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Armadillo Aerospace News Update: Quad updates, cooled engine, methane, modular vehicle

Published by Ekkehard on Tue Dec 5, 2006 8:42 am
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Matt put together some high-res photos of Pixel suitable for desktop wallpaper:
Wall Papers

I did an interview with c-net after the X-Prize Cup that covered a lot of ground:


Quad Upgrades

The most obvious thing that we had to improve post X-Prize Cup was the landing gear. I spent most of the flight back to Dallas sketching different landing gear concepts and assessing the tradeoffs. There was a strong temptation to make the landing gear into sealed pneumatic pistons that use the tank ullage as the reservoir. A decent sized check valve with a tiny back bleed for each leg would give a nice linear landing force with no spring return, and with a blowdown system like ours, the shock force would be somewhat related to the weight of the vehicle, with a 400 psi liftoff pressure and a 150 psi landing pressure.

The downside would be adding another potential leak path (with a sliding o-ring seal), which could easily result in a loss of vehicle failure. With our paired tanks, if ullage gas started leaking out of one tank, propellant would be pushed over from the other side, and it could quickly exceed the ability of the gimbal to balance the vehicle. This is the primary weakness of the current quad architecture, and it didn’t seem wise to tempt fate. I considered having an extra tank just for the shocks, but that adds a fair amount of complexity, and it wouldn’t get the blowdown variability benefit.

In the end, we decided to just make extremely sturdy side-load adapters for a 2” stroke commercial hydraulic shock. The shock also needed to be insulated to prevent the lox tank from freezing the hydraulic fluid. The sliding piston was made out of bearing brass, the insulators from garolite, and the fixed part from aluminum to allow us to weld it to the tanks if we choose. Currently they are just held on with set screws, and but up against the old shock mount point.

We tested a lot of landing combinations by hoisting Pixel up in the shop with various water loads to various heights, and optionally tilting or swinging the vehicle before pulling the Sea-Catch release to drop it. We did drops from over two feet high (3.5 m/s impact speed) and everything seems rock solid. Phil sat on top of the vehicle for some tests, and there is a very large difference in the landing loads. An empty vehicle dropped squarely on all four shocks lands pretty much like it hit the concrete without any shocks, but a fully loaded vehicle that lands on one shock gets a pretty gentle set down. A computer adjustable gas shock or some kind of sensing shock would be an improvement, but at our targeted descent rates (2m/s) this will work out fine.


We have replaced the big pin-and-block main thrust u-joint with a UJNL 20-20 (forged, 1 1/4″ bore) from Boston Gear. This is lighter, shorter, and has zero play. http://staging.smartcats.com/bost_root/web/pdf/product_sections/bearing_pp_127_133.pdf

James has been working on mounting a seat on Pixel. We quickly decided that a standard racing-car seat was not the way to go. After joking about using a saddle, we settled on mounting a motorcycle seat on a framework above the computer. The rider will probably be leaning over onto the seat and gripping some hand holds, rather than sitting upright.


I suspect we will have our next-generation vehicle(s) flying before any important manned flight opportunities arise, but we will probably let Russ ride Pixel under a tether just to see what the experience is like.

Cooled Engine

While the carbon reinforced graphite chambers are getting the job done, cracks in the chambers and resulting leakage have been continuous concerns. The cracking is almost certainly due to thermal expansion as the 20” long chamber gets really hot and is restrained by tie rods that don’t get very hot. We started using spring washers, but it was more of a token gesture as it would take an almost absurd number stacked up to hold the tension we want and still have room to compress enough to absorb all the expansion..

Our previous plan was to design a chamber that had an integral flange on it, so it was clamped at the top, leaving the hot part to essentially dangle unrestrained below the injector. Unfortunately, this wasn’t as straightforward from a manufacturing standpoint as we had hoped. The team at Cesaroni Aerospace was worried about a lot of possible failure modes, and we dithered around for a while without a clearly winning design appearing.

We decided to go ahead and try putting a cooling jacket around a simple graphite chamber. The cooling doesn’t really need to be very good, because the graphite is happy operating at extremely high temperatures, and we don’t have to worry about making a saddle section around the throat. Having fuel around the outside also puts the graphite in compression at all times, so it shouldn’t need any additional reinforcement.

