Shop space...

I spent the last weekend and most of Monday evening working in my Garage. Prior to this weekend all of the CNC tools were sitting in the middle of the room. I've now got them in their proper places.
The Lathe ..














And the Mill are in place...






















There is still much to do all the welding stuff is still stacked in the corner.




In addition to working on the shop I've been having a lively E-mail design debate with a number of people. At one point it spilled over on Arocket.
I really enjoy the arocket discussion group, it a valuable resource that sometimes drifts off course...

Todays arocket topic If you are an American living in china and have some Chinese shop machine rocket nozzles out of oak logs have you violated ITAR and will you go to jail? Don't laugh it is seriously debatable that oak would make a good cheap ablative nozzle for a short burn solid rocket.

The "go to jail" part is not debatable ITAR is deadly serious and is slowly choking the life out of American space technology. Many European space projects have specific rules saying you can't use American parts because they then impose ITAR on everything.

Thanksgiving and chambers.

For the truly obsessed a day off means spending the morning undisturbed with your CAD program and spread sheet. The virtual vehicle I presented last time has a lot of assumptions. One of my tasks is to verify the assumptions and reduce the uncertainty. One area of uncertainty is the weight of the thrust chambers and injectors. I spend the morning building a detailed cad model of my thrust chamber concept and using the CAD systems mass properties calculator to add up all the weights. Not counting the structure that ties the thrust chamber to the vehicle I get a fully accounted weight of about 5.3 lbs / 2.40 Kg This give me a chamber thrust to weight of better than 45. So my model virtual vehicle value of 30 seems reasonable. This weight is pessimistic as I have not figured out the details of the injector yet and used a 1 in (2.54 cm) thick copper plate as a astand in for the injector. The real world injector will almost certainly be lighter than this.


The basic chamber design is conceptually a copy of several different chambers that have been built over the years. It is a copper tube inside a slightly larger aluminum tube.The chamber is regen cooled with a graphite throat. I've been given drawings skectches and cad files from a number of people that have built similar motors. most of these motors have been larger, from 500 to 2000 lb thrust. Dues to scaling laws a smaller motor is harder to regen cool.
I hope to actually fire a small regen motor before the end of January.

A virtual vehicle

There are so many ideas and trade offs that it's hard to pin down a design. My current best guess for a vehicle is shown on the left. Its a lox ethanol vehicle with 4 250 lb thrust chambers. I'll try to describe it. The gray cylinders are aluminum irrigation tubing with hemisphere end caps. The red cylinders are the 4 motors. The green box is the payload and avionics. The blue lines are either spectra or kevlar lashings. Landing gear is not shown, but may be as simple as foam blocks under the tanks with additional shock absorption from give in the lashings. I like this design because I can test the tank and motor systems in a configuration identical to the final vehicle. I can even build up spare tank and motor assemblies. The other leading design candidate is a vehicle using 4 spherical tanks with four motors on the corners. In some ways a 50% scale pixel.

This is the first time I've actually drawn the vehicle in CAD. It has existed in many, many spreadsheets. The spreadsheet that goes with this vehicle can be found in PDF or XLS formats. The rest of this post won't make much sense unless you follow along on the spread sheet.

The top left of the spread sheet has a table of weights. I'm doing this both in Kg and pounds. To do any real physics type math MKS systems are better, but being an American I grew up with pounds and just can't kick the habit. The weights are listed along with the number of them on the vehicle. Several of the weight items are calculated. for instance the weight I assign to each thrust chamber is the peak motor thrust divided by 30. As the tanks have the most mass the tank calculations are more detailed. I use aluminum tubes where I can enter the length diameter and thickness, the spread sheet then calculates the weight and interior volume for a single tank. These numbers end up in the weight table. Finally I use the weight table and propellant mass. (labeled fuel in the spread sheet) to calculate the delta vee (DV) and number of hover seconds.

Some general notes:

• Things labeled fuel really mean propellant. I've used an average fuel and oxidizer combined density of 0.8 gm/cc (At 1 ATM this would actually be about .92)
  
• I only plan to fill the tanks to 90%.
• The design includes 2 carbon fiber wrapped cylinders and a regulator to provide presurant gas to the point where the tanks are 60% full and then it runs in blow down mode from there on.
• I've hidden a whole bunch of stuff in the "structure" item I hope to not need much structure beyond the tanks.
  
