Non-pyro system

[Hakan Demir]

[Hey guys, I am hakan and study at university as a mechanical engineer. Next year we are planning to attend HPR competition in Turkey. We have to do parachute system by non-pyro( which is called CO2 enjection charge ) we must not use any exploded system. That’s why we are going to use non-pyro. My question is that how can l assemble CO2 system in my rocket body ? And I want to design my system. Could you give me any advice for this ?
Thank you

 

[OverTheTop]

Hi Hakan. I don't personally do gas ejection but would suggest you just keep an open mind to the possibilities. CO2 is only one gas, there are many others that may be useful. Scuba air tanks can be a great source of pressurised gas, but be careful as the serious pressures can be quite dangerous. There are also mechanical release mechanisms that you may want to consider. So keep an open mind as it helps with the creativity!

So, welcome to the forum. Thanks for posting and I hope you get lots of answers here .

 

[DAllen]

Elsewhere someone mentioned using water bottles. Inflate them enough and it doesn't take much to rupture them. I'm talking like those 1/2 litre bottles (or whatever volume they are) with the super thin plastic. I bet an ematch taped to the side would do the trick.

 

[MClark]

Elsewhere someone mentioned using water bottles. Inflate them enough and it doesn't take much to rupture them. I'm talking like those 1/2 litre bottles (or whatever volume they are) with the super thin plastic. I bet an ematch taped to the side would do the trick.

Not water bottles, pop bottles.
Water bottles have the grooves and cannot be pressurized. Pop bottles (not coke brand, they ate also a bad shape) with cylindrical sides can be pressurized. Use caution.

 

[rocket_troy]

Getting back to the spring & line idea mentioned in other threads. I think it has real merit, so I’ve whacked together a very quick spreadsheet to perhaps assist in gathering an idea of which direction to head when looking for a suitable *compression* spring.

I’m assuming (regarding the compression spring) we’re looking for:

(1) Enough energy/force/work/impulse/momentum to achieve a reliable axial separation.
(2) The lightest possible solution

So, we’ll be needing a spring that needs to be long enough to ensure there’s plenty of “available compression” ie. the length of travel between the uncompressed “free length” to the fully compressed state. This ensures plenty of margin for things like laundry compression (which can absorb energy) and separation clearances etc.

We also need the “spring rate” to be high enough to ensure the spring is providing enough force as we want the spring to provide as much “work” as possible. Work = Force * distance. For compression springs, the “spring rate” is a measure of how much force the spring produces per distance compression, so if a compression spring had a spring rate of 10 N/mm, that means that the spring would produce a force of 10 newtons for 1mm compression. For 10mm compression, the force produced by the spring will be 100N and so on. As the spring releases (travels), the force acting on the separating pieces reduces respectively.

So, for the example just mentioned, if we were, say, pushing a 1Kg mass (using a=f/m) at 10mm compression the acceleration on the 1Kg mass would be 100N/1kg = 100m/s/s. At 5mm travel, the acceleration on the mass from the spring force would be 50m/s/s and so on. Between the lines, it also relates that the longer the compression, the more *time* the object is being accelerated so final velocity = v0+ a*t would be higher.

Now, intuition might initially suggest we need a large diameter spring with thick wire to provide a higher spring rate.
Intuition might also initially suggest we could utilise a lighter material like titanium to achieve higher spring rates from lower spring mass.

Let’s investigate.

Looking at the equation I used in the SS for spring rate
=((Wire_Diam^4)*Shear_Modulus)/(8*Active_Coils*Mean_Diam^3)


As we can see (as expected) the spring rate is significantly related to the wire diameter used . What’s also evident is that it’s also significantly related to the mean diameter of the spring but inversely related. What this means is that we can increase the spring rate by:

Increasing wire diameter
Increasing wire stiffness
Decreasing active coils (increasing coil pitch)
Decreasing mean diameter of spring.

The important point to note here (with spring weight in mind) is that we can both reduce the mean diameter of the spring and the wire diameter without necessarily costing us spring rate.

The other point mentioned above relates to the material selection – can I use a titanium spring to reduce spring weight?

Now, upon initial inspection in comparison with something like spring steel, we see that Titanium’s shear modulus is only ~54% that of spring steel whereas its specific gravity is ~58% that of spring steel which might indicate that Ti alloys offer no weight benefit. However, on closer inspection, to bring the Ti-alloy’s spring rate up to that of the spring steel spring (with everything else being equal) we need to increase the wire diameter and remember that spring rate is proportional to wire diameter^4 when the gain in cross-sectional area only rises to the square of the increase. This translates to potential non-trivial weight reductions for Ti-alloy based springs.

Other factors to consider:
Larger length:diameter ratio springs are more likely to bend (off axis) during compression and will likely require guidance/support to limit this undesirable likelihood.
Obviously titanium alloy springs are going to be significantly more expensive than spring steel springs and are unlikely to be worth the extra expense for this application.
Springs shouldn’t be over pitched – design the pitch (distance between each coil) to be less than 50% of the mean diameter. The SS uses 40% as a maximum (conservative).

Spreadsheet link:
www.propulsionlabs.com.au/spring_stuff/Spring_Estimator.xlsm

TP