We got some 8” ID aluminum pneumatic tubes for this from: http://www.scotindustries.com/

We had Cesaroni make us a few bare graphite chambers similar to the reinforced chambers we had been using. They just showed up today, but we haven’t done any fit-up yet. We went with a shallower converging angle in the nozzle (15 degrees instead of 30), since the only place we were seeing any erosion at all in the previous engines was on the converging section, and we believe that the shallower angle will help the film cooling stay attached better. Lutz Kaiser had also mentioned that the OTRAG chambers used shallow entry angles because it reduces organ-pipe combustion stability problems. We haven’t had any stability problems, but it still doesn’t seem like a bad idea.

I made all the various manifolds and flanges in the last couple weeks, and we should be doing water tests soon and hopefully trying to fly Pixel on the new engine next weekend. It turns out that the jacket, flanges, and manifolds are almost exactly the same weight as the tie rods, phenolic insulators, and retaining plate on the old engine. We should be able to reduce the film cooling and get a little bit better Isp, but the main thing we are looking for here is increased durability.

Our current injector pattern has both good face cooling and good performance, but we had to modify the design to allow all the fuel to come in from the sides, and we also wanted to avoid the “buried weld” in the current design, where a single internal weld separates part of the fuel and oxidizer manifolds. This is a classic point of concern, and we did have one engine that popped the op off on first ignition that could have been due to a leak there.

The solution was to build the lox manifold as an interlocking piece that can be welded in from the chamber side of the injector. This involves more machining steps, and the welding to the injector face must be done halfway through all the operations, rather than just at the end, but it does resolve the dangerous leak path. Unfortunately, I still have some concern about those welds cracking, which, while not dangerous, would leak unmetered fuel into the chamber. I initially milled the surface flat before drilling the injector elements, but we found that the welds were very weak and prone to cracking. We decided to just leave the welds at their full height and drill the elements through them (with spot facing first, of course), but I worry that the spot facing has exposed some areas below the weld penetration. We should be doing water tests soon to find out.



We are seriously considering trying out methane as a fuel instead of ethanol. Methane mixture ratio by volume is close to 1:1, just like ethanol, so we would get to keep the identical tank sizes, and we are working on converting the VDR chassis over to a methane testbed. Just on raw performance for a ground launched vehicle (for upper stages it is a pretty clear win), Methane only barely outperforms alcohol because of the low density, but our reasoning is based on operability issues, not delta-V.

We are considering using lox / methane in a self-pressurized system, where, instead of using helium to pressurize sub-cooled propellants, we allow the propellants to reach their boiling point and provide their own pressurization gas. Although almost all nitrous oxide hybrids, including SpaceShip One, use self-pressurizing propellants, it is not common for biprops. The AirLaunch company with their QuickReach rocket program are using self pressurizing lox and propane (which must be heated to reach the desired pressure), but I don’t think any significant vehicles have yet flown with this arrangement. AirLaunch and some old papers term this “VaPak” for Vapor Pressurization. Gary Hudson was kind enough to answer some questions for me about their experience, and it does seem worth pursuing.

Getting rid of helium as a consumable would be a big win for us. The helium costs more than the other propellants, but it is also bulkier to transport, and we have lost a couple crucial testing days due to running out of helium. If we can just travel with a big lox tank and a big methane tank, field operations will be a lot easier. We would only need to connect two hoses to the vehicle for the filling process, and we could top up the vehicles repeatedly without venting if desired.

Using methane may also allow us to simplify a few other systems. We currently have purge ports so that when the engine is shut off, helium is blown in right after the throttle valves to force any remaining liquid out of the plumbing and into the engine. With self-pressurizing liquids, the fluid in the plumbing should vaporize almost immediately, and an atmospheric pressure gas mixture shouldn’t be much to worry about. If this does still turn out to be a problem, it will be a pretty big drawback, forcing a gas bottle just for purging. We also think the augmented spark torch igniter can likely be removed, and replaced with just a couple spark plugs. Cracking the valves will result in nothing but gas coming out of the injectors, and I think a spark will light that without any problems.

