Nov 16th, 2011 by Jonathan Goff
Since I’m not yet ready to talk about some of the neat ideas we have in the hopper at Altius, I figured it might be fun to do some blog posts on some of the cool-but-not-very-well-known space technologies that are being worked on these days, particularly ones being developed by other companies here in the Denver, CO area. After all, we can’t let Brian Wang over on Next Big Future have all the fun writing about cool new technologies. So, without further ado, I’d like to introduce you to a cool line of MLI technologies that Quest Product Development Corp of Arvada, CO is developing for NASA.
[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 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.
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.
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:
- 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.
- 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.
- 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).
- 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.