[Note: I wrote this article based on publicly available information, without consulting with the Quest guys, so any errors are probably my misinterpretations.]

But before we jump into the technology, let’s do a refresher on what MLI is for those not so familiar with the area:

What is MLI?

MLI on a Mars probe (Credit: NASA)

MLI or Multi-Layered Insulation is a form of thermal insulation that uses alternating thin layers of metalized plastic (typically Kapton or Mylar) and spacer nets (usually made of Polyester or fiberglass) to slow the radiative heat transfer into or out of a spacecraft in a vacuum environment.  When you see pictures of spacecraft covered in what looks like gold foil, that gold-colored stuff is usually MLI.  Wikipedia has more details here.  MLI has been around for almost half a century, and is one of the key elements of spacecraft thermal control.

MLI is one of the best thermal insulators known to man, but there are several hitches to existing MLI:

• The thermal insulation performance of MLI tends to be variable.  One of the ULA guys (Frank or Bernard) related an anecdote that they once flew two back-to-back Atlas or Titan missions, using MLI as the thermal insulation on the Centaur tank, and they actually had 30% difference in boiloff rates, in spite of solar levels and mission profiles being almost identical.
• MLI is very structurally fragile because it is basically a bunch of plastic thin films only held together at the edges.  This means that they can’t be exposed to flight aerodynamic loads without getting quickly destroyed.
• MLI only works in a reasonably good vacuum (under 1 mPa or 8×10^-6 Torr). When combined with the previous challenge, this means that even though they’re much better insulators than say SOFI (Spray-On Foam Insulation), they can’t be used for insulating external faces of launch vehicle propellant tanks.

Quest, working with Ball Aerospace, has come up with a clever technology which they call Integrated MLI (IMLI), and several derivative technologies including Load Responsive MLI (LR-MLI), MMOD-MLI, and Launch Vehicle MLI (LV-MLI), which solve these weaknesses of traditional MLI, enabling many neat new space technology applications.

Integrated MLI

The core innovation that Quest and Ball came up with was the idea of replacing the plastic netting “scrim” layer with evenly spaced “micro-molded” snap-together polymer supports.  These micro-molded supports keep the MLI layers consistently spaced, transfer loads, and greatly reduce the conductive heat transfer between MLI layers.  To give you an idea of what these things look like, here are some pictures (borrowed from this brochure made by Quest’s micro-molding partner, Phillips Plastic Corporation):

[Editors Note: I'll have a picture here later when I can fix an upload bug with WordPress.  For now you'll have to read the brochure linked above to see what I was going to put here]

The neat things I see about this approach are:

• The IMLI blankets are now a lot more thermally deterministic, repeatable, and analyzeable.
• The IMLI blankets have much smaller thermal conduction contact area between each sheet, demonstrating around 30% better insulation than traditional MLI of a similar number of layers.
• The micro-molded snap elements tie the layers together and are anchored all across the surface you’re trying to insulate, instead of just along the edges, making IMLI significantly more robust than traditional MLI.
• By replacing the scrim layer netting with a few discreet micro-molded pieces, they’ve probably cut weight compared to traditional MLI blankets of the same number of layers.
• There’s some real potential for mass production and automated assembly that could drive down costs significantly.

And the technology behind IMLI also serves as the foundation for the other three derivative technologies.

Load Responsive MLI

While Integrated MLI was a big improvement over traditional MLI, you still could only use it in vacuum environments.  Quest and Ball developed what they call Load Responsive MLI (LR-MLI) to enable customers to have the benefit of MLI even in an atmosphere.  Basically, LR-MLI consists of a thin vacuum shell supported by some spring-loaded spacers, with a vacuum pulled on the space between the vacuum shell and the underlying structure. When the external pressure is non-vacuum, the spacers are forced flat, where the center of them rests on the center of the spacer below them.  This increases the heat leak through the spacers, but still provides a much better insulation than SOFI (their .25in thick test part provided better insulation than a 16in thick layer of SOFI!).  Once the external pressure starts falling off, the spacers push back apart in a way that greatly reduces the conduction path, resulting in a really good thermal insulation on orbit.  See this page and page 22 of this FISO presentation for illustrations of the concept.

