Pressurant tanks, crude vehicle model

For the vehicle, I’m considering downsizing from 5 gallon propellant tanks to 3 gallon tanks and from 250 lbf thrust to 180 lbf. Low Rider Depot makes a 3 gallon version of the 5 gallon tanks I’ve had success with and I purchased one for burst testing. Assuming engine thrust = 180 lbf, Isp = 140 seconds, O/F = 1.1, and runtime is 20 seconds, I’ll need 1.865 gallons of ethanol and 1.414 gallons of LOX. I’ll need high pressure tanks to hold the pressurant and equations from Sutton Chapter 6 and H&H Chapter 5 yield similar answers for the required volume: 0.79 to 0.92 gallons of nitrogen at 2000 psi per propellant tank (pressurantSupply). Based on the dimensions I could find (OD = 4.3 inches, height = 16.5 inches), the volume of a D size medical oxygen cylinder is ~1 gallon, so I’ll need 1 of those cylinders per propellant tank. Here’s a crude version of the vehicle with the 3 gallon propellant tanks and the D size medical oxygen cylinders. The vehicle is roughly 50 inches (129 cm) tall.


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Tanks, Heatsink Engine

I purchased two more 5 gallon aluminum tanks from KMW Performance and proof tested them to 400 psi (27.6 bar). I’m presently looking for somebody to clean one for LOX service; the local welding supply stores could not recommend a vendor for cleaning parts for LOX service. These tanks are going on a test stand, but I plan to use the same ones on my vehicle.

I also started machining the 100 lbf heatsink engine. I decided drilling the chamber and converging portion of the nozzle would be easier than working it with a boring bar, though the surface finish on the chamber ID is a little rough. I bought a 2 1/8” drill bit from eBay, a local machine shop sharpened it for $35, and the sharpened bit made quick work of drilling the chamber. Huzel/Huang say the convergent half angle ranges from 20 degrees to 45 degrees – my drill bit gave me a 60 degree half angle. The next step is to step drill a conical diverging section (15 degree half angle) and clean up the profile with a boring bar. Richard Nakka’s website has a handy calculator for forming nozzles.


Drilling the chamber and converging section. Stock is 4″ OD 1018 carbon steel. 


Looking into the chamber and through the throat. 

IMG_1503I bought this tiny trailer from Harbor Freight and I’m going to turn it into a portable test stand. Every Harbor Freight purchase is hit or miss: the 3/16″ steel plate I bought for the top cost more than the trailer kit and the wheel bearings get disconcertingly hot after short trips. 

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Another aluminum tank burst test

This week, I burst this 5 gallon aluminum air tank. I like this tank because it comes with five NPT ports and two brackets welded to it.

It failed at 46.5 bar (674 psi) in the heat affected zone of the longitudinal weld, basically the same location as the aluminum Harbor Freight tank.


I bought a 1000 psi (69 bar) pressure transducer on eBay for $35. I recorded the pressure with an Arduino Uno and SD card shield. pressureplot5galburst

Dry mass = 3.2 kg (7.0 lbs).

OD = 8.196” (20.819 cm)

t = 0.098” (2.489 mm)

UTS = 27,512 psi (189.69 MPa)

According to the manufacturer, the tank material is 5086 – H116. From what I’ve read, the strength of the welded material is no less than the –O condition, but my UTS is less than all of the published 5086 UTS I’ve seen. I think the difference can be attributed to local thinning at the burst location.

I want to mount the tank using the brackets that come welded to it. I ran a quick analysis to see if one bracket is strong enough to hold a tank full of LOX (they appear sturdy). The weight of the tank and 5 gallons of LOX is 243 N (54.7 lbf). fea1 fea2Based on the UTS I calculated from the burst test, I have a factor of safety of 22.9 at the bracket/tank weld.

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Aluminum air tank burst test

I bought this hydrostatic pressure tester from Amazon and used it to burst a 7 gallon (26.5 L) aluminum air tank I bought on sale from Harbor Freight. I thought the aluminum air tank might make a good LOX and fuel tank.

