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What I have been doing (Novice endeavours with rockets)


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#1 Baldor

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Posted 04 March 2018 - 11:30 AM

A lot of time without posting, but I have been reading you every day.

 

After some rushed attempts last summer solstice, I decided to make some serious but leisurely progress.

 

 

This were some of the rockets I made last solstice.

 

First I needed was some real tooling, so:

 

Attached File  lathe.PNG   1.31MB   2 downloads

 

And of course, I had to learn to machine. All my experience hands on was a few hours learning to face a lot of years ago. All my experience was with CAM software, complex geometries but not challenging to machine.

 

So, this is my first attempt to a 15mm core burner tooling:

 

Attached File  15mm_Tooling_1.PNG   944.06KB   1 downloads

 

The spindle is stainless steel, all the other parts are aluminium. I must redo the spindle, I'm having troubles machining the draft correctly. Nozzle diameter is 7mm.

 

So, now that I had the tooling, I needed some way to test the motors and get meaningful data and not only rely on eyeballing the launches.

 

Attached File  stand1.PNG   1.04MB   1 downloads

Attached File  stand2.PNG   703.31KB   0 downloads

 

Almost all material used was scrap I had laying around. I only purchased the 40KG load cell, and the sparkfun openscale . I need to calibrate it more preciseley, but for now it´s enough to make comparisons. Error not seems to be more than 10g.

 

So, time to make some motor and test it.

 

Test A:

 

15mm core burner.

60/30/10 BP, milled, pine charcoal.

Total fuel (including delay): 31g

Total weight: 64g

Sample rate: 80SPS

 

 

Attached File  20180304-TestA.PNG   31.73KB   0 downloads

 

Attached File  20180304_TestA.txt   310bytes   3 downloads

 

 

 

 

 

 



#2 Baldor

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Posted 04 March 2018 - 12:49 PM

Now, a very similar motor with top fusing:

 

Test B:

 

15mm core burner.

60/30/10 BP, milled, pine charcoal.

Total fuel (including delay): 35g

Total weight: 67g

Sample rate: 80SPS

 

 

Attached File  20180304-TestB.PNG   34.5KB   0 downloads

 

Attached File  20180304_TestB.txt   554bytes   2 downloads

 

Seems something have gone wrong. Max force arround 3500g, more than double the burning time, and some stuttering around max.

 

I have a previous test, bottom fussed as the first, and the results are consistent with the test in the previous post. I must try some more top fused motors, and see if this was a dud.

 

¿Some ideas explaining this last test?

 

¿Some sugestions in general?

 

To Do:

 

-Load motors to 1/2ID above spindle only. No delay.

 

-Make spindle without coned nozzle.

 

-Try hotter BP.

 

 


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#3 pyrokid

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Posted 04 March 2018 - 03:43 PM

Very nice work! A lathe opens a world of possibilities!



#4 stix

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Posted 05 March 2018 - 02:57 AM

So first lets get this straight - when we are referring to "top fusing" we mean inserting the fuse all the way into the core toward the top of the motor (bulkhead) then igniting, and bottom fusing means igniting near the nozzle end?

 

I guess looking at the video it does look like that, with the tube on Test B directing the ignition into the core. The strange thing is, and I see your confusion Baldor, is that you would think the results would be the opposite. In that normally ignition further up the core would result in more initial pressure and therefore faster burn time. Ignition at the nozzle end means it may take a while for the flame to propagate up the core and therefore slower burning. Very puzzling.

 

The other thing to look at would be if the fuel density was the same, or perhaps there was nozzle erosion - do you make your nozzles consistent - did you measure the nozzle id after the tests? Both will affect pressure and therefore burn time.

 

For what it's worth, I checked the data and both motors had a "similar" total impulse of around 18-20 Newton Seconds. ie. Average thrust x burn time is the same as 1/2 average thrust x twice the burn time. Nevertheless, it's good to be consistent.

 

Also, although the delay segment was included in the fuel weight, my analysis shows that Test A produced a specific impulse of around 65 and Test B around 52, but we can already see that in posted graphs and data. An efficient black powder motor and fuel should produce around 80. But, like I said, if we subtract the delay segment, the specific impulse figures would be better.

 

Specific Impulse is the Total Impulse divided by the Fuel Weight. (Using the same units - in this case Newtons).

 

Sorry I can't offer any other useful info. I've not made that many BP motors, Sugar Rockets are my thing. Hopefully someone else can shed some light on the issue and give some advice for moving forward.