• I'm assuming that I use aluminum tanks for both LOX and Ethanol, if I get into weight trouble I can build composite Ethanol tanks and probably save 16 Kg
• I have a burst safety factor of 2.3 in the aluminum tanks. If I need more performance I can increase the tank and chamber pressures as the system can be pressurized remotely. The CF cylinders are DOT rated for 2200 psi, with a SF of better than 3 so I can probably get the extra pressureant I need from these. This would imply a remote presurant tank top off.
  
• As per my communications with the XPC rules people any AST required safety shutoff valves etc.. are counted against the 25 Kg payload.
• I used 25% of Cp for system and injector pressure drop.
• I used 80% of the Cpropep frozen sea level ISP for my target ISP.
  

Comments are welcome.

This weekend.

I indirectly worked on project related stuff all weekend. On Friday night My son and I drove out to the FAR site and camped out. The stars were amazing. All day On Saturday
we worked on the FAR large vertical test stand. My son Paul welded and I did whatever to help. There were 8 of us who worked most of the day. The test stand should be ready to pour in another two work days. The picture shows the outside of the forms, Its going to be u shaped and the flame bucket faces the picture. there is going to be a 30 foot steel tower on top. Its being built for a nominal capacity of 30Klbs but it could probably do more for short periods.

Sunday, now that my final inspection is done on the house, I spent the whole day working on organizing things that could not be done before final inspection.

A Personal Milestone.

On December 27th 2005 we started demolition for a major remodel of our house. We tore the interior down to bare studs. The plan was to finish by April 2006 , today November 16th 2006 we passed final inspection! This finally allows me to set up my shop properly as I can put up storage cabinets, move machines against the walls etc... I could do none of this until we passed inspection because the wall in question abuts part of the remodel and the inspector told me he needed to have access to the walls for final inspection! My wife is still painting walls and picking out flooring but the major part of my involvement is complete!

Not much work to report I spent the evening cleaning up the shop after the weekend. I do have an idea that sounds crazy.

Mylar is a type of polyester film. According to the article at Google Answers.

"Speaking with Dupont Technical Service this morning,
I found that PET films are virtually industructible
down to -250 degrees C.  That is to say, the biaxially
oriented films (such as Mylar, Melinex etc) do lot
lose physically properties significantly as
temperature is reduced. "

Polyester film is closely related to PET soda bottles. It can be readily glued with the proper glue (gorilla glue is one brand) The water rocket people have explored this area extensively.
A gentleman at the 2006 space access had done some experiments with LOX and plastic soda bottles. He found they worked well and remained flexible atLOX Temperatures.

One of the problems of building low temperature composite structures is that they get brittle, and then crack loosing their strength. They also suffer from dissimilar temperature coefficients between their carbon fiber structure and their, usually aluminum, liner.

What if your structure was just a bag inside a mesh support? This is much like a counter pressure space suit instead of a full space suit. One needs to create the mesh out of a fiber that is very light and stiff enough that the liner will yield into the fiber before it fails.

The strongest fiber currently known is spectra 2000, it has a yield of 3.2 Gpa and a young’s modulus of 124 GpaIts also lighter than water. For comparison Titanium has a modulus of 120 Gpa so spectra size for size is stiffer than titanium. It’s also size for size 2X as strong as steel. Spectra is slippery so it’s hard to glue into a matrix, but a woven sleeve of spectra over mylar might make an interesting tank. A high perf tank made of soda bottles and kite line.Yea that is crazy.

A weekends work.

I worked on this all weekend and I've machined the parts for 4 valves. I've fully assembled one and done some preliminary testing, so far it looks good. I promissed some pictures. The first picture on the is the Ryobi drill taken apart.





The next picture is the set of machined parts nevessary to turn this into a valve driver. The part on the left consists of a custom piece bolted to the remanats of the drill chuck that was turned down.



The next picture is an assembled valve.


The motor power wires are on the left, the small wires exiting the top go to the magnetic position sensor that sense a small magnet glued to the top of valve shaft. I've built for of these. When I get the electronics to operate them done I'll test one with LN2 and see how they work.

The finished valve weight about 1100 grams.
The machined parts are about 170 gm and the just the motor without any thing else is 350.
so there is some room to use a better (brushless ) motor and trim some weight.
The final valve could probably end up under 1Kg.