I selected this hydrostatic tester because of the price and it has its own reservoir (I don’t always have access to a hose spigot). My only complaint is that it came with a flimsy and suspect pressure gage; I replaced the stock gage with a nice, liquid-filled one and I haven’t had any other issues. I used flexible aluminum tubing to connect the tester and vessel because it is rated to high pressure, it can be coiled for storage, and is *much* cheaper than hydraulic hose.

The tank burst at 580 psi (40 bar) in the HAZ of the longitudinal weld. I was careful about getting all of the air out of the line and tank; the actual “burst” was rather anticlimactic. The tank diameter is 10.05” (25.53 cm) and the thickness is 0.095” (2.42 mm); this gives an ultimate strength of 30,679 psi (211.5 MPa). IMG_0117 IMG_0118

Amazon has a few 3, 5, and 7 gallon aluminum air tanks with NPT ports welded to them and the vendor advertises a 600 psi burst pressure.

The hydrostatic pressure tester is also handy for calibrating pressure transducers. Here it is calibrating a 250 psi transducer I bought on eBay:IMG_0122

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100 lbf LOX/alcohol heat sink engine

I designed a 100 lbf (445 N) LOX/ethanol engine, similar to Robert Watzlavick’s heat sink kerolox engine.

The analysis is nothing more than the 1D isentropic compressible flow equations and thermodynamic data from RPA: 100 lbf engine

Propellants: 70%ethanol and liquid oxygen
Thrust: 445 N (100 lbf)
Mixture Ratio (O/F): 1.1
Specific Impulse: 221 s
Chamber Pressure: 1.379 MPa (200 psi)
Exit Diameter: 3.03 cm (1.192 inches)
Throat Diameter: 1.76 cm (0.691 inches)
L*: 119 cm (46.9 inches)
Oxidizer Orifice Diameter: 0.762 mm (0.030 inches)
Fuel Orifice Diameter: 0.787 mm (0.031 inches)

I used the free version of Rocket Propulsion Analysis to get the thermodynamic properties of the LOX/ethanol combustion products (temperature, specific heat ratio, and density) as functions of mixture ratio, chamber pressure, and alcohol concentration. RPA has a very intuitive user interface and the free version is surprisingly versatile and powerful. Version 2 includes film and regenerative cooling analysis. The student version is $200, but there’s a 15 day free trial.


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LatLong to XY

I have been using the UTM code from WHOI on an Uno to convert GPS latitude/longitude to UTM easting/northing. On an Uno, double and float are the same, so the easting and northing values are slightly different from the website. On a Due, double is 64 bits and the Arduino and website values are the same. latlon2xy is the doc with the Arduino code. Since I last looked at GPS, Mikal Hart has an updated GPS library that computes distanceBetween() and courseTo(). It might be worth pursuing.

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Modest helicopter control progress

I bought an Arduino Due to use as the flight computer and a LIDAR-Lite from PulsedLight 3D to use as an altimeter.  Preliminary experiments with the LIDAR look promising; it’s certainly much better than the barometer/accelerometer complementary filter, but it’s a little pricey (for a hobbyist sensor) at $90.

Converting latitude and longitude to Cartesian coordinates is not trivial and I’ve been giving myself a crash course in geodesy and the Universal Transverse Mercator coordinate system. My controller uses Cartesian coordinates (makes sense since the NGLLC profile is up 50 m, over 100 m, and down 50 m), so I’ll use the UTM transformation to take latitude and longitude measurements from my GPS and turn them into (x,y).

Also, I’ve been (slowly) porting TJ Bordelon’s FreeSpace IMU algorithm into an Arduino library.

TODO (this week):

  1. Finish porting the FreeSpace IMU to Arduino.
  2. 3D print replacement landing gear for helicopter.
  3. 3D print special landing gear with mount for the Due, GPS, IMU, and LIDAR.

TODO (next week):

Helicopter system ID: Need to relate ESC command signal to thrust and servo command signal to “gimbal” angle.

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