Monday, February 29, 2016

SteamFlash Rocket Motor 10 - Preventing Burnout - the answer is in the hair dryer

A challenge, see video below, is a risk of "burnout". Normally the energy needed to change water to steam keeps the coil at 100 deg C or a little above. If we run out of water or lose water then the coil glows and can melt nearby plastic, or worse.  I searched for fuse wire with the idea of making the coil out of a low melting point wire so it would act as its own fuse. This search found something better, the "thermal fuse". This sends current through a little bead of low-melting point metal. On temperature rise the bead melts switching the current off. This is used for a similar safety purpose in hair dryers and hair straighteners. Looks great - I am thinking of embedding this in the wall of a boiler made of epoxy resin - or in a cutout in a polypropylene tube reinforced with epoxy resin and carbon fibre.  Example ref:

Friday, February 19, 2016

SteamFlash Rocket Motor 09 - Second thoughts - I can be less like Elon Musk

I was going to keep Rocket Motor Number One as a historical treasure until I met ex student Dave who tells me he likes this blog and hurry up please to give us all Test Number 2 (This is Dave 2.0 - the number of interested people has grown to the point where there are 2 Daves)  The quickest way to Test 02 is to modify the motor from Test 01. Ordering a few goodies online - like a pressure gauge and digital hanger scales for only about 5 dollars each which is a pleasant surprise. I checked out pumps but small electric pumps only go up to about 100psi and I calculate that tension springs or rubber bands acting on a syringe plunger can do better as well as being simpler, lighter and easier. Aiming for a static test of a more complete design with thrust and pressure measurements.

Thursday, February 18, 2016

SteamFlash Rocket Motor 08 - I can be like Elon Musk!

After the SpaceX historic recovery of a rocket booster for re-use, Elon Musk announced that he would not re-use it because it was of historic significance and should be saved for display like in a museum.
I am therefore inspired to save the rocket motor of Test Number One for museum posterity in case it turns out to be the start of something wonderful - or even the start of something.

Wednesday, February 10, 2016

SteamFlash Rocket Motor 07 - Video long version

Video of the first test run.  Full version 8 min 34 sec includes setup and tech details.
(Short version in the previous post below is about 3min if you only want to see the motor running.)

SteamFlash Rocket Motor 06 - Video of First Test Run

Short Edit (about 3 min) of video of the first test run.
See previous post (05) for notes.

Tuesday, February 9, 2016

SteamFlash Rocket Motor 05 - First Test Run

First test run 7pm, Sunday, 07 Feb 2016 NZDT (8am, 07 Feb 2016 GMT).

Run at about 100W (peak 5.9 V across 0.36 Ohm).
This is the Flash Boiler only loaded with 6 ml of water for a short run.
The plan was to run for 20 sec then measure how much water had steamed away.  In theory this should be 1ml leaving 5ml.

The boiler ran strongly for 18 sec, giving a good proof of concept as a rocket motor.
At 18 sec the pressure opened the spring loaded o-ring joint at the bottom of the unit and we lost the water. The coil started to overheat and I cut off the power.

Starting setup - showing setup outdoors and remote from the control table.

This is me doing my introduction speech to my invited audience of neighbours.

Close-up of boiler running.  I held it together with spring loaded clamps so that excessive pressure would open the O-Ring seal. High pressure did happen and this "safety valve" did activate and do its job - see bottom of 2nd photo below.

End of run.

It was excellent value to have 2 x videos of this run. Between them we can see what happened and learn a lot about taking the next step. It is critical to keep the coil wet and the above photo shows what happens if it goes dry. Any flying model will need excellent methods for guarding against this.  This is the next big challenge - some early ideas are:

  • We may already have a safety method in that the coil is similar to a fuse. We need a test to destruction of the coil to see if it does a rapid burnout like a fuse.  Changing to a lower melting point wire may help.
  • Theory says that the resistance increases when the wire gets hot.  We can see on the videos that the voltage suddenly rises from 5.90V to 6.06V when this happens. Detecting that could be a safety method.
  • Thermistor = temperature sensor
  • Sensor to detect pressure drop
  • Limited run time timer with battery cutout
  • Change to heat resistant materials for the boiler - eg ceramics
Possibly a combination of most or all of the above.

The plastic and epoxy glue handled the 100 deg C temperature and steam pressure well.
Part of the syringe plastic body did melt a hole at the moment shown above with the glowing coil but that is to be expected in this not-by-design overheating moment.
Video currently editing - coming soon.

Lots of discussion happening here on next steps. A big question is the size of the nozzle hole and the nozzle shape. For this run we simply went with the hole that happens to come with the syringe body. 100W gives a surprisingly powerful result and looks like it may be near the upper limit we can use with these materials. We may be able to run with a smaller, lighter and cheaper LiPo battery than originally planned. 

SteamFlash Rocket Motor 04 - What is my (low) resistance?

Getting close to a first test run.
We need to know the resistance of the heater coil.

Simple following of the theory -  R = V divided by I  - gives a circuit like this which will not work for measuring a low resistance. The high current will give heating effects to the point of burnout for either the coil or the battery or even both.

Instead I used a standard component resistor that is large enough to get the current down to a moderate level, but still enough current to create a voltage drop across the coil that my meter can accurately measure.  I do not have 2 meters - the one electronic multimeter takes turns measuring the 2 resistances.

39 Ohms still lets through enough current (0.3A) to get warm so I run it in a water bath.
Of course you should know that as a general rule we do not mix electricity and water. This is however reasonable with a low voltage battery circuit and clean water.