Benefits of this approach as I see it:

• You now have a non-SOFI method for insulating a tank that works in both atmosphere and in-space that doesn’t have the popcorning problems SOFI has, which both eliminates debris falling off during launch, and also eliminates the risk of insulation flaking off once in orbit.
• LR-MLI is a significantly better insulator both from a mass and a thickness standpoint compared to SOFI.
• You get rid of the need for GHe or GN2 purges on the ground.
• Enables fairly lightweight dewars to be constructed for applications that need it.

My only concern is the challenge of maintaining a vacuum for a long duration on the ground, though I guess dewars are used a ton in industry, so maybe this isn’t a huge deal.

Launch Vehicle MLI

The latest improvement on the IMLI theme, for which Quest just finished a Phase I SBIR contract for last year, is an MLI technology capable of being used on external aerosurfaces of launch vehicles.  This Launch Vehicle or LV-MLI appears to be a combination of the LR-MLI concept with a thin aeroshell surface.  There aren’t as many details on the concept, since it’s still in active development (here’s to hoping things went well and they get a Phase II award next month!), but here are the SBIR abstract and briefing chart.  The goal is to have an insulation system that weighs about a third of what a 1.9cm SOFI layer would, but with 85X the insulation value.

MMOD-MLI

One other related concept that Quest and Ball developed is an IMLI variant that includes integral MMOD (Micro-Meteor/Orbital Debris) protection capabilities.  This MMOD-MLI includes layers of Kevlar and Nextel cloth between layers of insulation, providing the same sort of multi-shock shielding capability that is what makes Bigelow’s modules so much more robust than older ISS designs, while still packaging things in a neat, multi-functional structure.  Basically, an incoming piece of MMOD would hit the outer layer, instantly vaporizing the MMOD, which would then have its energy absorbed and the momentum distributed as it passes through the multiple shield layers.  Like LV-MLI, MMOD-MLI just finished a Phase I SBIR a few months ago, so the results aren’t all out, but the goal was a design that would give a propellant depot a 95% chance of surviving its design lifetime without an MMOD-induced failure, without adding substantially to the MLI mass, or significantly decreasing the MLI thermal efficiency.  The neat thing about this technology is that it looks like it can be integrated with LV-MLI or LR-MLI without much additional effort.

So you could theoretically get an MLI shield that can function in both atmospheric pressure and on-orbit, could take aerodynamic loads, and when on orbit could double as a very effective MMOD shield.  Think about that one.

Some Random Applications

This is far from all the space applications enabled by these technologies, but here are a few less-obvious ones that I think are worth mentioning:

1. Cryogenic-fueled Air-Launched Rockets: One of the big challenges for externally-carried cryo-fueled air-launched vehicles (including even LOX/RP-1 designs) is that the heat transfer environment during the flight from the ground to the launch point is substantially worse than a vehicle experiences on the ground, due to convective heat transfer from air flowing over the launch vehicle during flight, which may very well be an order of magnitude or more than what is experienced on the ground.  And unfortunately, air-launch vehicles are typically much more sensitive to losses due to boiled-off propellants. The traditional thought on how to handle this is to have some sort of Airborne Service Equipment (tanks and plumbing and stuff) that either keeps the tanks topped up, allows you to only load the cryo propellants at the last second, provides some sort of sacrificial coolant, or provides an active cooling loop. With something like LV-MLI (or LR-MLI inside a separate aeroshell if your tanks aren’t conformal with the outside of the vehicle), you could cut down on the heat leak substantially.  Maybe to the point that you could eliminate or greatly simplify the required ASE complexity, cost, and weight.  Maybe combine pre-subcooling the propellants a bit with the insulation and you might be able to get rid of the ASE requirements entirely.
2. Wet Stations: One of the ideas that made the rounds a lot a few decades ago was that NASA should haul the Shuttle External Tanks all the way into orbit (instead of ditching them at just below orbital velocity to burn up in the Indian Ocean).  Two of the technical issues with this idea were that the insulation on the ET was liable to flake and pop off in orbit, potentially creating space junk issues.  There were probably solutions to this problem, but they likely involved either a lot more mass, or a lot of added complexity compared to just using an insulation system that isn’t prone to flaking off.  The other issue is that I don’t think SOFI makes a very good MMOD shield, meaning that a structure that big had a pretty likely chance to have an MMOD failure during its lifetime if some external MMOD shield wasn’t added.  Using a combined LV-MLI and MMOD-MLI solution, you could lower the weight of the insulation system overall, increase payload on normal flights, and completely eliminate this problem.  And this isn’t just limited to SLS, this could also be the case for any other core stage or large upper stage that reaches near orbital or orbital speeds, such as the core stage on a Delta-IVH or eventually an ACES upper stage. As an added bonus, you could even get the 60s-Retro Black-and-White stage coloring scheme without the weight penalty.
3. Bigger Single-Launch Propellant Depots: Along a similar vein, this approach could allow you to do one of the ULA single-launch, dual-fluid depot concepts where the LH2 tank is built into the upper stage’s payload fairing outer mold-line, enabling a 70-75mT LOX/LH2 capacity using Atlas’s Centaur, or over 100mT using Delta-IV DCSS as the starting point.  The nice thing is that not only do you get lightweight, high quality insulation, but you also get MMOD protection at the same time (which is critical for a depot).
4. Super Jumbo Single-EELV-Launch Propellant Depots: On the crazier side, you could combine ideas number 2 and 3, and say have a Delta-IVH place its core stage (with an LV/MMOD-MLI combo in place of its current SOFI) into orbit, with what payload remains being a docking node, temporary stay habitat, or additional propellant tanks for other more storable propellants if you want a multi-propellant depot.  That gets you up over 200mT of LOX/LH2 capacity in a single launch, without requiring an HLV to do it…  Though admittedly, a Delta-IV CBC would take a lot of modifications to get the passive cooling right compared to the Centaur-derived approach that ULA proposed originally.

