Jun 19, 2001 Meeting Notes

 

In attendance:

 

John Carmack

Phil Eaton

Russ Blink

 

New Supplies:

 

Analog Fiber Optic Gyros

Larger porous sheet metal

Ice pack cooling vests

0.8 second motorized ½” ball valve

 

To Get:

 

Loctite

70% semiconductor grade hydrogen peroxide

Re-gear the 1.3 second motorized valve to 0.8 seconds

More 568-216 Teflon O-rings for bottle manifolds

O-ring cord stock for large diameter motor O-rings (not offered in Teflon, so I am getting silicone)

 

Motor Test

 

We fired the new 50 lb thrust motor with the externally threaded closures tonight, but we ran into some problems.

 

The initial push-tests worked fine, but when we fired the motor it only had a few valid samples, then all the samples started reading over 300 lbs (on a 100 lb load cell).  We may have broken a leg of the strain gauge bridge by banging into the load cell on the initial startup.  Tonight I’ll pull everything apart and check it against our 500 lb load cell.

 

The other thing that happened was that the new attempt to internally thread the catalyst pack retaining plate into the motor didn’t work out.  The initial burst actually stripped the threads, and the retainer was cocked down for the rest of the run.

 

 

New Motor Designs

 

We are going to try using the perforated metal for the injector plate and the catalyst pack retaining plate.  Clamping it between flanges will probably work, but we may try some tongue and grove machining to help it stay put.  There are also a couple local companies that do chemical etching that may be appropriate for some of our plates.

 

We are considering increasing the diameter some, because 50 pounds out of the same size that we were using for 15 pounds does put the flow through the catalyst retaining plate closer to the nozzle velocity than it should be.

 

The engine length can be cut down a lot, because we know that we don’t need more than 15 foam discs thickness.

 

Higher Performance Propulsion

 

While I am occasionally tempted by the nitrous / ethane combination, I still think that catalyzed hydrogen peroxide is the oxidizer of choice, largely because of the great simplification of using it for attitude control engines.

 

Our current manned vehicle propulsion system is set up so we will easily be able to convert the main lifting engine to either a biprop or hybrid configuration to nearly double our Isp, and leave the attitude control engines exactly the way they are.

 

In biprop form, my preferred design would probably use isopropyl alcohol instead of kerosene.  There is a slight loss of Isp, but there are several advantages:

 

Cooler combustion temperatures, which can be reduced as low as desired (with reduced performance) by watering the alcohol.

 

XCOR makes a good argument for using alcohol based on the fact that kerosene will contaminate plumbing, requiring very thorough cleaning if it might ever touch oxidizer, while alcohol will completely evaporate away.

 

“Ignition!” reports that the heat flux to the chamber walls can be reduced 50% by mixing 1% silicone oil into the alcohol, because it acts as a continuously deposited ablative layer on the inside of the chamber and nozzle.

 

Hypergolic ignition could be experimented with by mixing 15% manganous acetate with the alcohol.

 

For hybrids, just about any old grain will work, with polyethylene and HTPB being the common choices.

 

The big problem with any higher performance motor is that cooling becomes a significant issue, especially since we are looking at 60+ second burn times for future vehicles.

 

While peroxide is a very good coolant, I have some reservations about doing regenerative cooling with it in a motor that will be doing restarts, because when the engine shuts down, I am pretty sure that heat soak will rapidly cook off the peroxide in the cooling tubes.  If the engine valve is before the cooling passages, that will probably cause the peroxide by the nozzle to blow the peroxide around the chamber into the catalyst pack, causing an unexpected burst from the engine.  If the engine valve is after the cooling passages, it would probably cause the entire tank to go off.  I would not want to consider dedicated purge gas plumbing.

 

Hybrids probably won’t need much in the way of chamber wall protection, as long as the grain isn’t burned completely away, and they should have some degree of automatic film cooling of the nozzle if the grain extends all the way down, so they should have things a bit easier.

