion systems

On Saturday I investigated SEP systems for spacecraft going from LEO to the Moon and having a relatively small LEO mass of 20 tonnes. In hindsight, some of my numbers were a bit dodgy; more realistic values are for 15 tonnes of non-propulsion equipment.

But what happens when there's a possible initial mass of 100 tonnes, as could be launched to LEO by a Saturn V?

At this scale, SEP doesn't make sense. Sticking to my maximum transfer time of 2 years, the math works out as follows:

mf / m0 = 0.843

So 84.3 tonnes of payload and systems and 15.7 tonnes of fuel. Using just one ion thruster as before, and

Δv of 6.33x103 ms-1

would take 100 years. That means that to transfer in 2 years, I'd need 50 thrusters, corresponding to 250 kW of power consumption.

That's an awful lot of power, and a solar panel array that big is just ridiculous, without even taking radiation degradation into account. So, nuclear power could be used instead. Up to 1993 as part of the Space Defence Initiative NASA developed a reactor designated the SP-100 [13]. This was capable of delivering power levels up to about a MW at between 10-50 kg/kW (better specific mass at higher power). By the way: for the environmentalists, the SP100's nuclear fuel container is designed to reach the ground intact in the event of the reactor unexpectedly breaking up in the atmosphere. Additionally, I'm assuming a 1000 km starting orbit, and an orbital height of 700 km is considered by NASA to be "nuclear safe".

Pulling figures out of thin air, let's assume that a 250 kW reactor can be built at 30 kg/kW. That means that 7.5 tonnes would be power source. Additionally, I'd need 250 kW of ion thrusters, at about (yep, more made-up numbers) 13 kg/kW, another 3.3 tonnes.

Adding all that up, for 100 tonnes at LEO, I'd get 15.7 tonnes of fuel, 7.5 tonnes of nuclear reactor and 3.3 tonnes of thrusters, leaving 73.5 tonnes for payload. Very nice.

The real problem with what I'm doing here is the fact that no-one's done any research on really big spacecraft in the last decade or so, because there's been no requirement for such research. Everything's been kept small, cheap and efficient as far as possible (apart from the ISS, which is small, expensive and inefficient). I want to see things that are big, not too expensive and extremely efficient.

18th June 2004 - More about Magnum

I mentioned a month or so ago that I'd been told that NASA was developing a new launcher called Magnum, and that I was going to investigate. Here's where I've got to.

Firstly, searching for the keyword "Magnum" in a space-related context brings up a lot of results referring to NASA launching a series of ELINT satellites in the late 1980s, which isn't very helpful. It's also hard to filter out references to the Colt Magnum series of pistols.

A document from 1998 provides some interesting information:

  • Design is derived from that of the space-shuttle (this could be a replacement for the Shuttle-Z programme)
  • Performance is 80 tonnes to ~400 km altitude LEO at 28.5 inclination
  • Core is 8.4 m diameter to allow Magnum to launch from existing shuttle facilities (cost-cutting measure, I guess)
  • Target cost is $1000 per kg (and if you believe that you'll believe anything)

The document also mentions something that I hadn't picked up before, that NASA are planning to replace the current solid-fuel shuttle boosters with liquid-fueled boosters capable of autonomous fly-back and landing. I'll believe it when I see it.

Quite a lot of these details are corroborated by this Space.com article, which also contains some snazzy publicity imagery - including Lockheed-Martin and Boeing concept renders of what the fly-back boosters might look like.

Earliest information I've found is a set of 1997 lecture slides which, although interesting, are probably hopelessly out-of-date by now.

I've also found some comments from various people, many of which run along the lines of, "Why are we spending all this money on massive launchers so we can send spacecraft straight to Mars Apollo-style, when we could use [insert name of current, low performance launcher here] and assemble at ISS?" The fact that the ISS is in a hopeless orbit for insertion into a Martian transfer orbit, and the fact that we don't have the know-how to be able to assemble large structures in microgravity, makes this viewpoint rather an odd one.

And that's pretty much it. I've been able to find very little or no useful information dating more recently that 2000. I'm going to try e-mail MSFC directly, and see what they can tell me. It's worth a try.

Solar Radiation

23rd April 2004 - Solar Radiation

Now my Easter break is over , I'm going to pick up with a more detailed analysis of the Solar radiation environment [#stark1].