I think this is potentially a really positive move forward that might open the doors for commercialization of technologies that NASA is helping fund development for. Just thought I’d pass along the thoughts.

[Update: I had a glaring error on one of the slides pointed out. It turns out that an inclined plane in LEO will only line up with an arbitrary departure asymptote once every 360 degree revolution of the ascending node. This means departure opportunities happen once every 50-70 days. Option #1 gets hurt the worst (requiring 4-6 depots to get the coverage needed instead of 2-3), but the other two still have merits. More details later. And I'll eventually update the presentation (or flesh it out on Selenian Boondocks in corrected form)]

Very much looking forward to reading the article. Thanks for the link, Keith!

With that introductory post out of the way I wanted to share a variant on the theme that I looked into about a week ago–using the 5m diameter DCSS instead of the Centaur as a baseline for single-launch depots.

I ran the numbers for this concept, and if you:

1. Did the “depot LH2″ tank as just a stretched version of the 5m diameter LH2 tank on the DCSS, using existing tooling
2. Used both the LOX and LH2 tanks from the DCSS as the “depot LOX” tanks

You could store somewhere between 100-103mT of LOX/LH2 (at a MR of ~5.8-6:1) using a depot based on existing stages and existing tooling.  This would give you ~90% of the capacity of an ACES derived depot, even if ACES never gets funded.

A couple of quick notes:

1. The existing DCSS 5m LH2 tank is load-bearing.  It actually connects at the bottom of the tank to the interstage, and at the top of the tank to the PLF.  This means that a depot LH2 tank built using the same structure, and with no payload on top, could actually replace a good chunk of the cylindrical part of the PLF for a depot flight.

   

2. One of the challenges that will need to be addressed is that with the tank built into the PLF diameter, you can’t put normal MLI on the outside of the tank, since normal MLI can’t take aerodynamic loads.  A deployable sunshield is one solution, but might only work at La Grange points.  Another interesting one being developed by Quest Product Development Corp (in conjunction with Ball Aerospace) is LV-IMLI, an advanced version of MLI that might be able to be up to the task. A third option would be qualifying a larger diameter, “hammerhead” style fairing with say 6-7m diameter.
3. You don’t necessarily need to use both the DCSS tanks as “Depot LOX Tanks”.  It is also possible to say use just the DCSS LH2 tank, and leave the DCSS LOX tank for use in storing Argon, Xenon, or some other SEP propellant.  This would make more sense for L1/L2 based depots.  You’d still be at somewhere around 74-75mT of LOX/LH2, but you’d also have about 28mT of liquid argon storage capacity.  And you could store up to 60mT if you used Xenon instead of Argon.
4. The 100mT number was assuming a target depot O/F ratio of 6.0, which is actually probably on the lean side.  The actual PLF volume is big enough to support a much bigger LH2 tank, so you could account for boiloff, etc.
5. You’d still use a CRYOTE-like module between the DCSS and depot LH2 tank to handle all of the actual depot operations functions such as power, control, rendezvous and docking, propellant transfer, etc.
6. DCSS LH2 tanks are less nice from a heat transfer standpoint because they have isogrid ribs on the inside that serve to lower the thermal resistance between the LH2 and the tank wall (more wetted surface area to conduct heat through–think of the ribs as “heating fins”), but the surface are to volume ratio of a squatter larger tank might make up for this somewhat.

