just bought 100. We'll see if they arrive........
Hybrids 2023
- Thread starter DRAGON64
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They take all the fun out don't they?
OK Time to build a Hybrd. I just popped in to the forum to learn about CG shifts in Hybrid tanks and learned more in this thread then I have found anywhere else. Thanks for the discussion.
I will start a seperate build thread as I will love some help along the way. Designing my first Hybrid is a bit scary.
cfb_rolley
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Fingers crossed... if they do arrive, let me know because I'll order a pack myself for some club launches!
Nitrous bulbs order got cancelled and refunded. They cannot ship internationally...... @cfb_rolley @fanga
Ahhh bugga
Indeed.... I'd already planned how I was going to make an adapter to take 3 bulbs to increase power available.... Probably a good thing. I've got enough unfinished projects.
drewnickel
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Haven't checked the forum in several weeks so missed out on a bunch. Going back to the Cg shifting issue someone pointed out before, the way I deal with that is simulating motors with the hybrid simulator I wrote (which is fine for EX motors, but can also give some idea for commercial motors if you know enough about the motor). HRAP has the option of calculating center of mass location relative to the front of the motor, with the assumption being the motor hardware mass and cg doesn't change, the fuel grain cg remains at the center of the grain, the liquid is a cylinder with a diameter equal to the tank and volume equal to that of the liquid remaining in the tank, and the vapor is evenly distributed in the remaining space above the liquid. It also allows exporting of a .rse file, so that the weight and cg location can be accounted for when doing flight sims. OpenRocket and RockSim both allow the use of .rse files, and both apply the over-ridden cg location in simulations.
As a side note, I have finally started my big hybrid project. 98mm minimum diameter ~N3000, hoping to fly at BALLS in 2024 to ~60,000 feet. Basically a 6 foot long snap-ring hybrid motor with fins and a nosecone. ABS fuel, U/C valves with CNVAs, internal vent, and literally half the rocket's weight at liftoff will be propellant.
As a side note, I have finally started my big hybrid project. 98mm minimum diameter ~N3000, hoping to fly at BALLS in 2024 to ~60,000 feet. Basically a 6 foot long snap-ring hybrid motor with fins and a nosecone. ABS fuel, U/C valves with CNVAs, internal vent, and literally half the rocket's weight at liftoff will be propellant.
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drewnickel
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Current progress. I have the upper airframe and have purchased a Featherweight Blue Raven.
What's the light green bit next to nozzle? Small for secondary combustion. I assume from the grain profile, you're printing it in ABS
drewnickel
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Light green is a block of G3 fiberglass machined into a mixer chamber. Idea is there is a cruciform pattern that forces the oxidizer rich core flow into the fuel rich outer flow to get the same benefits of a post-combustion chamber in a smaller package. TU Delft in the Netherlands has used graphite mixers in the past on their Stratos rockets, and The University of Waterloo in Canada tried it out with G3 this past year (bumping specific impulse from ~140 seconds to around 190 seconds without any other significant changes).
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Any details? That's an impressive performance improvement in a small package......
Like your stretching length spring vent line.
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drewnickel
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I credit Tom (Contrail Rockets) for the internal vent design, it's simple and effective. As for the mixer device, I don't have a lot to reference as I only know of a handful of motors that have implemented them, but the TU Delft Design had three radial holes spaced out on a graphite plate with a diameter about the same as the nozzle throat. They were able to get to around 92% combustion efficiency with their sorbitol-aluminum-paraffin blend, which is pretty incredible as a typical U/C hybrid may get to around 70% and a higher end amateur motor with post combustion chamber may be 80-85% efficient.
drewnickel
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Mine will have a lot more open space and four ports so that I am less likely to snag a fill hose as they are ejected at ignition.
Seems like a good project for a small CNC. Which I have. I'll book in for Balls 2024. See you there. At least I know there'll be nitrous available...
I'll book in for Balls 2024. See you there. At least I know there'll be nitrous available...