Because the propellants boil as liquid is removed, the tank pressure does not drop as much as you would expect. We did some tests with liquid nitrogen at 225 psi saturated pressure to confirm tank filling procedures and pressure drops, and the pressure only dropped by a third, from 225 psi to 150 psi, going from a completely full tank to a completely empty tank. The expelled liquid is also getting colder and denser as the tank drains, so the actual mass flow is dropping even a bit less than that.

Cooling a chamber with saturated methane will be more challenging than with a subcooled liquid, but I think the graphite chambers will work fine even if they get nothing but film boiling along their surface.

There are two primary performance disadvantages with saturated propellants: The higher temperature / pressure propellants are less dense than in their normal sub-cooled form, and the gas remaining in the tanks at liquid expulsion is very heavy compared to helium. You can get graphs of pressure/temperature/density at http://webbook.nist.gov/. At 20 bar, completely full 36″ ID spherical tanks (14.13 cubic feet) would hold 777 pounds of lox and 284 pounds of methane. At liquid depletion, the gas remaining in the tank at 15 bar is 51 pounds of oxygen and 21 pounds of methane. In expendable booster applications it should be possible to burn the gas remaining in the tanks all the way down to vacuum, but the transition point when the first propellant goes to gas is scary from a combustion flame-out standpoint, and the immediate drop in thrust by a large factor may make powered vertical landing near that point dangerous.

I had been concerned about free-venting methane, but an LNG consultant that we spoke with really didn’t think it was much of a problem, especially if we are shooting it up with a couple hundred psi behind it and a nozzle on the end.

We are still tracking down all the things we need to actually do engine firings with methane. Apparently LNG (commercial methane, “liquid natural gas”) dewars are built to a different spec than LOX / LIN dewars and they aren’t very popular around here. We are probably going to have to pay to get one made just for us, especially if we want to use it at 300 psi or so. We need to decide which connectors we are going to use for filling, since apparently there are three different “standards”. We need to develop a little resistor based sensor that can sense cryo liquid level before the computer controlled vent valve, which will allow us to have both automation and a good telemetry log of the filling process.

I have started working on a coaxial injector for saturated lox / gaseous (from the cooling jacket) methane:

We will probably wind up with both internal and external tapers on the posts for better mixing. The manufacturing aspects of this are so nice (no welding or angle table work) that we may try making a coaxial alcohol injector, even though that is usually not considered a high performance liquid-liquid injector.

Modular Vehicles

We have gone through a few design iterations for our next-generation, modular vehicle system already, and fabrication will be starting next month.

The biggest decision is the propellant tanks.

As a baseline, our current 36” ID welded spheres cost about $2000 each if you include labor costs. They hold 100 gallons, weight 90 pounds, and burst at around 750 psi.

On the high end, Microcosm gave me a rough quote for 25” diameter x 59” long linerless carbon fiber lox tanks at $13000 in quantity for a 100 gallon, 55 pounds, 1200 psi burst tank. For an upper stage, these would pay for themselves, but probably not in a booster.

I spent a while investigating flowformed tanks, similar to those used in the OTRAG project. http://www.flowform.com/ has some good information on the process. Most of the products that are made with flowforming use two different sets of expensive tooling: a forging tool to create the pre-forms, and the actual flowforming mandrel that the pre-forms are spun over. Going from scratch would be well into six figures, so we tried to find a combination of existing tooling that could meet our needs. We found that we could start with 14” sch 40 seamless 304 SS pipe and machine it to a precision pre-form shape to avoid the need for forging, and they had an existing 13.305” diameter mandrel that could create 168” long tubes. The process would allow the ends to be left thick, so aluminum bulkheads could be held in with snap rings or threads. The 304 SS would reach about 170ksi UTS when flowformed, so the walls would only be about 0.038” thick for a 1000 psi burst pressure. This resulted in a 100 gallon tank that would weigh about 85 pounds. The price was $7000 each in quantity.

The mass ratio would only be a little better than the current aluminum spheres, but for a vehicle with many modules it would have much better aerodynamics due to the aspect ratio. Conversely, the aspect ratio would make testing small, four module configurations as vertical landers much harder, requiring broad spacing interconnects and/or extremely wide legs. Flowforming may yet be a good technology when coupled with a more exotic material, like aluminum-lithium or maraging steels, which would give mass ratios competitive with the composite pressure vessels, but making the forging tools for it and sourcing the materials would be significant up-front costs.