Anyhow, this isn’t a thoroughly modeled idea, but it’s one way to get a 100mT capacity depot in a single launch using existing stages and existing tooling.  Whether you actually need a depot this big or not is open for debate, but I wanted to point out that it was possible, even if ACES is never funded.

I was going to write a short post today about a variant on the Dual-Fluid, Single-Launch Propellant Depot idea, when I realized I had never actually gotten around to explaining the idea here on the blog.  While this idea has been now discussed in a few of our AIAA papers ULA and I have published on the topic, I figured it was still worthwhile to do a brief blog post for those who haven’t had the time to read any of these papers.

Back in early 2009, Frank Zegler (of ULA) and I both independently came up with the concept for a LOX/LH2 propellant depot of decent propellant capacity that could be launched on a single Atlas V.  The basic idea, illustrated in the picture below (lifted from Bernard’s presentation from last month that is the third link above) is pretty simple:

Dual Fluid Propellant Depot (Credit: Bernard Kutter, ULA 2010)

• You have three parts, the Centaur upper stage that is used to launch the depot, a central section that holds the depot controls, docking interfaces, propellant transfer interfaces, etc, and a depot LH2 tank that is manufactured using the same tooling and processes as the Centaur tanks.
• When you get to orbit, you transfer all the excess LH2 from the Centaur to the depot LH2 tank, you then vent the Centaur LH2 tank down to vacuum, and permanently seal off the connection between the Centaur LH2 tank and the depot LH2 tank.
• You then close off the Centaur LH2 tank, and transfer the remaining LOX from the Centaur into it.  It now becomes the LOX tank for the depot.
• In order to reduce propellant boiloff to reasonable levels, ULA has typically suggested either the use of MLI and/or deployable sunshields to cut down on the heat flux into the tanks.
• Boiled-off hydrogen is used as a “heat sponge”, to intercept heat flowing into first the LH2 tank than the LOX tank.  Eventually the now much warmer GH2 is run through a rocket nozzle to provide settling force and station reboost.  It turns out that the boiloff rate achievable with good passive system design is lower than the amount of propellant you need to use anyway for stationkeeping/reboost, so for depots in LEO or L1/L2, you get the benefits of a ZBO system with separate reboost capabilities without the difficulty of building a ZBO system.

The end result is that with a Centaur diameter depot LH2 tank, you can store around 30 tonnes of propellant.  It’s a bit on the small side, but enough to fully refuel a Centaur or Delta-IV upper stage in orbit.  It has both propellant.  It requires no orbital assembly, no EVAs, no new tank tooling, etc.   The depot tank and depot center section together weigh less than 2 tonnes, so you can actually use the Centaur to deliver a depot like this to anywhere in cislunar space, and with good passive shielding (sun shields etc) to Mars orbit or to a NEO you want to visit.

There are ways of going bigger than 30 tonnes that have been discussed, the two primary ways suggested have been having the depot LH2 tank built into the 5m diameter fairing (instead of keeping it Centaur diameter).  That gets you up to ~60-70 tonnes of propellant.  Going with ACES style tanks can get you up into the 110-120 tonne capacity, but require the development of the ACES stage.  Even 30 tonnes of propellant, if you have depots in both LEO and at L1 or L2 is getting into the range that you can do very interesting things.

Anyhow, just wanted to introduce people to the idea if they hadn’t heard of it before.  Read the papers for more details.

That exchange was annoying, but utterly predictable. What really torqued my screws though was the HEFT presentation that was released yesterday. On pages 26-27, they list a bunch of key technologies needed for exploration, and which missions they were applicable to. The only technology that was included in the list that was shown to be not applicable to any of the missions was In-Space Cryogenic Propellant Transfer…

The dirty little secret most people don’t know is that the only HEFT study that was actually well within budget goals was the one based on the original FY11 proposal, which focused heavily on propellant depots and advanced technologies. I hope Chris Bergin doesn’t get too mad at me for posting a teaser from L2 of NASASpaceflight from back in September:

HEFT DRM 1 Budget Sandchart

As you can see, the only point at which it breaks the “budget bogey” is near the end of the commercial crew development, but for most of the exploration phase is well below the line.  Now admittedly, this DRM is not compliant with the now-signed NASA Authorization Act, however the HEFT team had abandoned this idea long before that Act was signed into law.  The only reason I could find for this was that this approach required “an excessive number of commercial launches”.  The next two DRMs (DRM 2A and 2B) also featured propellant depots, but combined with a “modest” HLV.  They ended up costing a lot more, but were still at least close to hitting budget targets.  Unfortunately, they also got rejected for requiring “too many commercial launches”.  The HLLV focused option (which dropped depots and any new technology) completely blew the budget guidance across the board, much like what NASA proposed in its report to Congress this week.

