Mixed propulsion idea

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strud
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Mixed propulsion idea

Postby strud » Sat Dec 22, 2012 4:20 pm

I'm toying with an idea for an L3 project than is a large diameter rocket (say 10 to 12") which has a central M900 hybrid but is surrounded by a number of high thrust high pressure water rocket motors to act as boosters.

The Hybrid would be lit and when burn through occurs, the multiple water rocket motors would be 'started' to get the large vehicle up to a reasonable launch velocity which would then be sustained by the central hybrid motor.

Before I can go very far with design, I'll need some help from the water rocket guru (George) to come up with some rough estimates of thrust levels for a range of 'motors'.

Seeing that this was going to be a largish diameter vehicle compared with the motor diameter (64mm) the addition of some water rocket motors in the empty space around the motor tube won't add too much mass and will give a great initial kick.

Of course I have no idea if this will 'fly' with Tripoli as a cert flight..... I have a feeling not....

CS

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Re: Mixed propulsion idea

Postby air.command » Sat Dec 22, 2012 7:09 pm

Hi Strud,

That sounds like an interesting project! I'm happy to run some numbers through a simulator, but would need to know a few more details about the design. The main one will be how much the whole thing is going to weigh. Assuming that the body is 12 inches you could probably fit 5 water "motors" around the central hybrid. In the diagram below the central tube is 70mm and the motors are 110mm.

If the rocket weighs say 20Kg with hybrid loaded including nitrous (no water) and say the water rocket boosters are pressurised to 500psi. The water rocket motors are say 1m long giving each motor a capacity of around: 9.5L

You would want the nozzles to be about 20mm. I run the sim as a single motor that needs to lift 1/5th the total mass.

Bottle Volume 9500 cc
Diameter 110 mm
Water Fill 3300 cc
Launch Pressure 3446 Kpa (500 PSI, 34.01 Bar)
Nozzle diameter 20.0 mm
Nozzle viscous losses 0.16
Dry mass 4000.0 grams
Coefficient of drag 0.30

at 500psi

Results
Launch and thrust phase
Initial thrust 1916.6N
Initial burn acceleration 251.7 m/s2 (25.7G)
Average acceleration 360.5 m/s2 (36.8G)

Burnout
Burnout after 160 milliseconds
Burnout Velocity 57.6 m/s (207.5 kmh, 129.0mph)
Burnout Altitude 3.2 metres (10.4 feet)

Burnout acceleration 240.3 (24.5G)
Drag force at burnout 2.1 newton (0.2kgf)
Deceleration due to drag 0.1G
Speed increase due to air pulse 20.0m/s

Coast
Drag-free coast 169.4 metres to apogee at 172.6 metres after 6.0 seconds
Actual apogee at 161.10 metres (528.5 feet) after 5.78 seconds
Total flight time 11.57 seconds


At 300psi:

Results
Initial thrust 1152.3N
Initial burn acceleration 132.5 m/s2 (13.5G)
Average acceleration 143.8 m/s2 (14.7G)
Burnout
Burnout after 268 milliseconds
Burnout Velocity 38.6 m/s (138.9 kmh, 86.3mph)
Burnout Altitude 4.4 metres (14.5 feet)

Burnout acceleration 114.1 (11.6G)
Drag force at burnout 1.4 newton (0.1kgf)
Actual apogee at 77.79 metres (255.2 feet) after 4.13 seconds
Total flight time 8.13 seconds

I can run the sim again for various combinations if I know more about the design.

Some notes:
- What is the peak acceleration you would want on liftoff?
- Building 500psi capable pressure chambers at 110mm is not easy to keep lightweight enough, but do-able.
- Holding the rocket down at that pressure is going to be a good challenge. Each nozzle will exert around 1200N on the launcher which you have to hold down, add to that a bit of a safety margin and you are looking closer to 1500N per nozzle! Simultaneous release of all 5 is a must.
- There would be lots of high pressure water splashing back at the hybrid during launch. You would need to protect it.
- You could do 5 through the wall fins.

