July 3, 2005 notes:

 

Spherical Tanks

 

I was having difficulty finding commercial aluminum pressure vessels in the size and pressure range we want, so we decided to make our own. It turns out that our vehicle size is a very good fit for spherical tanks. A 3’ diameter sphere holds about 100 gallons, so two 3’ spheres is about the right amount for a single-man-to-100km vehicle, and a 3’ diameter cabin is pretty comfortable.

 

Spherical tanks are nice in that you only have a single weld bead, and with metal, you get roughly a third higher mass fraction than you would get in a barrel section due to not having hoop stress at twice the axial stress.

 

I tried to get pricing information from http://www.northlandstainless.com/products/heads.php about their hydroformed hemispheres, but they didn’t return my email, so I would up with the metal spinning shop http://www.amsind.com/.

 

We are initially assuming no post-weld heat treatment, so our alloy selection was going to be from the 5XXX series of work-hardening, highly weldable aluminum alloys. The heat of welding anneals the nearby metal, greatly reducing the strength, but the effects vary from alloy to alloy.

 

5052 is the most common aluminum used in rolling and welding fabrication operations. If you ask for something formed out of sheet aluminum and don’t specify otherwise, you will probably get 5052. All of our cones and cylinders for various rocket bodies have been from 5052.

 

5083 and 5086 are more highly alloyed versions with higher strength, but somewhat more expensive and less readily available. 5083 is a little higher strength and a little less formable. The spinning shop preferred 5086.

 

Tensile strengths in ksi (from AMS Metals Handbook, similar data here: http://www.thermaflo.com/engref_tensile.shtml ):

 

Yield Ultimate

5052-O 13 28

5052-H38 37 42

5083-O 21 42

5083-H116 33 46

5086-O 17 38

5086-H116 30 42

 

The prices were quite reasonable, only $370 per hemisphere for a 3’ diameter, 1/8” wall 5086-H116. I ordered four for testing.

 

The order arrived with a sheet of actual metal test results for that heat of aluminum, showing 45 ksi UTS. In general, tested values are expected to be slightly higher than book values.

 

We had a bit of a surprise on the sphere weight -- strictly from surface area calculations, a 1/8” wall 3’ diameter hemisphere should weigh 25 pounds. The hemispheres we received only weighed 18 pounds each. I was under the impression that manual sheet spinning operations, as opposed to automated shear-forming, didn’t produce much thinning of the metal. This was incorrect for something as deep as a hemisphere. The metal at the polar boss was the full 0.125” thick, but the metal at the girth edge was thinned down to 0.100”. That thinning alone wouldn’t account for the low weight, so it must have thinned even more higher up the dome. When we later sectioned a dome, we found that it got down to 0.060” at the thinnest.

 

The thinning during spinning would be accompanied by additional work hardening which could additionally strengthen it, so it might be a wash higher up the hemisphere, but only having 0.1” at the weld area was going to reduce the strength for sure.

 

Getting two 3’ hemispheres with weld-beveled edges to line up for welding is a bit of a trick. Our first attempt was to put the hemispheres together inside the 3’ diameter tubular section we had fabricated for an inter-tank, and do the initial tack welds through cutout holes. We then pulled the sphere out and ran a bar through the entire thing so it could be rotated on two sawhorses. James was out of town this weekend, so Russ did all the welding. We used 5356 filler rod instead of the 4043 we use for all of our normal work.

 

We welded fittings on both ends and hung the tank on a scale under a forklift as we filled it with water for hydrotesting. The tank was under 40 pounds, and it held over 800 additional pounds of water, for a mass ratio of 21.

 

The tank burst at only 210 psi, popping open right along the weld. Inspection showed that it ruptured at a section where the weld had bridged a small gap between the two imperfect hemispheres. We also saw that the back sides of the weld didn’t show good closure, with a visible line down the bead in many areas.

 

We prepared the other two hemispheres more carefully. Russ tacked about 16 little tabs on the inside of one hemisphere so that the mating hemisphere was forced to meet up a lot more tightly, and we spent some time hitting problem spots with a hammer until we got almost perfect conformance. We cleaned the weld area more aggressively, and we made a new support rod that filled the interior of the sphere with helium backing gas during the weld operation. Russ had one problem with a weld blowout due to having the helium flow rate too high, but overall the welding went very smoothly.

 

This tank ruptured at 340 psi, and it tore in the base metal right above the weld in the heat-affected zone, exactly where we expected it. The back side weld beads showed good closure, but there were a couple spots that were still a little less than perfect. This was still lower than what we had hoped for (0.1” metal at 38 ksi should give 420 psi), but there are a lot of possible explanations: increased cross sectional area and forces as the tank stretched, multi-axial tensile stress may be lower than single direction test results, the metal may not have been a uniform 0.1” all the way around, and one other thing that we can easily correct: The hemispheres came with a 60 grit brushed finish (“spinning lines removed”), and the tear seemed to follow the brush marks. We are going to take the next set of hemispheres “as spun” and bead blast the area around the weld to remove stress risers.