To give the latest HEFT report some credit, they did list depots as a potential commercial partnership with NASA.  If that meant something COTS-like where NASA helped fund some of the risk maturation on a FFP milestone basis, but basically let the commercial companies drive most of the technical decisions, that would be great.  I’m worried though that what NASA really means is the same “support” Griffin gave with his “we’ll buy propellant if you guys make it work on your own dime” comments.

But it’s really frustrating to see that it looks like depots were rejected for the same flawed reasons given in the ESAS report. Problems that industry is actively proposing good solutions to.  It’s also interesting that NASA’s NEA missions end up being so big and bloated.  I asked Josh Hopkins about this at his presentation last month, and he said part of the problem is that NASA decided that most potential NEOs were “too small” to be interesting, and therefore were focusing on the bigger, rarer, and harder to reach asteroids…and letting their whole architecture bet contorted by these initial assumptions.  Just like ESAS.

Ultimately, I think the whole HEFT process illustrates once again the danger of having secret teams at NASA doing conceptual architecture development in a vacuum, and without public transparency.  Instead of openly analyzing things, getting frequent feedback, or seeing if industry has ideas to deal with supposed show-stoppers, early decisions are made that drive things off the rails.  When those early assumptions drive the analysis in completely unaffordable directions, there isn’t a good mechanism to rein things back in.  Or at least, it’s hard to tell from the outside, because all the public gets to see is occasional summary reports released at the end, long after the flawed assumptions have been buried deep into the analysis in a way that will take years to pick out.

I guess the good news is that even though there are some elements in NASA that still don’t get it, there are a lot of other programs, particularly stuff in the Office of the Chief Technologist that give me some hope.  If Congress insists on setting NASA up for failure again by forcing them to build their Zip-Code Engineered Ares/Shuttle Zombie Rocket, at least some of the commercial work will be funded that will enable us to pick up the pieces when this all flies apart another 5 years and $10-15B down the road.  I’m hoping between the rendezvous and docking work we’re trying to do at Altius, depot work being done at ULA and Boeing, NEO exploration concept work at LM, inflatable station stuff being done at Bigelow, and all the commercial crew development projects, many of these excuses and wrongheaded assumptions will be impossible to make with a straight face next time NASA decides to do another internal, non-transparent, echo-chambered, insufficiently vetted paper-study project to figure out what they should do next now that the last Congressionally underfunded project goes flying off the cliff.

Background

As I mentioned on my Space Show appearance earlier this week, my first appearance there in January ’07 elicited a lot of discussion both on the blog, and offline. I mentioned that a guy approached me with the question along the lines of “is there a way to wrap a business case around propellant depots”. That would be Colin.

We batted around a lot of ideas over the months, and came to the conclusion that while it was probably a bit too challenging to wrap a business case around depots directly. Not unless you had the backing of a wealthy philantrocapitalist like Musk, Bezos, or Bigelow (if any of the readers happens to be such a philantrocapitalist desperately in search of ways to invest a couple hundred $M to try and one-up Elon, my email address is jongoff@gmail.com, just in case you were wondering). The problem is that making depots really pay off probably requires at least two, and possibly three miracles. And when you have to raise outside money from non-philantrocapitalistic sources, you’re only allowed at most one miracle per business plan.

So, we backed up and looked to see if we could find any businesses that enabled depots that might be profitable in themselves. The idea of prox-ops tugs seemed to fit the bill. The idea is that one of the miracles needed for a depot is the development of a good prox-op tug that could take the complexity out of bringing “dumb” propellant tankers from their delivery orbit to the depot, hooking up the transfer lines, and then sending the tankers home empty when they’re done. If you could find a way to develop such a tug (or something close enough that it was clear that you only needed to trick-out your already existing tug with some slightly different add-ons), in a way that paid for itself, that would both make you some respectable amounts of money while simultaneously making the fielding of commercial depots that much closer to reality.