 

There are four major materials directions for us to consider:

 

Graphite nozzle and phenolic chamber liner.  Your basic experimental rocketry combination.  Long burning motors with graphite nozzles have been reported to cause problems with heat conducting through the graphite to the casing or retaining hardware.

 

Cooled copper, either regenerative or simple water jacket.  If a simple water jacket with a reservoir worked without having boundary layer boiling problems, that might be a nice solution, but I don’t think we are up to making a proper regeneratively cooled engine.  Adding heat-sink style fins to the chamber and nozzle inside a large water jacket might be helpful

 

Silica-phenolic ablative nozzle / chamber as used in several noteworthy projects ( http://members.home.net/danmoser/ ), and have given lifetimes of over 240 seconds with hot burning lox / kerosene engines.

 

An Iridium / Rhenium nozzle / chamber from http://www.ultramet.com is probably very high dollar (I am waiting for a quote), but would be the best solution – it just wouldn’t care about the temperature, and there wouldn’t be a worry about wearing it out or having a burn-through.

 

 

I have been able to convince myself either way on the hybrid / biprop question in the past, but I think I have finally come down conclusively on the hybrid side for our upcoming phase of development.

 

In the long term, cooled or Ir/Re biprop will be the way to go, because it will have better testability characteristics, will be easier to refuel, and will have a small Isp advantage, but in the near term, I think an ablative hybrid solution wins due to simplicity and crash safety.  If a vehicle comes down hard and breaks some plumbing, having a lump of plastic covered in peroxide is just a hell of a lot better than having a pool of peroxide and alcohol.

 

 

New Vehicle Design

 

Flying people is annoying for a high performance vehicle design.

 

A standing pilot on top of 2’ diameter tanks would be good packaging, but could only tolerate a couple G’s of acceleration.

 

A supine pilot makes a very non axisymmetric shape.  You might be able to put them in a 2’ by 4’ capsule and use a pair of propellant tanks underneath them, but the airframe and nosecone would be strange shaped.

 

Putting them in a conventional circular cross section results in a much wider vehicle than you would really want for a single person.  Even a spherical tank of 4’diameter would hold a lot more propellant than you need for a 100 km ride for one person.

 

If you go with a 4’ diameter cabin, you might as well leave room to stack three passengers, because you are going to be biting the drag anyway, and your propellant tanks are going to be large enough.

 

Another option is to fill out the side volume beside the passengers by using two 12” to 16” diameter propellant tanks on either side of the seats.

 

If we go with a hybrid main engine, the chamber length may get quite long with a single port.   The normal approach is to go to a multi-port grain at that point, but that involves additional issues with a supporting web, and we may be able to use the extra length in a useful way.

 

We want to have passive aerodynamic stability on the vehicle.  Fins would be highly stressed, and a failure would be catastrophic.  A simple tubular airframe can be stable even without fins if the CG is far enough forward.  If half the vehicle length is the relatively thin hybrid chamber, but it is enclosed in the same diameter airframe as the cabin / peroxide tanks above it, the vehicle should be stable.  There would be lots of “wasted” volume in the bottom half of the vehicle, but the mass and drag would probably be at least as good as having four stout fins, and the construction would be much easier and more robust.

 

The attitude thrusters would be sideways firing at the bottom of the vehicle, and would have a nice long lever arm to act on.

 

The only downside is that it would probably not be reasonable to vertically land such a top-heavy vehicle, unless you deployed wide landing gear, and gear deployment scares me.

 

The next vehicle after the manned VTVL will probably be a demonstrator of this configuration.  We will take the propulsion system from the manned VTVL, enclose it in a 12” diameter by 10’ airframe and fly it to test sideways firing attitude thrusters, high mach flight conditions, and recovery options.  It should be quite a good performer even as a monoprop, with a mass ratio of 2 and reasonable aerodynamics.  If we add a hybrid grain, we may need to begin getting familiar with the big launch sites to fly it.