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Visible Mach Diamonds in IgniterV3 Test2 on 2014-10-23

Visible Mach Diamonds in IgniterV3 Test2 on 2014-10-23

Introduction

This is my first attempt at producing a 3D Printed igniter. I started working on this after running into problems testing my IgniterV2 in early September 2014. I am really happy with the way this turned out and plan on building more 3D printed igniters to refine the process.

Objective

The goal of this igniter is two-fold
  1. Build something that can be used in a variety of configurations to find the optimal placement of orifices and to help resolve the issues plaguing IgniterV2.
  2. Produce a design that can be freely shared among peers.

To elaborate... Partially as a result of being stuck with my IgniterV2 not working as I wanted it to, I decided that it may be a good time to work on a fresh design. I’ve been experimenting with 3D printing rocket engines for the last year but I had yet to properly test one.

Traditionally, when building an engine I spend a few weekends working in the FUBAR machine shop. This was time consuming and we don’t have the most fantastic equipment. Also, to be fair, I’m not the best machinist in the world either.

Building a 3D printed igniter allowed me to quickly design something and send it off to be printed. Meanwhile, I used the extra time to do some propellant flow testing and prep my test stand. While the cost is somewhat higher in dollar terms the extra time (in my view) I have from not fabricating this myself more than makes up the difference.

IgniterV3 CAD Drawing

IgniterV3 CAD Drawing


Coming from a software background I’m used to working with open-source software and I like the idea of sharing work among peers in order to make things better. Especially when there is essentially no commercial implication. While this approach is less accepted in the hardware world (sometimes for good reason) I felt that a 3D printed igniter would be candidate for a shared design.

The reason I believe this is because a source of ignition is a component of most liquid rocket engines. Igniters can also be difficult to build. Engine builders are more interested in building engines than worrying about complicated igniters. Creating an igniter that could be easily downloaded and printed means that my peers could save time working on the mundane tasks and instead focus their time on the more important process of building engines.

Design

The igniter was printed via the Select Laser Sintering (SLS) 3D printing process. I typically use either Shapeways or ExOnefor printing. They are both local companies for me (North East US). A view of the model can be found on Shapeways here here. As of the time of this writing (Nov 2014) the cost to print the igniter is $75.41, which I consider a fantastic deal given how much time goes into building a proper one.

The igniter is designed to run at 100psia. It has a 2.5” chamber length, a 0.398” diameter and an L* of 18.8”. The throat diameter is 0.1645”.

The throat divergence angle is only 5o. This allowed me to build a more efficient igniter and also make it so that the long divergence section can have thicker walls where the 7/16”-20 thread that mates the igniter with the injector is. This was a problem in my IgniterV2. Since it was much harder to build such a small angle on a lathe the thread walls were very thin and partially melted in one instance.

An Open Source STL file can be found on GitHub. This can be freely re-used, etc... Additionally contained on GitHub is an Excel file showing the design and calculations as well as a Rocket Propulsion Analysis (RPA) file showing the design parameters. There is some slight divergence between RPA and my Excel design most likely due to less accurate assumptions made in my analysis.

5 degree divergence angle

5 degree divergence angle


Performance Characteristics

  • Fuel Orifice Diameter: 0.014”
  • Fuel Mass Flow: 0.0024 kg/sec
  • Oxidizer Orifice Flow: 0.038”
  • Oxidizer Mass Flow: 0.0032 kg/sec
  • Total Mass Flow: 0.0056 kg/sec
  • Calculated Force: 2.5 lbf
  • Design Temp: 3026 Kelvin
  • Design Specific Impulse: 203 Isp

Ports

Port Configurations on IgniterV3, shown in both mounted and un-mounted configurations

Port Configurations on IgniterV3, shown in both mounted and un-mounted configurations


  • 2 Fuel Ports
  • 3 Oxidizer Ports
  • 2 Igniter Ports
  • 1 Pressure Port
  • 1 Flame Tube 7/16”-20 Thread Connector

This igniter is more of a development piece of hardware since it has more ports than are actually needed. There are a pair of fuel/oxidizer ports that directly impinge and another set where the oxidizer is slightly behind the fuel in order to push the fuel forward. There is one oxidizer port in the back of the igniter to test a perpendicular impingement. One of the igniter ports is close to the impingement point and the other is closer to the throat. Finally, there is one pressure port and 1 flame tube connector.

The purpose of these extra ports is to test various configurations to see which works best. This was partially borne out of the issues faced with IgniterV2 but I think it’s generally useful since I’m not aware of any one "correct" way to design the ports.