For future measurements, I plan to use a higher resistance like 390 Ohm. That would give a very low current (0.03A) with no significant heating effect and no need for a water bath. This also gives a very small coil voltage drop like 0.01 V but what I have learned now from doing this is that the modern digital multimeters of today do a good job of measuring very small voltage values.

Photo here shows what the circuit really looks like.
Running here with the multimeter connected to the 39 Ohm resistor.

The 39 Ohm resistor has a voltage drop of 11.29 V

The coil has a voltage drop of 0.103 V

The calculation by proportion gives a value of 0.36 Ohm for the coil.
I am surprised at how much higher that is than the 0.23 Ohm I was aiming at - although I was adding  some extra length to "err on the safe side".  But works out well with my aim to start with low power test runs.

Question: Why is the coil not running underwater?  Isn't it supposed to be a heating coil?

Answer: In this circuit, the 39 Ohm resistor acts as a power hog, taking nearly all the power so there is very little power and therefore heat dissipation happening with the coil.

The coil will get its turn to get the power in our next episode.

Monday, February 8, 2016

SteamFlash Rocket Motor 03 - Materials and Construction

Focus on a prototype flash boiler to run at low power.
The working temperature is 100 degrees C or a little above.
A later version may need to be made out of metal and ceramics.
However I want to make the prototype out of transparent plastic so I can video what happens inside for early low power trials.
Human observers will remote-operate from a distance!

The disposable syringe body sold by chemist shops for 2 dollars is a good size and shape.
Info online suggests without being definite that syringe bodies are made of heat resistant plastic.

Online articles also suggest that "epoxy resins" have good heat resistance.
I will try holding the coil in place with the common and easy to work with "Araldite 5 minute" epoxy glue.

Experiment done. Placing a syringe and a glued object in boiling water then checking strength.
Both seem to stay at full strength or nearly full strength.

Such a plastic device could fail after a short time but it will fail by splitting rather than shattering.

Coil setup with connectors from electrical connector strips.

Tighten the screws on the connector inner ends then cut the heads off to enable fitting into the syringe tube.

Starting on the base-plate to hold the coil and block the end of the syringe. Material is circuit board which is epoxy resin with a copper backing.  Mmmmm - possible problem with the copper?

Scrape the copper away from one of the terminals to prevent it causing a short circuit.

Trial assembly

Trial assembly

Sealing the coil in place on the backplate with epoxy glue.
The wire springiness in the coil is pulling unevenly on the connectors. I need to use a scrap of wire to  lift up the near connector.

SteamFlash Rocket Motor 02 - Power and Resistance

A SteamFlash Boiler needs an electrical heater.
This is a coil of nichrome wire.

How long should it be?  MATH!

Candidate model aircraft “LiPo” battery
14.8 V
– voltage drop under load?
- estimate 14V

1.5 Ampere-Hour
"45C" rating which advises max continuous current of
45 x 1.5 = 67.5
Let’s say 60 to allow some contingency and be realistic.

P = VI
Power (in Watts) = Voltage X Current
P = 14 x 60
P = 840 W

For this to happen, coil resistance is:
R = V divided by I
R = 14 divided by 60
R = 0.23 Ohm

Nichome wire diameter 0.5mm information supplied states 4.5 Ohm per metre.

Therefore length needed is:

L = 1 x 0.23 divided by 4.5
L = 0.051 m
L = 5cm

Let us say 6cm for connection at ends - also for the first prototype good to err on the higher resistance side.

I am thinking about the need to transfer energy from the wire to the water. I am guessing based on life experience with similar setups that this needs more wire surface area to be in contact with the water. We can get this by the idea of "2 resistors in parallel".  eg 2 x higher resistances in parallel give a lower resistor.  Each higher resistor is 12cm long, the 60A current is split to a more moderate 30A each and the total wire length is now 24cm.

Coil beginning.  A centre "tap" of wire twisted together runs from halfway along the coil to the base. Also running down the centre - careful that they don't touch! is the top end of the coil going back to join the bottom end.

SteamFlash Rocket Motor 01 - Only a little less than nuclear

My "maker" attention was well captured by the SpaceX booster retrieve success of 21 Dec 2015. Thinks, can we on a smaller scale do exciting tech by putting ideas together in interesting ways? And what more practical activities can we come up with to get school students excited about tech? Needs a name - "Project HiTechFromLoTech".

That train of thought quickly led to the amazing Peter Beck.
Peter made a breakthrough by observing that batteries and electric motors have improved recently so it is now possible to simplify and advance liquid fuel rockets with electric fuel pumps.

I started gathering info about those amazingly powerful hobby – model – aircraft etc LiPo batteries.
Thinks .. can we go further than a support system - can a powerful battery directly propel the rocket? Some calculations suggest that it MAY be possible to run a rocket directly from a LiPo battery via an Electric Flash Boiler.

This is NOT like the fearsome “Skycycle X2” flown by Evel Knievel where the water was kept in a high-pressure heated container.
More effective (and safer) here to carry cold (or possibly warm) water which feeds into a small “flash” unit heating a small amount of water to steam at any one time.

As of today, Sat, 06 Feb 2016, I have made a trial nichrome wire heating coil and I am close to the first water boiling experiment. Watch this space! Or should that be - "Watch this .. Space!".

References and Links:

Zuppero, A. Schnitzler, G. Larson, T. (1998). Nuclear-Heated Steam Rocket Using Lunar Ice.

SpaceX (2016) "The Falcon Has Landed" | Recap of Falcon 9 launch and landing.

Wikipedia (2014). Steam Rocket

Youtube (2013). Snake River Canyon jump 1973-EVEL KNIEVEL Awesome video!!