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Deployment charge pressure, volume and black powder

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RocketTree

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In this thread, I would like to discuss ejection charge pressures, and deployment area volume limits with disposable/reloadable motors. Unlike smaller model rockets, the recovery gear volume must be optimized for sufficient deployment pressure on larger diameter models - for example 4 inch dia.

Assuming we can not add more black powder, what are the limitations of a factory deployment charge in disposable or reloadable motors?

If a disposable Aerotech G motor has 0.7g black powder and we are building a 4 inch diameter rocket:

What is the maximum recovery gear volume to achieve 10-15psi?
3.9" (ID) x 7" = 16.25psi (83.6 cu in) 194lb force
3.9" (ID) x 8" = 15psi (95.6 cu in) 179lb force
3.9" (ID) x 9" = 13psi (107.5 cu in) 155lb force
3.9" (ID) x 10" = 11.5psi (119.4 cu in) 137lb force
3.9" (ID) x 11" = 10.5psi (131.4 cu in) 125lb force
3.9" (ID) x 12" = 9.5psi (143.3 cu in) 113lb force

With a suggested 10-15psi, it looks like our maximum volume can be determined.

We should also consider the volume of the motor mount tube itself.
Motor tube 1.1" x 18" = Volume 17.11 cu in
This would drop the pressure by an additional 1psi.

From here, we know the recovery bay should optimally be 7 inches (up to a maximum of 10 inches)

How can we increase pressure without adding more black powder or reducing recovery bay size (assuming we can't add BP)

- Seal-off factory nose cone (big hole in the bottom)? (but leave a small pinhole for equalizing)
- Add solid mass? (Large styrofoam ball, etc)
- More stuffer tube and plate?
- If recovery gear itself (parachute, nomex, cord, etc) adds internal mass, reduces volume - we can assume it will cause a higher pressure than previously calculated.

How much black powder in various motor types/brands?
- Do larger disposable motors have more BP? (eg. M1350, H180 ,I205....)
- How much BP charge in Aerotech RMS vs DMS
- How much BP charge in CTI and other brand motors?
- What is the BP equivalent of an Estes A - F black powder motor ejection charge?

Should a small diameter rocket use less than 0.7 BP than included from factory?

Minimum diameter rocket 29mm (1.1") @ 24" long with Aerotech G80-13T

0.2 grams BP = 15psi and 14 lb force on the nose cone base.

0.7 grams BP = 50psi and 57lb force on the nose cone base.

If the entire BP charge is not necessary, some of that BP could instead be used on larger rockets.

Do we take a chance and reduce the amount, or just use it all?

How relevant is the lb force on the nose cone in small vs large diameter rocket?

What is the maximum amount of BP you can SAFELY use in a DMS (disposable motor system) cartridge?

Any ideas or previous experience?

----------------------------------------------------------------------

Looking for your input.

Thanks
-RocketTree
 

shockie

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well there's basically 3 ways to increase gas pressure:

1. Increase the amount of gas. Since BP is a poor gas generator, (40% gas/60% solids) compared to single-base smokeless powder (single-base, ie nitrocellulose based smokeless powder have 6x the gas generation as compared to BP).

Both BP and NC produce pressure pulses. BP produces its gas much more rapidly resulting in a larger initial pressure pulse. Smokeless powder produces much more gas than BP but it produces it more gradually resulting in a lower pressure pulse. Perhaps there is some way to take advantage of this pressure pulse?

Is it possible to add a little NC based smokeless powder to an ejection charge and have the combination produce more gas, sort of like an additive? or synergy? way back in the late 30's early 40's Jack Parsons experimented with rockets that had a fuel made of BP + Smokeless powder.

PS When I talk about smokeless powder I mean single-base NC smokeless powder, not Pyrodex or Triple 7.

1 gram of black powder gives you 718(721) calories of heat, 270cc(258mL) of gas, and about 1/2(0.4) gram of residue.

2. Increase the Temperature of the Gas

I believe if you compress the gas, this will raise it's temperature

3. Decrease the Volume of the gas

Gases can be compressed. If the same amount of gas can be put into a smaller area, it will exert a higher pressure. Perhaps a "stuffer" tube will do this?

I will refer you to this:

https://www.rocketryforum.com/threads/black-powder-ejection-charge-calculating-c.107742/ see post #1 and #11
 
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afadeev

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How much black powder in various motor types/brands?
- Do larger disposable motors have more BP? (eg. M1350, H180 ,I205....)
- How much BP charge in Aerotech RMS vs DMS
- How much BP charge in CTI and other brand motors?
- What is the BP equivalent of an Estes A - F black powder motor ejection charge?
Yes, BP charge weights do vary with motor diameter, manufacturer, and impulse. And time of manufacturing (answers vary over time).
IIRC:
  • Most Aerotech 29mm and 38mm motors come with either ~1.5g or 2.0g of BP provided in a plastic vial to be poured into ejection well.
  • Most AT 24mm motors come with half that, or ~0.75g of BP.
  • Q-Jets have either 0.3g or 0.5g of BP, depending on time of manufacturing (early ones had more) and motor impulse (and/or something else).
  • Estes 18mm A/B/C have ~0.6g of BP.
  • Estes 24mm D has ~0.8g of BP.
  • Estes 29mm E/F have ~ 1.0g of BP.

