Pressure Differential Question

[TheBru]

I had a Saturday evening thought and I just can't find the correct words to google to get the equation I need to answer this, so I turn to the forum for help.

My thought is:

If you had a sealed compartment of air (think closed sealed coupler) with an actuated release valve that is closed at sea level / the launch site - would it build up enough pressure to be used as the "ejection charge" for a vented laundry bay using shear pins at +10km, +25km...?

What if you add a bike tire stem and pressurize to 100psi (or some other value) before launch?

Effectively, is it possible to use the pressure differential between sea level and a given altitude to allow a sealed vessel to be used as an "ejection charge" negating the need for BP?

 

[David Schwantz]

Don't think that would have enough volume. But CO2 cartridges do. I have seen them online here but never in person.

 

[heada]

CO2 cartridges can run in the 2000psi range. PCP airguns run in the 3500 psi to 4500 psi range. They have all the equipment to charge a composite gas cylinder up to those ranges and have the valves that can trigger the release.

A 400cc cylinder filled to 4000psi would be able to deploy pretty much any hobby rocket at any altitude. Servo could be setup to trip the valve so that it is 100% pyro free.

 

[OzHybrid]

So.. Sea level atmospheric pressure is 14 ish PSI. If that were held in a suitably sealed container with shear pins and launched, at some point it should be able to break the pins. Say a 2 inch rocket area =Pi*2*2/4. If your rocket was in a complete vacuum now and rounding numbers you have Pi*14 you get 44 pounds of force available to break your pins. So in theory, it's doable.
In practice, however, you're not going to end up at a total vacuum altitude.
Shear pins break better when the load on them is applied rapid-ish. Black powder does that. CO2 cylinders do that.
This method would be difficult to implement and if you did not reach your target altitude, you wouldn't get an ejection as the air pressure wouldn't have gone low enough.

 

[cerving]

Sea level air pressure is about 14.7 psi, so if you can figure out the difference in pressure and the area of whatever would separate (presumably the nose cone), it MAY be possible to have it separate solely due to air pressure. It would have to be TOTALLY sealed... of which very few rockets are. The slightest leak will compromise such a system.

 

[OverTheTop]

We see this happen sometimes in practice, typically with a fast-burning motor to a decent altitude. If the compartment isn't vented with a small hole it can separate during ascent. It is a bit unpredictable due to variable amounts of sealing on the airframe. The cure is a small vent hole in each deployment compartment, or shear pins.

If you are looking at CO2 ejection systems check out the Raptor CO2 system. The uni team I mentor used that at Spaceport America Cup recently.

 

[cbrarick]

Ahhh, Pascal's Law https://en.wikipedia.org/wiki/Pascal's_law

However, the above posts are correct. you need a vent, there's no simple way to make airframes completely airtight.

AND why are you trying this - reinvent this https://shop.fruitychutes.com/products/peregrine-co2-system-kit-8-12g
but at much lower pressures? The problem would be, could you get a solenoid operated valve that would let enough volume out all at once to cause a rapid spike of pressure in your leaky airframe? How much volume would you need? (think ideal gas laws here)

 

[OverTheTop]

However, the above posts are correct. you need a vent, there's no simple way to make airframes completely airtight.

You don't need a vent, but if you don't then you need shear pins to resist the differential pressure during ascent.

 

[Reinhard]

I had a Saturday evening thought and I just can't find the correct words to google to get the equation I need to answer this, so I turn to the forum for help.

My thought is:

If you had a sealed compartment of air (think closed sealed coupler) with an actuated release valve that is closed at sea level / the launch site - would it build up enough pressure to be used as the "ejection charge" for a vented laundry bay using shear pins at +10km, +25km...?

What if you add a bike tire stem and pressurize to 100psi (or some other value) before launch?

Effectively, is it possible to use the pressure differential between sea level and a given altitude to allow a sealed vessel to be used as an "ejection charge" negating the need for BP?

The words you're searching for are: "adiabatic expansion". You can use various calculators found online, like this one. (This one assumes both volumes stay separated by a piston, resulting in different temperatures, but this can be neglected here).