 

btw. Nice Lathe - wish I had one :)

 

Cheers.


Edited by stix, 05 March 2018 - 06:11 AM.

I just start the conversation - someone else has to question them.


#5 Baldor

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Posted 06 March 2018 - 01:29 AM

Yes, top fusing near the bulkhead, bottom near the nozzle.

 

Yesterday I made another test, this time with no delay. I'm still formatting the results, but total impulse is consistent, max force was around 6500g, and the start and end of the curve were almost vertical, like a top fused motor should look.

 

All the test have been done with the some batch of BP. Consolidated to 6000PSI. I don't see erosion in the first motor, but see some erosion or cruft in the other two. Must measure to decide what is. In my todo list is making some more spindles to compare geometries.

 

Specific impulse was 57... There is a lot of room for improvement. I will make two batches of BP, one 60/30/10 as the one I used, and one 75/15/10. I will test different combinations, and when I find one that doesn't cato, stick to it. It could be different proportion depending of spindle geometry.

 

Your input is very informative.  You are pointing me  were to look.

 

And the lathe.... These chinese lathes are not very expensive, but need continuous maintenance and can only make light cuts. Search for CJ18A for example. Problem could be shipping to Australia.



#6 Baldor

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Posted 06 March 2018 - 04:07 PM

I repeated the top fusing test, with better results.

 

 

Attached File  20180304-TestC.PNG   20.95KB   0 downloads

 

 

15mm core burner.

60/30/10 BP, milled, pine charcoal.

Total fuel (including delay): 32g

Total weight: 60g

Sample rate: 80SPS

 

Total impulse: 18.2Ns

Specific impulse: 57.7s

 

Attached File  2018030C_TestC.txt   424bytes   3 downloads

 

The curve seems more normal for top fusing. Am I right?

 

The bump at 0.075s seems to be the moment the paper for the fuse is expelled. I found the paper tube intact after the test.

 

Also, after the end of thrust, it doesn't go to zero. It drops to 3.4N, and keeps droping slowly for a time. Seems I used a little to much BP over the end of the spindle, and this "delay" is showing as a little thrust.

 

One think I don't like of the test stand is the rubber support for the engine. In the video you can see some movement. In the next test it will be replace by a HDPE sleeve 1/3 the length of the motor. The sleeves are easy to make, and I can make an specific one for any tube size.

 

For now I will keep the spindle geometry the same, and try to dial the BP. Tomorrow I will make a 200g batch of 60/30/10, pre-drying the KNO3 and the charcoal. Last batch made a clump in the jar. I will make also a 200g batch of 75/15/10, and try different mixes of both to find how hot BP I can use without catos (Tests will be performed out of the stand, of course)

 

And a question: I can get vineyard charcoal instead of pine, but its 4x the price. Should I see a big diference?

 


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#7 OldMarine

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Posted 06 March 2018 - 07:25 PM

Vineyard coal got a very bad rating from Danny Cregan's tests.What other woods do you have available? (Don't know your location)


Come on! Name one other hobby in which you cheer as your money and hard work go up in smoke!

#8 stix

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Posted 07 March 2018 - 02:07 AM

That last Test C is pretty good. I agree that there must have been too much bp over the end of the spindle, which caused the last third of the thrust curve to have not much thrust. Although, it still did have around 300grams but heading down.

 

So we know you are using a 15mmID nozzle throat. What is the ID of the tube casing, also the length of the spindle etc.?


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#9 Baldor

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Posted 07 March 2018 - 07:23 AM

Nope... 7mm nozzle, 15mm ID casing. :-)

 

The nozzle is conical, about 40º (20º per side), from 7mm to 12mm.



#10 Baldor

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Posted 07 March 2018 - 07:50 AM

OldMarine:

 

I'm in Spain.  What I can get is wood for the fireplace, but are usually "hard" woods. Holm oak, olive.. The softer ones are pine and vineyard. Maybe I can get my hands on some common beech (Fagus sylvatica) but I don't know if will be an improvement over pine. For now, I'm buying this http://www.granvelad...l-en-polvo.html

 

#11 stix

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Posted 07 March 2018 - 07:52 AM

Oh, Sorry. All this time I thought from your previous posts that you are working with a 15mm ID nozzle... Ok, thanks.... hmmmmm...