The challenge is that while there are probably a half dozen companies in the US that have the technical competence to design a tug-like spacecraft, and most of them want to do so pretty badly, none of them have really tried to go after this new market entrepreneurially. Now admittedly, and to be fair, it’s got to be really hard for a large, publicly traded company to take entrepreneurial risks like that in even the best of cases. But part of the problem has been the challenge of finding some initial toehold market that you can really get into the space tug business with.

There are a couple of aspects to this problem:

1. You need a customer who has a need that’s bad enough they’d be willing to actually buy servicing from you.
2. You need a technical approach that is non-scary enough that said customer will actually be willing to let you try
3. You need a customer with money, and an approach that’s cheap enough that the value of your service to them is sufficiently less than your internal cost to develop and deliver that service that you can actually make money.

That’s a bit harder than it sounds, and at least part of the reason why nobody has actually delivered on orbital servicing using space tugs is that this is a tough nut to crack.

There are people out there who could probably benefit a lot form servicing (NRO and USAF LEO satellites, GEO comsats, etc), but their spacecraft are so expensive that they really don’t want to be the first guinea pig–second or third maybe, but they’d rather see you demonstrate your capabilities on someone else. In both cases we looked at options like just attaching the spacecraft and providing some extra maneuvering capability–not even trying to actually transfer fuel or power or anything. But the reality kept coming back to the fact that the amount of engineering work it would take to go directly after such a mission immediately would likely cost so much that you couldn’t raise the money. If you could demonstrate your capabilities on something easier first, it might be realistic to start servicing these bigger players, but that means you still need to find a toe-hold market. They’re great second markets, but not good toe-holds.

Iridium For The Win?

In the end, we started looking at the Iridium constellation, due to the collision they had with the Russian Cosmos satellite in February of 2009.

Colin hit on many of the reasons why Iridium looks like an interesting first customer for space tug servicing. But let me add a few more:

• Iridium provides a large number of identical servicing targets.  Which means that you can design the servicing hardware once, and provide dozens of missions, reusing hardware, software, and experience.
• Not only does losing satellites start cutting into Iridium’s ability to retain and grow their customer base, but satellite losses that turn into more debris could actually make it harder to successfully replace the current constellation.  They really need to keep those orbits clean enough that they can continue to use them.
• While they really don’t want to lose many satellites, the fact that they do have some spares means that they can actually afford to take more risks on something like orbital servicing [Note: an article in Space News yesterday indicated that they felt they could still survive as a business if at least 36 of their satellites were still available when Iridium Next starts launching].  The bar is a lot lower than it would be with $1B NRO satellites, or GEO comsats that are bringing in hundreds of $M/yr/satellite.
• The government has a huge interest both in Iridium continuing to exist, and in Iridium not having more of their satellites become collision targets.  At Iridium’s altitude and inclination, debris from any collision is going to increase the danger to just about all LEO orbits of interest.  Remember, unlike GEO where all the satellites are going in the same direction at nearly the same speed, LEO orbits all tend to cross each other.  It wouldn’t take too many repeats of the Cosmos collision to start really making a big mess.  So the government might have a legitimate reason to help try and encourage ventures like this.
• With Iridium’s satellites getting old, the odds of one or more of them running out of propellant or having its batteries die before it can be disposed of is probably not insignificant.
• The LEO environment is a lot easier to design an initial tug for, and a lot easier to affordably reach.  Not to mention you can probably get away with a smaller spacecraft.  All of these lower the amount of investment you need to raise up front.
• Iridium actually has enough revenue that they might be able to afford a sufficiently low-cost servicing option.
• Their only other option is really to count “hope” as an operating plan. The analysis I linked to above indicates that they think they can survive as-is, but having an orbital servicing option that allows them to continue at full capacity might still be very interesting to them.

What the NRO Doesn't Want to See a Repeat Of

How Best to Be Of Service?

So, we started looking at various options of servicing the Iridium satellites.  Our initial idea was to provide a small tug for each of the satellites.  If a debris conjunction appeared likely, or if the satellite’s attitude control system failed, or if the satellite ran out of propellant, the helper tug would take over, and either help the Iridium craft dodge the debris, or provide station keeping or end-of-life disposal services. While such an approach would eliminate the complexity of actually doing anything to the Iridium satellite other than just grabbing it, it would entail trying to make 66 small spacecraft cheap enough that you could launch them, hook them up, and perform their missions all for a price-point Iridium could afford…which quite frankly seemed unlikely.