If you only start with ambient pressure, you will find that you need a rather large reservoir, relative to your parachute compartment (and pressures will still be a bit lower than recommended). But in principle it can work.

Reinhard

 

[TheBru]

Thank you all for your thoughts and help. For me, this was mostly a thought exercise as I mentally toy with various ideas to remove the number of consumables used on my flights. I have seen CO2 systems before but they used carts that were punctured and not re-usable.

PCP airguns run in the 3500 psi to 4500 psi range.

This is a great thought! I had not considered an airgun. Could be an interesting set of tests.

Ahhh, Pascal's Law https://en.wikipedia.org/wiki/Pascal's_law

However, the above posts are correct. you need a vent, there's no simple way to make airframes completely airtight.

AND why are you trying this - reinvent this https://shop.fruitychutes.com/products/peregrine-co2-system-kit-8-12g
but at much lower pressures? The problem would be, could you get a solenoid operated valve that would let enough volume out all at once to cause a rapid spike of pressure in your leaky airframe? How much volume would you need? (think ideal gas laws here)

I think the main difference in my thought vs CO2 is that I am contemplating ejection methods that don't require consumables. Either you need to have BP on hand or CO2 carts, how do we get to a space where flying DD is like motor eject? Just show up with a motor and fly. And while I must admit that Pascal is a fascinating rabbit hole, I do not believe it's the right equation for this instance. I think you and Reinhard might have gotten it right with ideal gas law / adiabatic expansion. I could be wrong Fail fast and learn....

The words you're searching for are: "adiabatic expansion". You can use various calculators found online, like this one. (This one assumes both volumes stay separated by a piston, resulting in different temperatures, but this can be neglected here).

If you only start with ambient pressure, you will find that you need a rather large reservoir, relative to your parachute compartment (and pressures will still be a bit lower than recommended). But in principle it can work.

Reinhard

This feels like the rabbit hole I was looking for and I shouldn't be surprised that it leads back to thermodynamics... I knew I should've taken that one when I had the chance but I just find fluid dynamics more applicable day to day and chose that instead. Live and learn.

 

[rocket_troy]

This feels like the rabbit hole I was looking for and I shouldn't be surprised that it leads back to thermodynamics... I knew I should've taken that one when I had the chance but I just find fluid dynamics more applicable day to day and chose that instead. Live and learn.

You might be able to avoid the thermodynamics stuff *if* there's enough *gas* volume ie. the volume available to your energy source (atmospheric gas) not stuff like chutes/pistons/shock cord etc.
If there's enough gas within the system, then you can tentatively simplify things by just comparing the inside-outside pressure differential to whatever deployment pressure you're aiming for from the various BP calculators; because the pressure drop from initial deployment to the couplers clearing each other should be manageable.
I would highly recommend you still pressurize your volume beyond 1 atm even if that's only 1/2 a bar above and have a means of detecting leakage on the pad. The additional energy will also be useful for extra margin.

TP

 

[rocket_troy]

I think the main difference in my thought vs CO2 is that I am contemplating ejection methods that don't require consumables. Either you need to have BP on hand or CO2 carts, how do we get to a space where flying DD is like motor eject? Just show up with a motor and fly. And while I must admit that Pascal is a fascinating rabbit hole, I do not believe it's the right equation for this instance. I think you and Reinhard might have gotten it right with ideal gas law / adiabatic expansion. I could be wrong Fail fast and learn....

I've deployed many times from compressed air only using one of those paint ball high pressure hand/foot pumps - purely for the novelty of avoiding consumables. I've now resorted back to CO2 purely for the convenience of carrying my tiny CO2 filling manifold vs the more cumbersome HP hand/foot pump.

TP

 

[cbrarick]

it was a neat idea but I was concerned about the practicallity of it. Plus I had visions of a schrader valve (or presta) sticking out the side of a rocket.
who knows, someone will take the idea and run with it, someday.

