HYBRID ROCKET MOTORS
HPR HYBRID ROCKET HELP CLINIC - THE FAQ
- What type of valves do I need ?
- Where can I get these types of solenoid valves ?
- Where can I get these types of ball valves ?
- What types of threads, greases or seals are needed with Nitrous Oxide ?
- What are all these acronyms for fittings ? What does NPT, BSP etc mean ?
- What type of fittings do I need for my hybrid ground support ?
- What is the dump valve for and is it needed ?
- Where can I get Nitrous Oxide in the U.K. ?
- What type of tank do I need to store my Nitrous Oxide ?
- How do I get Nitrous Oxide from the tank to the rocket fill fitting ?
- Are there any rules and regulations for storing Nitrous Oxide ?
- Why is Nitrous Oxide used as the oxidiser in commercial HPR hybrid rocket motors?
- What is the boiling point of Nitrous Oxide ?
- What is the density of Nitrous Oxide ?
- What pressure is the Nitrous Oxide in the tanks held at ?
- How dangerous are hybrid rocket motors ?
- Is a pressure gauge neccesary on a Nitrous Oxide tank ?
- Does it take longer to prep a hybrid rocket than a solid rocket ?
- How does a hybrid motor differ from a solid motor during flight ?
- Does a rocket have to be built differently to accomodate a hybrid rocket motor ?
- How do I build a 2-stage hybrid rocket ?
HYBRID GROUND SUPPORT
HYBRID FLIGHT OPERATIONS
1. What type of valves do I need ?
The standard type of valves used with hybrid rocket fill systems are high pressure, Nitrous Oxide safe solenoid valves such as the solenoid valves manufactured by NOs (brand names are names such as Powershot). These solenoid valves require 12 volts to operate, and are available with 1/4 inch NPT and 1/2 inch NPT threaded connections. Normally, 1/4 inch NPT is the standard connection thread used with High Power and Amateur hybrid rockets, for no reason other than it seems to be the size people have standardised on.
Note that the solenoid valves used for hybrid rocket launch systems should be high pressure solenoid valves, since lower pressure valves will not be capable of handling the pressures involved with pressurised Nitrous Oxide. low pressure valves could stick, or not open, and in extreme cases, could rupture or blow apart.
It is also possible to use high pressure ball valves coupled to an electric motor (either directly, or through a gearing asembly). This is a lower cost alternative to using solenoid valves, but requires some careful design and mechanical construction. Additionally, there needs to be a means of reversing the motor, to ensure the ball valve can be both opened and closed.
2. Where can I get these types of solenoid valves ?
In the U.K. performance car or bike shops tend to sell the solenoid valves suitable for High Power and Amateur rockets, since they are designed for use with Nitrous Oxide injection systems for car and motorbike engines. In London, shops such as Customville in Goodmayes, Romford, sell the NOs valves.
3. Where can I get these types of ball valves ?
In the U.K. high pressure ball valves able to be used with hybrid rocket motor fill systems are available from suppliers such as Swagelok (through distributors such as North London Valve and Fitting Company), RS and Farnell. Note: Appropriate ball valves need to be high pressure ball valves, and have either PTFE, NBR or Nylon threads, or the ability to fit PTFE, NBR or Nylon threads.
4. What types of threads, greases or seals are needed ?
Appropriate threads are generally PTFE, NBR or Nylon. PTFE tape can be used for sealing. Krytox grease is the recommended Nitrous safe grease - it is not cheap, but it is safe. Petroleum Jelly (Vaseline) is not recommended as a grease.
5. What are all these acronyms for fittings ? What does NPT, BSP etc mean ?
- NPT stands for National Pipe Taper, and is the most common U.S. standard for pipe fittings. NPT fittings are measured on the internal diameter of the fitting.
- AN fittings were designed originally for the U.S. Military (The "A" standing for Army, and the "N" standing for Navy). AN fitting numbers refer to the outside diameter of fittings in 1/16 inch increments, thus an AN 4 fitting would have an external diameter of approximately 4/16", or 1/4", and an AN 6 fitting would have an external diameter of approximately 6/16" or 3/8". The approximately is important, since the AN external diameter is not a direct fit with an equivalent NPT thread, and the table below, should provide some guidance.
- Dash or - fittings are interchangeable names for AN fittings, thus a Dash 8 fitting would be the same as an AN 8 fitting.
- BSP stands for British Standard Pipe and is the U.K. standard for pipe fittings.
| NPT Size | Nearest AN Size | Nearest Dash (-) Size |
|---|---|---|
| 1/8" | 4 | 4 |
| 1/4" | 6 | 6 |
| 3/8" | 8 | 8 |
| 1/2" | 10 | 10 |
| 3/4" | 12 | 12 |
| 1" | 16 | 16 |
| 1 1/4" | 20 | 20 |
| 1 1/2" | 24 | 24 |
6. What type of fittings do I need for my hybrid ground support ?
The fittings used mainly in Hypertek and R.A.T.T.Works hybrid motor fill systems tend to be 1/4 inch NPT fittings for connections to the solenoid valves, with Dash-4 fill hose fittings.