Here's the link for the paper from Delft. pdf download
https://repository.tudelft.nl/islan...076-9833-e4e3e675b0ee/datastream/OBJ/download
If you go to the RattWorks site and look at the K240 injector tests, they managed an isp of 203 in test 4 without secondary combustion. ABS grain and 24-hole injector. I attached the file too. Save you looking.....
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What is the lowest pressure that you should fly a hybrid at? I have some old SkyRipper hybrids but my club can only fly in the winter here in North Alabama. Even though this week had 80 degree temperatures it is always barely above 60 degrees during the one time a month we have a launch.
drewnickel
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Short answer:
I wouldn't fly under like 600 psi.
For a more detailed answer:
Because the density of nitrous goes up as temperature/pressure goes down, tank pressure dropping doesn't have a huge impact on mass flow rate through the injector and so tank pressure drops faster than chamber pressure would. Below a certain pressure drop (difference between tank and chamber pressure), you start to get chugging instabilities that could lead to flash-back where the flame travels back into the nitrous tank. Reason for these instabilities is even in a well designed motor there will be pressure spikes from variations in the grain and from turbulence and other factors, and when you have a low pressure drop those same pressure spikes are a larger percentage of the pressure drop. If the pressure increases suddenly, you get a reduction in flow rate because the pressure drop decreases. Since the flow rate has decreased, the chamber pressure will drop which then increases pressure drop and flow rates again and that repeats over and over again. Its why on some hybrids you can hear a buzzing noise. With a low enough pressure drop, which is usually because the tank is too cold, those pressure spikes can be large enough to cause back flow into the tank which will almost certainly lead to a CATO.
drewnickel
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On the opposite end of the scale having tank pressures too high (over ~900 psi) can mean the tank is mostly filled with vapor, and so you will have more thrust because of the higher pressure but will have a lower total impulse because of having less nitrous on board.
You're assuming the over pressure to 900psi is because the Nitrous is too hot. There are multiple ways you could increase the pressure without increasing the temperature. Nitrogen tank for pressurisation as an example.
When you fill generally, there's a first quick pre fill which chills the tank followed by the fill.
I've launched in Australia on a very hot day. As long as you can keep your bottle cool and your rocket cool ( a wet towel around the rocket tank area and another around the bottle in a breeze is the easy way) there is no problem. But you need to pre plan for those high temp days. At the lower end, I've heated a bottle up in the shower on the morning of the flight and then kept it warm wrapped in dry towels on a cool day.
Controlling your fill temp is critical. However you manage that.
I’m not aware of the recommendations for SkyRipper motors, however Contrail recommends no less than 550 psi and no more than 900 psi. Typically I don’t use below 650 psi. In Winter my fill tank pressure can be around 500 psi on a frosty day. I generally leave it out in the sun to warm up for an hour or two prior to use. It usually gets to 650-700 psi.
I have considered using a 12V electric blanket, but the techniques mentioned by Norm are simple and easy solutions too. Even wrapping the tank with a towel and pouring warm water over it will work. I generally have hot water in a large thermos flask at the site.
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RainierWolfcastle
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I think assuming pressure being temperature related is pretty safe in this case because it was in response to "I have some old SkyRipper hybrids", and as far as I am aware none of those came with additional nitrogen pressure as part of their fill and fire instructions.
The Rocket flew great with the SRS H78. The outside temperature was about 60 degrees and the tank pressure was right at 700 psi. Now to fly the SRS 54mm motors that I have. The ABS fuel grain is 38 grams new and after the flight it weighs 25 grams So it used 13 grams of ABS.
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If the temperature at the various pressures had been mentioned... Nothing.
Assume nothing... Experimental.....? Don't mix up temperature and pressure.
drewnickel
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Most all amateur hybrids are self pressurizing and operating off just the vapor pressure of nitrous, so temperature/pressure are interchangeable for probably 90% of amateur hybrids (and much closer to 100% of those at TRA launches). All the hybrid motors that have ever been certified for hobby use through TMT or equivalent organizations have been self pressurizing.
drewnickel
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If you are just running off the vapor pressure, there is a one-to-one correlation between the temperature of the nitrous and the pressure in the tank (so long as it is still a saturated mixture of liquid and vapor). If you know pressure, you know the temperature and vice versa.