 

[Steve Shannon]

Thank you all for your thoughts and help. For me, this was mostly a thought exercise as I mentally toy with various ideas to remove the number of consumables used on my flights. I have seen CO2 systems before but they used carts that were punctured and not re-usable.


This is a great thought! I had not considered an airgun. Could be an interesting set of tests.


I think the main difference in my thought vs CO2 is that I am contemplating ejection methods that don't require consumables. Either you need to have BP on hand or CO2 carts, how do we get to a space where flying DD is like motor eject? Just show up with a motor and fly. And while I must admit that Pascal is a fascinating rabbit hole, I do not believe it's the right equation for this instance. I think you and Reinhard might have gotten it right with ideal gas law / adiabatic expansion. I could be wrong Fail fast and learn....


This feels like the rabbit hole I was looking for and I shouldn't be surprised that it leads back to thermodynamics... I knew I should've taken that one when I had the chance but I just find fluid dynamics more applicable day to day and chose that instead. Live and learn.

There have been folks who have designed and used spring powered systems for deployment, triggered by either a solenoid or a servo.

The simple fact is that for all these schemes you carry all the mass to apogee and chemical storage of energy is still much more efficient than mechanical, pneumatic, or electric.

If you truly want the simplicity of motor eject, but the benefits of dual deployment you may have to invent a better mousetrap, such as some kind of a drop in ejection charge “cartridge” system with charges for apogee and main deployment and which plugs or screws into your choice of avionics . Of course someone still needs to load the charges, etc., but you could have several of these loaded up and ready to use. If you do a very good job maybe it would even become a standardized system.

 

[techrat]

A party balloon, inflated at sea level should pop on its own at 10,000 feet. Don't know if that's enough pressure to cause ejection, but someone smarter than me can probably do the math on that one.

 

[rocket_troy]

A party balloon, inflated at sea level should pop on its own at 10,000 feet. Don't know if that's enough pressure to cause ejection, but someone smarter than me can probably do the math on that one.

Well, at 10kft the pressure differential between sea level and that altitude should be in the ball park of 31600Pa or 4.58 Psi. If the internal diameter of the airframe is say... 100mm that translates to an area of 0.007854m^2 or 12.17in^2. Multiply out and you get about 248N (~25Kg) or 56lbs of force trying to separate.

TP

 

[TheBru]

There have been folks who have designed and used spring powered systems for deployment, triggered by either a solenoid or a servo.

The simple fact is that for all these schemes you carry all the mass to apogee and chemical storage of energy is still much more efficient than mechanical, pneumatic, or electric.

Spring powered is the actual route I’ve been iterating on for the last year and it’s nice to hear others have done it instead of just assuming and not searching. Attempting to design without influence to see where I get before getting stuck ( or succeeding )

Re: Mass - you are correct that BP is going to be lighter but you also need a number of other things to accommodate the boom. I haven’t added it all up yet but I would assume the extra epoxy, thicker bulkheads/airframe, charge wells, battery, and others(?) have a not insignificant weight. There might be benefits as a system vs comparing the specific item storing energy. I’ll be sure to share the answer when and if I figure it out.

 

[rocket_troy]

Yes, pyros will always win on mass, but for many HPR rockets, mass is not super critical and a spring energised system can be actually done pretty efficiently (mass wise) with careful design. Also, carrying the mass all the way to apogee (once you've done all the work accelerating it) is generally *advantageous* for altitude maximisation for HPR rockets. Mass is your enemy for the accelerating/power phase, but inertia is your friend for the coasting phase of the flight.
What non pyro really buys you (other than the obvious safety issues) is *reusability*. No stuffing around with Ematches. You can test the same system, same components, same mechanism on the ground a hundred times without the need of expendable components and you shouldn't really need redundancy for that part of your recovery.
A well designed system can dramatically shorten and simplify the whole prep process. For our launches, my rocket is normally 1st on the pad for the day's event (and I fly hybrids!) simply because of that reason. My entire recovery prep takes 1 or 2 minutes and if I wanted to push it, I could easily do it under 30 sec.

TP