We also looked at the idea of having only a handful of tugs (1-3) that would sit around in standby, and if an event came up, it would rapidly maneuver to the Iridium satellite in danger, rescue it, and then go back to standby mode.  While this cut down on the number of tugs you needed drastically, now the propulsion requirements goes way up.   Especially if you can’t put one in each orbital plane.  Switching from plane to plane requires a plane change if you have to do it in a hurry, and that can be crazy expensive from a delta-V standpoint (a 6o degree plane change done the quick way could cost you almost as much delta-V as it took to get into orbit).  You could also do ground based “rescue tugs” that could only launch on need…but once again none of these approaches was really realistic either from a launch frequency, propellant usage, or total cost standpoint.

What we finally settled-on was the idea of just refueling the satellites themselves.  If you did that, you could use one or maybe at most two or three tugs to do the whole job.  Plane changing between two planes with the same inclination can be done relatively cheaply if you have a lot of time.  You just go into either an elliptical orbit, or a circular orbit of different radius, and then your nodal regression rate will be either faster or slower than the Iridium satellite planes.  If you can be patient, you’ll eventually catch up with the next orbital plane.  Moving from satellite to satellite within a plane also requires some tricky maneuvers, but if you have time, they don’t have to use a lot of propellant.  One option we batted around was launching a little “mini-depot” (really mostly a dumb tank with some transfer systems and docking ports) into an elliptical orbit that would drift from plane to plane.  That way you wouldn’t have to accelerate and decelerate the whole propellant load each time you went into one of your drifting orbits.

The end result was that by doing it this way, you could cut down drastically on the number of launches, not have to be anywhere near as rushed, and cut down on the number of satellites.

For the paper one of the things we were going to do was analyze the different trajectories to figure out how much propellant it really took to move from satellite to satellite in a given plane (given various allowable travel times), as well as how much propellant it took to move between planes.  Once you know that, you can start getting a better handle on how many launches you would need, how long things would take, etc., which would start enabling you to get a realistic cost estimate for at least the marginal cost of the servicing.

Servicing Challenges

The one other big question mark was what it would take to service the satellites once you got to them.

The Iridium satellites weren’t made for servicing.  The propellant inlets are capped with the caps safety-wired on.  And the whole assembly is likely covered up with MMOD protection or MLI.  I tried to get a good look at the one Iridium satellite they had up in the Air and Space Museum while we were out in DC for the NGLLC awards ceremony last year, but the area where I”m pretty sure the fueling interface was at was somewhere you couldn’t readily see.  I was sorely tempted to see if I could sneak up onto one of the displays nearby to get a peek, but chickened out. Anyhow, the net result is you’re going to need to do some stuff that would take about 15 seconds for a dude on the ground to do, but is going to be annoyingly complex to do on orbit.  I won’t go into details, just to avoid pissing off the ITAR gnomes, but suffice it to say this is not a trivial task.

There’s also the challenge that propellant isn’t the only thing that these satellites need.  Batteries and solar panels wear out.  While the latest and greatest solar panels are a lot better than what Iridium was launched with, you still need to find a way to couple that power into the satellite itself without hurting it.  Do you collect the power and then just beam it onto the other solar panels (might work if it’s just the solar panel efficiency that’s dropping)?  Or is there some sort of power umbilical used for the satellite on the launcher that could be reconnected to to provide both power, and a backup battery?  How do you attach it all?  Once again, I have some ideas, but I think I’ll leave things at that.

Real Life Getting in The Way

So, we had some starting ideas, and a basic approach.  We were still unsure if we actually wanted to jump on this idea as a real business, but we were interested in at least seeing where the analysis went. We pulled together a team to write the AIAA paper to investigate the idea.  I can’t remember for sure, but I think Colin was planning on using this paper as part of his MBA that he’s been working on.  And then life got messy and complicated, and in the end we had to back out of the paper.  But we wanted to put these thoughts up here to spur people’s thinking. If someone is going to provide this service for Iridium, the clock is ticking.  Even a fast-paced spacecraft development project is going to take probably 2-3 years, and there’s really not too much longer that you can delay and still be of use to Iridium.  Basically, we wanted to get the idea out into public to see if someone could find a way to run with it.