Flame Tube Design

The flame tube design is the same 7/16"-20 thread as IgniterV2. However, as a result of me using a lower (5o) divergent angle I was able to keep the walls at a much thicker ~0.11". In future designs I may try and move towards a flange type connection to the chamber since the thread tends to orient the igniter in awkward positions when mated with the injector.

Spark Igniter

The spark igniter is the same one I used in IgniterV2. This approach seems robust and mature to me. I plan on sticking with it for the foreseeable future.

Test Results

Test results are below. Each test includes a description on the left and an image linking to a youtube video on the right. Click on the image to launch a video in a new tab.

2014-10-23: (Test2) Indirect Impingement with Far Igniter

Still frame showing mach diamonds

Still frame showing mach diamonds

  • Test Results
    • Ox Orifice/Feed psia: 0.037”/245
    • Fuel Orifice/Feed psia: 0.012”/211
    • Average Chamber psia: 77
    • Pre-Calculated Flow (kg/s) & OF Ratio: 0.00434, 1.33:1
    • (Estimated) Actual Flow (kg/s) & OF Ratio: 0.00452, 1.26:1
    • Test Duration: 1500 ms
    • Test Video
  • Analysis
    • This test looked very good. The chamber pressure was about right given that flow rates are about 30% below theoretical design. Mach Diamonds are visibly present in the exhaust and it has a somewhat orange/red tint to it. Using the indirectly impinging orifices with the igniter towards the throat does not seem to have caused any issues.

2014-10-23: (Test3) Indirect Impingement with Near Igniter

Another still frame showing mach diamonds

Another still frame showing mach diamonds

  • Test Results
    • Ox Orifice/Feed psia: 0.037”/246
    • Fuel Orifice/Feed psia: 0.012”/210
    • Average Chamber psia: 79
    • Pre-Calculated Flow (kg/s) & OF Ratio: 0.00434, 1.33:1
    • (Estimated) Actual Flow (kg/s) & OF Ratio: 0.00451, 1.29:1
    • Test Duration: 1500 ms
    • Test Video
  • Analysis
    • Another very good tests. The only change between test2 and test3 is that the igniter was moved closer to the impingement location. However, results are almost identical to test2 regardless, perhaps suggesting that igniter position has little to do with performance.
    • One very small difference I noticed is that if you look very closely there are about 2 frames where the mixture is not combusted and exists as a fine mist. However, in test2 there is only one flame. Perhaps keeping the igniter closer causes the mixture not to be combusted as quickly. Note - this may not necessarily be a bad thing as the non-combusted mixture would in effect simulate film cooling and could possibly help with the wall temp. Further analysis is needed

2014-10-23: (Test4) Indirect Near Igniter with Higher Ox Flow

Inital frame during startup showing a beautiful blue color

Inital frame during startup showing a beautiful blue color

  • Test Results
    • Ox Orifice/Feed psia: 0.037”/291
    • Fuel Orifice/Feed psia: 0.012”/210
    • Average Chamber psia: 80
    • Pre-Calculated Flow (kg/s) & OF Ratio: 0.00483, 1.6:1
    • (Estimated) Actual Flow (kg/s) & OF Ratio: 0.00493, 1.53:1
    • Test Duration: 1500 ms
    • Test Video
  • Analysis
    • This was the same as test3 but with a 300psia ox flow rate instead of 250psia in test3. the mass flow and chamber pressures are slightly higher and the flame appears to be a tad bluer perhaps reflecting the slightly higher oxygen to fuel ratio. I don't notice any significant differences otherwise.
    • In this test we do not see the 2 frames of wet flow. This is likely due to the increased oxygen flow which does a better job of sustaining combustion.

2014-10-27: (Test7) Connected to Engine

Night test with the ingiter attached to the main engine

Night test with the ingiter attached to the main engine

  • Test Results
    • Ox Orifice/Feed psia: 0.037”/296
    • Fuel Orifice/Feed psia: 0.012”/212
    • Average Chamber psia: 79
    • Pre-Calculated Flow (kg/s) & OF Ratio: 0.00483, 1.6:1
    • (Estimated) Actual Flow (kg/s) & OF Ratio: 0.00500, 1.56:1
    • Test Duration: 1500 ms
    • Test Video
  • Analysis
    • This is the last "igniter only" test on 2014-10-27. In this test the igniter is connected to the main engine to see how it performs. Note - the main engine is not started. Unfortunately, it was starting to get a bit darker when I ran this test so we could not see any of the nice mach diamonds from the earlier test. There is a slight whistling sound, which I suspect is the hot gas flowing through the main engine nozzle. Aside, from these minor differences the engine performed almost identically to test4.

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