I sampled a few (definitely not all) AT motors, but got the Estes #s from my notes.

If the entire BP charge is not necessary, some of that BP could instead be used on larger rockets.
If you need more BP, or custom BP amount for an electronically triggered deployment charge, you are in the market to buy quarts of BP. Sooner or later. Might as well do it now, and spare yourself the trouble of scavenging residual BP amounts from smaller rockets. Not worth it.

Should a small diameter rocket use less than 0.7 BP than included from factory?
How relevant is the lb force on the nose cone in small vs large diameter rocket?
Short answer - no, don't go below minimal BP amount included with the motor.

Long answer - it's not just airframe diameter that you need to take into account.
Way more details here:


What is the maximum amount of BP you can SAFELY use in a DMS (disposable motor system) cartridge?
The answer depends entirely on how much airframe volume you will be pressurizing, and how much force you will require to reliably dislodge the nose cone, eject the chute, and propel the nose cone with enough force to pull the chute way clear of the airframe.
The latter also depends on the presence of shear pins (which ones, and how many), and your particular technique of packaging and igniting the BP ejection charge. And the type of BP you are using.

More info here:

...and here:

If in doubt - ground test.
If not in doubt - please ground test anyway!

HTH,
a
 
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Nytrunner

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What is the maximum recovery gear volume to achieve 10-15psi?
3.9" (ID) x 7" = 16.25psi (83.6 cu in) 194lb force...
....
...

With a suggested 10-15psi, it looks like our maximum volume can be determined....
You're on the right track, but took a curve.

The ejection force resulting from that pressure is the important part. Especially when you start adding shear pins into the mix and need to ensure you'll develop enough force to break them and deploy properly
That 10-15 psi is variable. Since larger tubes have more area at the bulkhead, that may be too little for a smaller tube, or more than necessary for a larger tube.

How much black powder in various motor types/brands?
- Do larger disposable motors have more BP? (eg. M1350, H180 ,I205....)
- How much BP charge in Aerotech RMS vs DMS
- How much BP charge in CTI and other brand motors?
Both CTI and Aerotech publish their ejection charge amounts in their instructions and/or documentation for various motor sizes.
Someone gave an answer for Estes, but I haven't seen it written down officially

Should a small diameter rocket use less than 0.7 BP than included from factory?

Minimum diameter rocket 29mm (1.1") @ 24" long with Aerotech G80-13T
.......
If the entire BP charge is not necessary, some of that BP could instead be used on larger rockets.

Do we take a chance and reduce the amount, or just use it all?

How relevant is the lb force on the nose cone in small vs large diameter rocket?
Yes, sometimes you need to remove some of the included charge (and sometimes you need to add more if you have more bay volume and room in the charge well)
CAREFULLY calculate before doing this. Early Q-jets were a good example, the .5gram charges made some low power tubes pop and shockcords break. Too big a charge can stress te recovery harness, zipper your tube. On the infrequent occasion I have to remove some from a charge, I put it in a leftover charge vial and keep for altimeter ejection charges.

As mentioned above, the resulting force on the nose cone is the whole point.
 

Dotini

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Question regarding Estes ejection charge from A to D:

Does the ejection charge weight/impulse vary in some known and systematic way as we go up the line of Estes engines? Specifically, Is the ejection charge weight/impulse any different between a B motor and a C motor? Between an 18mm C motor and a 24mm C motor? Between a C11 motor and a D12 motor?
 

RocketTree

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Some additional info related to the original post. I recently weighed the BP ejection charge from a G74 Aerotech Economax motor and it was 0.68g. Maybe some margin of error with the scale... hard to say.


Question regarding Estes ejection charge from A to D:

Does the ejection charge weight/impulse vary in some known and systematic way as we go up the line of Estes engines? Specifically, Is the ejection charge weight/impulse any different between a B motor and a C motor? Between an 18mm C motor and a 24mm C motor? Between a C11 motor and a D12 motor?
I would also like to know this.
 

bronicabill

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Some additional info related to the original post. I recently weighed the BP ejection charge from a G74 Aerotech Economax motor and it was 0.68g. Maybe some margin of error with the scale... hard to say.
Well that just happens to be what I got when I weighed the BP charge in both Econojet F20W's and G64W reload kits! Scale was calibrated before weighing the powder, and at that light of a weight, I cannot imagine any discernable margin of error involved!
 

FredA

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BP is measured by volume, not by weight.
 
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