The Sun's outer three layers are responsible for the majority its electromagnetic radiation. The photosphere, at about 5900 K, emits visible wavelengths; the chromosphere above emits more intense ultra-violet radiation; and the corona, the Sun's outer layer, gives out X-rays. The chromosphere has temperatures up to 10,000 K, and the corona reaches temperatures greater than 2,000,000 K.

The radiation pressure of the Sun's electromagnetic emissions is sufficient to drive of huge amounts of matter in the form of the solar wind. At one AU, the solar wind has a flux of about 9 protons per cubic cm, travelling at an approximate mean velocity of 450 km/s.

Disturbances in the Sun's atmosphere are the cause of solar flares, large plumes of material thrust out from the Sun's surface. Associated with flares are increases both in radiation and high energy particle fluxes. Near the Earth the first observable change is a sudden and relatively brief increase in solar radiation about 20 minutes after the flare occurs, and then about a day later a longer burst of high energy particles, similar to the solar wind but more intense and at higher velocities, typically 1000 km/s.

As discussed previously, the enhanced electromagnetic and particle emissions pose a substantial risk to exposed equipment and personnel.

References

[1]De Angelis, J.W.Wilson, M.S.Clowdsley, J.E.Nealy, D.Humes and J.M.Clem, 'Lunar Lava Tube Radiation Safety Analysis' http://lowdose.tricity.wsu.edu/2001mtg/abstracts/deangelis2.htm
[2]Moon Miners' Manifesto: 12 Questions About Lunar Lava Tubes http://www.asi.org/adb/06/09/03/02/100/12-questions.html
[3]Artemis Project: Lunar Lava Tube Dimensions http://www.asi.org/adb/m/04/02/01/02/lava-tube-size.html
[4](1, 2) Coombs, C.R., and B.R. Hawke, 'A Search for Intact Lava Tubes on the Moon: Possible Lunar Base Habitats', in The Second Conference on Lunar Bases and Space Activities of the 21st Century (W.W.Mendell, Ed.), NASA CP-3166, Vol. I, p. 219, 1992.
[5]T.L. Billings, 'Lunar Lava Tubes via Clementine', sci.space.tech, 17-02-2004 http://groups.google.com/groups?selm=itsd1-23D9F4.12302818022004%40news2.west.earthlink.net
[6]T.L. Billings, 'Re: lunar resources/lava tubes', sci.space.tech, 25-01-1996 http://groups.google.com/groups?selm=itsd1-2501960102390001%40ip-pdx20-60.teleport.com
[7]D. Stephenson et al., 'lunar resources/lava tubes', sci.space.policy, 22-01-1996 http://groups.google.com/groups?selm=DLL6tA.I6z%40emr1.emr.ca
[8](1, 2, 3) P. Kokh, D. Armstrong, M.R. Kaehny, and J. Suszynski, 'THE LUNAR "HOSTEL": An Alternate Concept for First Beachhead and Secondary Outposts', The Lunar Reclamation Society, 1991 http://www.lunar-reclamation.org/hostels_paper1.htm
[9]P. Kokh, 'TransHab and the Prehistory of its Architecture', The Lunar Reclamation Society, 1999 http://www.lunar-reclamation.org/transhab.htm
[10]Arianespace, 'Ariane 5 User's Manual', Issue 3 Revision 0, March 2000
[11]The Boeing Company, 'Delta IV Payload Planner's Guide', October 2000
[12](1, 2) JPL Advanced Propulsion Technology Group, 'Advanced Propulsion Concepts', Island One Society http://www.islandone.org/APC/
[13]'SP100 Power Source', NASA SpaceLink http://spacelink.nasa.gov/NASA.Projects/Human.Exploration.and.Development.of.Space/Human.Space.Flight/Shuttle/Shuttle.Missions/Flight.031.STS-34/Galileos.Power.Supply/SP-100.Power.Source
[14]J. P. W. Stark (2003) 'The Spacecraft Environment and its Effect on Design' In Spacecraft Systems Engineering (J. P. W. Stark, G. G. Swinerd & P. W. Fortescue, ed.), pp. 11-47. John Wiley & Sons Ltd, Chichester.

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