I wouldn't be letting my nitrous get to that temp. Even in Australia where it's hot. That graph has an end to it because at that point you cannot contain the bottle contents anymore. I don't think our newbies realise that....... Stay away from that end of the graph. Temperature wise.
It's really easy to put a wet towel around the bottle. There is no excuse for not doing so.
As for pressure....
You can overpressurise Nitrous with various gasses and this is done in the motor racing community regularly. It's safe and easy. Obviously, you need to ensure your hoses won't go pop. 1100 psi is about the limit. This can make a massive difference to the fill time and reduces chilling of the tank which also aids in maintaining your tank pressure and temperature. You do need to ensure the pressure you choose to use is compatible with your valves and rocket nitrous tank. No one wants to see any component fail.....
It's really easy to put a wet towel around the bottle. There is no excuse for not doing so.
As for pressure....
You can overpressurise Nitrous with various gasses and this is done in the motor racing community regularly. It's safe and easy. Obviously, you need to ensure your hoses won't go pop. 1100 psi is about the limit. This can make a massive difference to the fill time and reduces chilling of the tank which also aids in maintaining your tank pressure and temperature. You do need to ensure the pressure you choose to use is compatible with your valves and rocket nitrous tank. No one wants to see any component fail.....
drewnickel
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Regarding the end of the graph, that's the critical point of nitrous where the liquid and vapor densities are equal and so beyond that point it acts as both a liquid and a gas (it can dissolve things like a liquid and diffuse through materials like a gas, and can make flashback far more likely). That critical point happens when nitrous reaches ~36.5 C or 72.45 bar (97.7 F or 1051 psi in freedom units). For self-pressurizing hybrids the ideal pressure is around 50 bar / 725 psi, which happens around 20 C or 68 F. You end up with enough tank pressure for a reasonable chamber pressure and the liquid is still dense enough to not just waste space in the tank.
Definitely agree that supercharging is beneficial in a lot of ways. You can also avoid the headache of trying to estimate injector flow rates with two phase flow by just supercharging enough that the nitrous remains liquid going through the injectors. A lot of the European college teams in particular seem to utilize supercharging in their motors (TU Delft, Stuttgart, etc), usually filling normally and then supercharging to ~62 bar with nitrogen. That being said it's not very common in hobby rocketry.
The phase change from liquid to gas at the critical temperature means that the bottle you have can no longer contain that same mass of N2O as a gas that it did as a liquid. And the bottle pressure safety disk ruptures. So stay away from temperatures that cause spontaneous phase changes.
As the HHGTTG would say, "always keep your towel handy and some beer" although in this case, the beer would be to dampen the towel. Not to dampen your nerves.
As the HHGTTG would say, "always keep your towel handy and some beer" although in this case, the beer would be to dampen the towel. Not to dampen your nerves.
If anyone in OZ is looking for a bottle for Nitrous, came across this.
https://www.41pintsofbeer.com.au/product-page/co2-gas-cylinders-2-6kg-full
Au$119
You'd need to arrange pick up in Sydney. In Australia, CO2 bottles are the same as Australian Nitrous bottle from BOC, without the dip tube.
They also do 450gm bottles for sodastream which could make a J/ K motor-ish for $27 bucks Au.
https://www.41pintsofbeer.com.au/product-page/co2-gas-cylinders-2-6kg-full
Au$119
You'd need to arrange pick up in Sydney. In Australia, CO2 bottles are the same as Australian Nitrous bottle from BOC, without the dip tube.
They also do 450gm bottles for sodastream which could make a J/ K motor-ish for $27 bucks Au.
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I'd find repressurizing with high pressure oxygen a little more interesting than with nitrogen. The oxygen has solubility in nitrous oxide and will end up replacing the ullage gas with O2 rather than N2O, which is then much safer. At least, safer in a relative rocket specific sort of way! Of course there are some etceteras involved... I'd not be top pressurizing anyway, but instead bubble the O2 through forming a Nytrox blend. Nytrox is less hazardous than N2O in a couple different ways, but is going to have its own issues just like any oxidizer suitable for using in rocket motors or engines.
Gerald
Gerald
