November 19 and 23, 2002 Meeting Notes

 

 

New Vehicle Work

 

We are intending to get a new vehicle in the air, at least hovering, by the end of the year.  Flights to altitude should follow shortly thereafter.

 

We hydrotested a new fiberglass tank.  Same behavior – lots of disturbing snaps and pops the first time we took it to 350 psi, but none the second time.  I am going to buy a few more of these, in case we need to rebuild again, and also to let us go ahead and test one to failure just to see what happens.  Our early testing with the new vehicle will probably be at very low pressures, because the quad engine configuration will have a much higher T/W ratio than we really need.  Supersonic flights will start with 300 psi tank pressures.

 

The new vehicle will have a rear ejection parachute instead of the rocket drawn parachute from the top.  We are going to try to make the vehicle completely pyro free, using electromagnetic or pneumatic actuators for the parachutes.  A small RocketMan Kevlar drogue will be pulled out by a slug thrown by a small spring or air cannon.  For our mid altitude tests, the drogue will immediately pull the parachute deployment bag out, but for high altitude tests, we will have a line release to

 

We are going with a rolled and welded aluminum cone for the next vehicle, instead of a composite cone.  It will be a 15 degree half angle, and we will have a reusable shock absorber at the top for final impact attenuation.  If a composite shop happens to already have molds/tooling for a shape you need, that seems to be fairly cheap, but otherwise, getting simple cones and cylinders made at a sheet metal shop comes out a lot cheaper.

 

A selection of various honeycomb core composite panels should be arriving soon.  We are going to fabricate the fins and some bulkheads out of these.  They probably aren’t going to be heat resistant enough for the main engine bulkhead, because the adhesives bonding the core to the panels is not terribly high temperature.  We looked at some resistance welded stainless or titanium honeycomb panels, but the prices were around 20x what the aluminum and fiberglass bonded panels were.  We will probably make the engine bulkhead out of a thick slab of aluminum, and use our CNC mill (which is finally going to get installed next week) to mill out a custom isogrid to lighten it.

 

It took a bit more work than I expected to get the simulator working with four throttled engines.  The decision to have a given engine valve continue opening or closing needs to be based on the derivative of angular rate, unlike solenoid attitude engines, which are just based on the rate versus desired rate comparison.  I have it basically working now, but initial liftoff from the ground is a bit problematic, because a slightly tilted takeoff angle will cause one engine to throttle up very far, because the vehicle won’t tip to straight until it has nearly lifted off.  I am currently limiting that with sort of a hack, but I want to find a good solution for “on-ground” detection, which would allow large differential throttling when it is desirable, as when a single engine is performing poorly, but disallow large differences when it is due to ground contact.  We could have four actual ground contact switches, or use the laser altimeter, but we weren’t expecting to have to add either to the vehicle for the altitude flights.  I may just go with a short timer, during which time the behavior is modified.

 

 

Full Size Mockup

 

Our big parts started arriving for full size X-Prize vehicle mockups.  We could have done some of the work with wood and foam type materials, but we are taking it as an opportunity to begin working with some new metal fabrication shops.

 

www.baldwinmetals.com is doing the rolling and welding work for both the upcoming 2’ diameter vehicle, and the 5’+ diameter X-Prize vehicle.  The mock-up cone is 62” base diameter and a 24” top diameter, with a 10 degree half angle, made out of 1/8” 5052 aluminum.  We are going to be welding and cutting on this a lot as we try different layouts, so we didn’t want to worry about dealing with real thin material.  It weighs 162 pounds, but we expect to make the flightweight vehicle with aluminum of less than half that thickness, and add a carbon fiber overwrap.

 

www.bakertankhead.com made the 62” diameter 2:1 eliptical tank ends we are using.  Because this is just for mockup, I saved money by just having them fabricate them out of the cheapest mild steel they had, instead of aluminum.  They weigh 250 pounds each in 3/16” steel.  I also had them fabricate some 12” diameter, 3/16” thick stainless steel dished “tank ends” that we are going to use as weld-on closures for our lightweight engines.

 

We are still debating three options for our main propellant tank.  We know that it will be a carbon fiber wrapped liner, but the exact fabrication of the liner is still up in the air.  We have a known-good solution with 3/16” thick aluminum tank ends from Baker, welded to a 1/8” thick barrel by Baldwin, but that it heavier than we would like.  www.acmemetalspinning has tooling to metal spin 60” diameter and 66” diameter heads, and they are comfortable going down to 1/8” thick, but the actual profile is a 132” diameter sphere section dish, which is not as good a pressure vessel end as an elliptical end.  We would ideally like the entire tank liner to be made out of very thin metal, like 0.060” or so, but that will be difficult to work with, requiring either pressurization, or internal stringers.  The third option is a 62” diameter polyethylene liner for the largest of the Pentair Composite tanks, but we don’t have weight or pricing for those yet, and we might have to actually buy a fiberglass tank to strip down to the liner.  On the plus side, they have an isotensoid contour actually designed for filament winding.

 

http://media.armadilloaerospace.com/2002_11_23/11-24-02_a.jpg

http://media.armadilloaerospace.com/2002_11_23/11-24-02_b.jpg

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Sparse Engine Test

 

We built an experimental motor today to test out a theory for low pressure drop catalyst packs.  The welded-in anti-channel rings in the last engine seem to be working very well, with the final screen not noticeably bowed down towards the nozzle at all.  In the older engines where the entire pressure drop of the pack is retained by a retaining plate at the bottom, ¼” thick perforated metal plates would be noticeably bowed after a couple runs.

 

The idea for the new engine was to use a brazed stainless screen on every anti-channel ring (instead of just the final ring), and only silver screens between the welded anti-channel rings.  If a pack is made of nothing but pure silver screens, it just crushes down onto itself, but the though here is that with positive support every 10 silver screens, the silver would not crush itself, and we could dispense with all the inert stainless screens we have been alternating with the silver screens.

 

The exact engine combination was:

 

2 loose ACR as spacers at the top

9 stainless screens as a flow spreader

welded ACR with brazed screen

10 silver screens

welded ACR with brazed screen

10 silver screens

welded ACR with brazed screen

10 silver screens

welded ACR with brazed screen

10 silver screens

welded ACR with brazed screen

10 silver screens

welded ACR with brazed screen

10 silver screens

welded ACR with brazed screen

 

The pack was compressed to 4000 on our press gauge (about 8000 pounds over the 5.5” diameter pack) after each ACR was inserted.  We use 5000 for the normal engines with the alternated stainless, but the silver is a lot softer, and we didn’t want to completely crush it.

 

Unfortunately, it wasn’t possible to maintain compression on the pack during the ring welding process, so while the screens did get to take a bit of a set, there wasn’t any active stress in the final motor. 

 

Welding all the ACR warped the engine a bit, so we had to drill the bolt holes out a bit to close it down.

 

On testing, the engine produced a fair amount of perfectly smooth thrust, but lots of peroxide came through completely undecomposed, so we need a combination of more catalyst or more pressure drop for clean running.  We will try adding another set of 10 silver screens on Tuesday, but we may need to work out some form of clamp that can allow Russ to weld the rings in a compressed state.

 

In other testing news, I am buying a fairly expensive coriolis mass flow sensor to allow us to make very accurate Isp measurements dynamically during test runs of engines over 5000 lbf, which will take us through our X-Prize development.  It is among the more expensive ways of measuring flow, but it is nothing but a stainless steel tube to the peroxide, it doesn’t care if you blow high pressure gas past it, and it has accurate start and stop behavior, with good bandwidth.