- George
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strud
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Re: Mixed propulsion idea

Postby strud » Sat Dec 22, 2012 7:37 pm

Hi George,

Thanks for the numbers !

Even the 300psi sim looks useful and the acceleration ample. The aim is to get a decent kick off the pad to ensure the vehicle is up to a good speed by the time it leaves the rail/tower.

I agree the hold down and release mechanism won't be simple with those forces.

This will have to be a non-cert flight, since the water motors won't be 'certified'.

The release timing can be reasonably simple and possibly triggered by burning through a polymer wire or similar when the hybrid begins to come to life.

CS

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Re: Mixed propulsion idea

Postby air.command » Sat Dec 22, 2012 8:29 pm

What would you expect the rocket to weigh without the boosters? 20Kg is probably a little on the heavy side, but you would expect about 1Kg of dry weight per booster. If the rocket is closer to 15Kg all up then you can use even lower pressures. You can adjust the acceleration profile by sizing the nozzles appropriately.

Timing is definitely going to be everything. You want the hybrid to start producing thrust when you let go of the boosters. Luckily there are no ignition delays with the boosters when you are ready to go. If the hybrid is also producing thrust at launch, then the boosters will be even more effective at getting the rocket up to speed.

- George
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Re: Mixed propulsion idea

Postby ROCKet STAR » Sat Dec 22, 2012 9:24 pm

Very interesting idea Craig, I'm keen to see how this one works out. Perhaps there would be a way of pressurising the water motors with gaseous nitrous oxide from the hybrid's vent?
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strud
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Re: Mixed propulsion idea

Postby strud » Sat Dec 22, 2012 9:50 pm

Yep using the 'waste' vent gas may be possible although one would need a pressure reg of sorts to limit the pressure and also ensure there is still enough differential to fill the hybrid.

Definitely something for fun/interest rather than max altitude :-)

CS

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Re: Mixed propulsion idea

Postby Halo » Thu Dec 27, 2012 7:41 pm

Hi CS

Mate why not one large booster like a paint ball tank or dive tank, tank holds air at stupid pressures attached to a single water tank so a two stage set up.

GL
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Re: Mixed propulsion idea

Postby air.command » Fri Dec 28, 2012 1:13 pm

Halo wrote:Hi CS

Mate why not one large booster like a paint ball tank or dive tank, tank holds air at stupid pressures attached to a single water tank so a two stage set up.

GL


One of the main problems I see with this approach is getting a high enough flow rate out of either the paintball tank or dive tank with standard tank valves. Your thrust phase will need to be under ~1 second. You would also need to have either a fairly strong water tank to hold the pressure during the initial phase when you open the tank valve, or use a high flow rate regulator.

Other than being quite heavy I think the dive tank may not work with Craig's configuration due to it's size. I think he was looking to fill the void around the hybrid motor.

Another safety aspect is having a fully pressurised tank on the rocket while you are handling it and setting it up on the pad. If the valve should suddenly open during setup, it may be an uncomfortable situation. It's always better to pressurise it remotely while the rocket is ready to go on the pad.

- George
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Re: Mixed propulsion idea

Postby strud » Sat Dec 29, 2012 10:48 am

Agreed, the main motivation for the multiple individual motors surrounding the central hybrid was to make use of the geometry ans use similar motors to what aircommand has done before.

With multiple water motors operating in parallel in this situation, could one fill them with water thru the nozzle but pressurise them from the top ?

The reason for asking is that if being filled with the waster N2O for pressurisation, a simple approach to pressure control might be to install a check valve on the fill line from the vent and then place a PRV set at the pressure limit you want the tanks to fill to after the check valve but before the lines split off to each tank.

One problem with this idea (there is likely many) is that the vent gas is going to be cold, cold enough to start chilling down and maybe freezing some of the water.
There may also be problems with the composite tank materials becoming a little unhappy at significantly reduced temps, however there will be a fair bit of water in there to act as a heat sink.