The fiberglass pressure vessels from Structural http://www.structural.com/base_pages/composite.htm that we used for most of our peroxide vehicles are so tempting for propellant tanks, because they are dirt cheap, about $10 / gallon across the entire product range from 60 gallons to 1600 gallons) and rather high performance, with the double-flanged designs holding over 1000 psi without bursting (the single flanged and screw on tanks will fail at 600 psi), at a weight of about 10 pounds per gallon. The liners are cross linked polyethylene. We have several of these in the shop, so we finally just said “what the heck, lets see what happens if you fill them with liquid nitrogen”. First we tested some small plastic containers to get a sense of what might happen. The first test was discouraging – an HDPE quart jar cracked at the base just from having liquid nitrogen poured into it. A PET water bottle didn’t have any problems holding LiN. A piece of one of the tank liners we had around did seem to hold up fine after being submersed in a cooler full of LiN, and still seemed to have decent flexibility, so we thought it was worth testing the full size tank.

We did the test with remote controls at our 100 acre site, because if the vessel did rupture at high pressure, it was going to be a pretty big kaboom due to the LiN having soaked up at least some heat and being at a saturation level above ambient pressure. It turned out to be pretty anti-climatic, with the liner cracking at only 60 psi and letting the LiN leak out. This was probably for the best, because it would have been a bit sporty to put LOX in a PE lined tank anyway, and these tanks do make the most distressing crackling sounds when you pressurize them above their 150 psi rated pressure the first time. Still, I wonder if they can rotomold the liners out of PET…

The last option was to improve on our existing spun tanks. AMS industries http://www.amsind.com/ has been great to work with, and I’m happy to hear that XCOR and Paul Breed (follow his work at http://unreasonablerocket.blogspot.com/) have also ordered tank ends from them based on our experience. We have gone over a lot of different options for improving the mass ratio.

The 36” ID x ¼” thick tanks have all been bursting in the heat effected zone by the girth weld, so we could achieve better performance if we started with a thicker plate, and machined it down everywhere except a thicker band at the weld zone. It always seemed to me that you should be able to do that while the part is still chucked up on the spinning machine, but AMS’s machines don’t have the ability to do that. Spincraft http://www.spincraft.net/index.html is the high-end aerospace tank spinner, and I bet they could do it, but they never return my calls or emails. I realized recently that we could probably pre-machine the spinning blank to different thicknesses while flat, then spin it. AMS thought that this would probably work, but getting the plate completely flat before machining might be challenging. We may give this a try at some point. A first step would be to just thicken the weld band, but later optimizations could try to correct for the thinning that happens at the point of maximum curvature as well. A 36” ID hemisphere is spun out of a 48” diameter flat circle, so there is certainly some additional room for evening everything out.

Our current tanks are made out of 5083 aluminum, but we have been considering other options. One of AMS’s supplier suggested that we look at 5383 aluminum, a relatively new alloy. This goes under the trade name “Sealium”, because it is designed to replace 5083 for aluminum boat building, and it does look like a very good alloy for us. It is basically just like 5083 for forming and welding, but the welded joints are 15% stronger in the HAZ. The alloy is available in the sizes and thicknesses we need, so we went ahead and chose this alloy for our next batch of 24 hemispheres. 5059 aluminum is an even newer alloy aimed at the same market, with a >25% strength increase across the board, but the availability isn’t as good. The shipbuilding industry is actually a lot closer to what we do than the traditional aerospace industry. http://www.aws.org/wj/feb04/anderson_feature.html

After a lot of searching, we did finally source 2219 aluminum (the material used for the space shuttle external tank before they moved to an aluminum-lithium alloy), but the price was very high at $1800 for a single sheet (that was the cheaper quote). If we are willing to heat treat the entire tank after welding, 6013 may also be a good alloy to consider.

Elon Musk put me in touch with their contact at Alcan that is supplying the Al-Li metal for Space-X, and I have a meeting set up with their rep this week. Apparently the minimum order is around 3000 pounds, and the lead time is six months, but that might not be out of the question, depending on the price. They are trying to find us some material to make a test article out of.

Availability is a bigger issue than most people think, and even as we start building honest space vehicles, we are still more likely to use a less-optimal material that we can actually get our hands on quickly and reliably.

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