 

Then given a reasonable known "standard", that is, nozzle diameter is preferable to be 1/3 of the diameter of the internal casing ID, one could put forward that 7mm does not create enough pressure. Alternatively 5mm would give more pressure.

 

7mm = lower pressure and perhaps part of your current problem. Sure, you can increase the fuel burn rate via a hotter mix - but I think smaller nozzle, shorter core = more efficient.

 

Others may disagree.

 

btw. I wouldn't call yourself a "novice". You've shown careful planning in what you have done. Not many people bother to do that.


Edited by stix, 07 March 2018 - 08:00 AM.

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#12 Mumbles

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Posted 07 March 2018 - 08:21 AM

Vine charcoal in Europe has always been sort of a different beast that vine charcoal in the US. It might be worth a shot. 


Just so you guys quit asking, here is the link to the old forum. http://www.xsorbit2....forum/index.cgi

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#13 Baldor

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Posted 07 March 2018 - 09:16 AM

Stix, I had the assumption tha 1/2 ID was the standard for BP core burners. The rocket tool sketcher ( https://www.amateurp...cher-by-pudidk/ ) asks for 7.5mm nozzle. If I had troubles making a 7mm spindle, making a 5mm on will be a nightmare. Must give it a try before I finish this round of tests.

 

Also, spindle lengt is 7.5xID. Spindle length should not affect pressure inside the chamber. Both chamber volume and chamber surface increases linealy with the spindle length. Idealy, a longer spindle will mean more gas volume at the same pressure. If my intuition isn't wrong, this will mean a higher effective pressure in the nozzle.

 

About the planning... This comes with the profession. I have been a mould designer for more than 20 years, the last five the technical office chief. Fortunately, now I'm in a more relaxed job. :-)

 

Thanks Mumbles. I will try with vine and compare. I don't feel like making a TLUD now, maybe in the future.



#14 Maserface

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Posted 07 March 2018 - 09:20 AM

For what its worth, standard "skyrocket" dimensions give a nozzle diameter that is 1/2 x the ID.



#15 stix

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Posted 07 March 2018 - 03:32 PM

For what its worth, standard "skyrocket" dimensions give a nozzle diameter that is 1/2 x the ID.

 

My apologies, yes, that's probably correct. That's why I should shut up when it comes to bp rockets :blush:

And that's why I can't have nice things :)


Edited by stix, 07 March 2018 - 04:22 PM.

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#16 Maserface

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Posted 07 March 2018 - 07:36 PM

Stix. I didn't say it was the most efficient or powerful core geometry, just that it's common. You are absolutely correct that it may be more effective to modify the core geometry than the propellant.
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#17 Baldor

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Posted 08 March 2018 - 01:21 AM

Maserface, I plan to play with both propellant and geometry. I enjoy working with the late, and the small spindles are something I have to work out.  But the BP I'm using is very tame, 60/30/10 with commercial pine charcoal, and it's easy to modify. My idea is to have a base of data with the current geometry and progressively hotter BP, then start comparing different geometries against this data.

 

Next test should be a couple of motors with 75/15/10 BP, outside of the stand, of course, and see if they CATO. It one of them fails, I will mix 50/50 the tame BP with the hotter one, This will give me a 67.5/22.5/10. Rinse and repeat until I find an adequate composition for this tubes and geometry. Of course, if the motors doesn´t fail with the hottest BP, this means  I have a lot of work to do with my BP.

 

Stix, you don't need to shut up. Even mistakes are informative, and as Maserface said, the standard doesn't mean the best, so yours wasn't a mistake.


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#18 Arthur

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Posted 08 March 2018 - 03:19 PM

European vine charcoal is regarded as very hot, comparable to balsa.



#19 stix

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Posted 10 March 2018 - 08:13 AM

One thing to consider when testing, especially with smaller amounts of fuel, is what I call "Fuel Loss Compensation".

 

This only applies to vertical test stands.

 

As the fuel burns, it is consumed and therefore should be taken into consideration, that is, the load cell reading will be incorrect as the fuel is consumed.

 

No big deal in the end, unless you want to do "accurate analysis" of your recordings - perhaps worth thinking about though.


Edited by stix, 10 March 2018 - 08:19 AM.

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#20 Baldor

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Posted 10 March 2018 - 10:32 AM

I thought about it, Stix, but 30g against a peak thrust of 5Kg don't seems much. Assuming a constant fuel burning rate (It's not constant, burning surface increases if perfectly lit), should be easy to subtract it from the results.






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