Of course this is sort of for nought if the volume of the waste is not enough to fill these motors, since then one would still need the compressed air tank to top up the system.

CS

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Re: Mixed propulsion idea

Postby Halo » Sat Dec 29, 2012 7:42 pm

Ok did not read Craig's original post correctly with wanting to use the void space, so I understand now why the multiple motor set up.
But reading your post won't you have issues with knowing the pressure of each tank prior to launch? With out telemetry from each tank.

What about large tank beside pad with fill line to each tank with a fill set up like the N20 fill rig then on launch you open valve it pressures the motors with non-return valves and each motor has a bust disc over the nozzles if all lines to motors are the same they should fill at same rate, that way motors could be filled with water prayer to launch, no need for air tank on air frame, no need for hold down set up, only issue would be cutting all air fill lines or you could use one fill line an a manifold set up for filling motors.

Or instead of bust discs one large cable tie in the center holding down air frame onto plugs in water motors then once the hybrid starts it burns through the cable tie and.

( oh I have put my cap guns away)


GL
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Re: Mixed propulsion idea

Postby air.command » Mon Dec 31, 2012 9:14 am

With multiple water rocket motors you want to ensure you have equal amounts of water in each and all are pressurised to the same pressure. You can fill each of the motors with water through the nozzle, but will need to isolate each of the motors from each other when doing this. This adds a little more complexity to the launcher, but is not difficult. Normally you use a little air pressure to push the water into the rocket from an external container. This will partially compress the air above the water and may be enough to offset any lack of volume from the vented N2O.

To equalize the pressures you need to have a manifold that joins the pressure chambers together. Either this manifold lives on the rocket or on the launcher. If the rocket is being pressurised with the vented N2O then it makes sense to put the manifold on the rocket. When we fly water rocket clusters we have a manifold on the launcher in order to keep the rocket weight to a minimum. Having the manifold on the launcher is a little more complicated because you need to have air fill tubes go up through the nozzle and go above the water level in the motor. This allows the pressures to equalize between the motors without transferring any of the water. (We learned this one the hard way - when we first started we just had a manifold on the nozzles and pressurised the whole thing, as the bubbles went up through the water inevitably one the motors would get slightly more pressure than another and the small pressure differential was enough to start pushing the water from one motor into the other. )

I think utilising the vented N2O definitely has some merit. One question though, if the N20 is increasing in pressure in the water rocket motors doesn't the back pressure make it harder to fill the N2O tank in the hybrid? I don't think the freezing issue in the water rocket motors would be too much of a problem because wouldn't the temperature of the N2O go up as it is being compressed inside the motors? We could potentially add some anti-freeze to the water to lower the freezing point.

I also wonder what would happen during the air phase of the water motor burn,... you have a burning hybrid, and all around it you are blowing high pressure oxidizer. You probably wouldn't want to have any of the rear of the rocket made from plastic. :) ... hmmm plastic water rocket nozzles would be cool if you could ignite them easily :shock:

Here is a crazy idea ... put the hybrid motor inside the pressure chamber and have just a single tank the diameter of the rocket. You could then perhaps have a number of water nozzles around the hybrid. The vented N2O from the hybrid would feed directly into the outer chamber.

- George
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Re: Mixed propulsion idea

Postby Stewart » Mon Dec 31, 2012 2:04 pm

George,
All other problems aside, I'ed be more worried about the N2O being ignited by the hybrid after the water was expelled, the Kn of a water rocket is way to high for hybrid N2O combustion, I envisage the water motors cato'ing
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Re: Mixed propulsion idea

Postby strud » Mon Dec 31, 2012 5:02 pm

After thinking about the physical layout and functional aspects of this a little more, I'm thinking that the water 'booster' should be simply that, a booster that runs for a short time before falling behind and detaching.

This booster is as large as discussed previously with thrust being transmitted on the inner diameter of the annulus made by the multiple motors.

Better do some drawings.


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