 

Based on our results, I am going to go conservative for our first high performance vehicle. We are going to use ¼” thick stock for the spun hemispheres, which should give us a weight of under 80 pounds, a tankage mass ratio of 11, and a burst pressure of 640 psi or greater if the stress relieving helps. Our operating pressure is 300 psi, so that gives us quite a lot of margin. I ordered six hemispheres, so we can burst another one, then put a vehicle together. Still very cheap at $650 each.

 

I am considering the possibility of machining a ring (probably in several pieces) of thicker metal to go between the two hemisphere halves. That way, the hemispheres would only have to meet up with the wider ring, rather than the beveled edge on the matching hemisphere. The cross section change would be a stress riser, but guaranteeing a full weld with no thin bridging sections would be a benefit, and our weld beads are already a stress riser anywat..

 

When we are ready to increase the performance, we can either use a heat treatable alloy like 2019 and heat treat the entire tank set after welding, or try and find a metal spinning shop that can perform CNC machining on the domes after spinning them, allowing us to start with a ¼” thick plate, but turn most of the hemisphere down to 1/8” or less except for a gentle increase to the full thickness in the couple inches near the weld line. AMS can’t do this, but I expect there is a shop somewhere that can (if anyone knows of one, tell me…).

 

Another page with lots of aluminum welding information: http://www.weldreality.com/aluminumalloys.htm

 

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Vehicle Work

 

Phil fabricated a base heat shield for the vehicle out of nomex honeycomb. The aluminum legs will be in ceramic cloth sleeves, but we are still a bit concerned about the engine plume cooking things during takeoff and landing. We will be finding out soon with a hold-down test firing.

 

Total vehicle weight is 350 pounds dry, and it can hold 140 pounds of propellant. This is a bit heavier than we guessed, but not too bad. When we get all the parts in for the duplicate vehicle we can start weighing various combinations and seeing how much we could save with various alternatives, like using aluminum for all the manifolds and hose ends.

 

We tested the computer control of the roll thrusters with the vehicle hanging from a hoist, and they worked well. I experimented with a couple different parameter values, and you can have a fairly generous dead-band between firing the opposing thrusters. It stops manually induced rolls pretty quickly and with minimal gas use, but we are still concerned about any off-axis engine thrust induced roll being difficult to counter with small thrusters. I will probably feel better with two engines gimballing to provide roll control.

 

We also did the final computer / IMU / gimbal actuator alignment and testing, liquid nitrogen pressure testing of the LOX plumbing, and pressure testing our high pressure plumbing. The only thing we still have to bench test before the first hold-down firing is the GPS system.

 

Engine Work

 

Our new engine was a disappointment. Going from 20 x 1/16” holes to 80 x 1/32” holes for each propellant and pulling the fuel injection point in from the side to the same radius as the lox injection seems to have brought on some degree of combustion instability (about +/- 20% fluctuation), and the Isp still isn’t any good.

 

On the bright side, we have made several 30+ second burns with it so far without a problem. We got a heavy professional hardcoat applied to the chamber, and we have been doing most of the tests with 1% by mass ethyl silicate added to the 95% (190 proof) denatured ethanol. The ethyl silicate leaves a crusty white coating inside the engine, but it is supposed to significantly reduce the heat transfer. We did do one run without the ethyl silicate, in case that was causing the combustion instability, but it didn’t change anything (or melt).

 

The single-billet construction of both the lox and fuel injectors and manifolds is holding up well, with no signs of localized flameholding or melting.

 

One new bit of data we were able to learn: previously with the side injection we couldn’t tell much about the liquid distribution, but now that the fuel flows into a manifold at the top of the chamber, we were able to water test the chamber before welding the top fuel manifold on, giving a straight-up fountain of water from each cooling channel. There was a quite significant variation in fountain height, with the highest point about 50 degrees forward of the tangent entry point, and steadily decreasing the rest of the way around. Varying the inlet pressure moved the high point around a bit, but it retained the steady decline. We will make a much larger fuel manifold on the next from-scratch engine. We expanded the LOX manifold on this engine a bit after seeing that, but we don’t have a convenient way of testing the distribution there.

 

An older edition of Sutton listed L* for lox-alcohol motors as ranging from 36” to 120”, and our existing motors have been at the very bottom of that range. A couple people have told us that we should be able to achieve high efficiency with that volume if we use a high performance injector, but since we are going to be throttling, we really don’t want twitchy injectors. We are making a chamber extension that adds 5” more barrel section to the existing motor, for 36” more L*. We are probably going to have to make a brand new injector to clear up the combustion roughness, but we’ll try it with the existing one first. For the next injector, I am going to just drop the hole count significantly to get more pressure drop across the orifices. Our previous engines ran perfectly smooth with very little injector pressure drop, but that is a classic recipe for stability problems.

 

All of our test stand instrumentation is now running through the flight computer, making the test stand completely wireless. This has a lot of benefits compared to our dedicated data acquisition system, and is working well. Keeping the flight computer active and exercised even when you aren’t flying a vehicle is a Good Thing..

 

The hard firebrick ( “Louisville dry press high duty brick” ) we have been using on the test stand is holding up remarkably well. One of these days we are going to put one of our Buran tiles (tossed in for free from the guy we bought our Russian space suit from) on our blast deflector and see how it holds up under the same circumstances. It wouldn’t be the lightest thing, but it looks like you could make a reentry heat shield out of this stuff…

 

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