7. What is the dump valve for and is it needed ?
A dump valve is used to rapidly release Nitrous Oxide from the hybrid motor's oxidiser tank in the event of a pre-launch abort. A dump valve is only really neccessary or desireable on hybrid motors without a vent line, and on hybrid motors of J-class total impulse and above. For smaller hybrid motors with vent lines, such as the R.A.T.T.Works H-70H and I-80H hybrids, the amount of Nitrous Oxide contained in the hybrid motor oxidiser tank is sufficiently small that it will be released through the vent line relatively quickly, and generally sufficiently quickly to negate the need for a dump valve.
If building a Nitrous Oxide oxidiser fill system, there is no requirement for the dump valve to be a solenoid valve either (or the fill valve too for that matter). A ball valve with a spring release system controlled remotely is quite adequate, as long as the ball valve is rated as a high pressure ball valve.
8. Where can I get Nitrous Oxide in the U.K. ?
As with the solenoid valves, the best source of Nitrous Oxide is likely to be performance car or bike shops. Again, since the Nitrous Oxide is used in engine injection systems, the Nitrous Oxide tends to be available from these types of shop.
In London, shops such as Customville in Goodmayes, Romford, supply Nitrous Oxide and Nitrous Oxide tanks. Uncle Bob's Rocket Shop is also able to source Nitrous Oxide for hybrid rocket motors.
For the micro-hybrid rocket motors, Nitrous Oxide sparklet bulbs are used. These are small pressurised cylinders containing Nitrous Oxide, and used in the catering industry for whipped cream.
NOTE: The larger suppliers of gases frequently claim that people are unable to get Nitrous Oxide "across the counter". This is wrong, and because of the frequently erroneous information supplied, it is probably better to approach the smaller suppliers, such as performance car and bike shops, from the start.
The following is a list of Nitrous Oxide sources in the U.K. with a web presence:
http://www.highpower.freeserve.co.uk/
9. What type of tank do I need to store my Nitrous Oxide ?
Nitrous Oxide is stored as a gas over liquid (750 psi at room temperature), and either commercially available tanks designed specifically for Nitrous Oxide, or high pressure tanks (such as a SCUBA diving tank or a CO2 fire extinguisher tank) are required for its storage. The tank has to be capable of withstanding the pressure of the Nitrous Oxide with a significant safety margin - hence the requirement for high pressure tanks. At room temperature, pressurised Nitrous Oxide is generally stored at around 54 bar pressure. A SCUBA diving tank is rated to approximately 300 bar pressure, and a CO2 tank is rated to approximately 350 bar pressure. In both cases, these are more than capable of holding Nitrous Oxide.
A Nitrous Oxide supplier will be able to advise on the most appropriate tank. A dip tube fitted to the tank is also often used, since this stops the need to tip the tank upside down to get the last bit of liquid Nitrous Oxide out of the tank.
10. How do I get Nitrous Oxide from the tank to the rocket fill fitting ?
The most common means of supplying the Nitrous Oxide to the rocket fill fitting is via a braided stainless steel hose. The actual pressure hose is enclosed within the braided stainless steel, so it can be used on its own, or with the braided stainless steel covering. To protect the pressure hose, it is preferable to get a pressure hose with stainless steel braiding, but if cost is an issue, the stainless steel braiding is not essential. Fittings at each end of the hose will depend on the fittings required for the Nitrous Oxide valves and tank fittings.
11. Are there any rules, regulations or restrictions for storing Nitrous Oxide ?
There are no restrictions on storing Nitrous Oxide in the U.K., as long as it is stored in a suitable high pressure tank.
In terms of rules and regulations, the only rules and regulations apply to the containers used to store the Nitrous Oxide, and these are Health and Safety Executive (HSE) rules and regulations for the storage of gases under high pressure in suitable containers. In the case of hybrid rocket motors, this applies to the tanks used to store and supply the Nitrous Oxide to the hybrid rocket motor. The manufacturers of the tanks used (whether Nitrous Oxide specific, SCUBA or CO2) will already have to have met the standards laid down by the HSE, not the end user.
12. Why is Nitrous Oxide used as the oxidiser in commercial HPR hybrid rocket motors?
Nitrous Oxide (N2O), also known as Dinitrogen Monoxide, NOx or Laughing Gas, is used as the oxidiser in commercial HPR hybrid rocket motors because it is probably the easiest oxidiser to handle, easiest Oxidiser to store and most readily available oxidiser. Liquid Oxygen is cryogenic, and requires another level of safety in terms of cleanliness of equipment, cryogenic capable flow lines, etc. Hydrogen Peroxide is far harder to obtain in useful concentrations, and is a more reactive oxidiser capable of causing some nasty damage to organic material such as people.
13. What is the boiling point of Nitrous Oxide ?
The boiling point of Nitrous Oxide is 97° Fahrenheit.
14. What is the density of Nitrous Oxide ?
The density of liquid Nitrous Oxide is 0.8 g/cm3 at 20° C, and rises to 1.22 g/cm3 by the time the temperature falls to -88° C.
15. What pressure is the Nitrous Oxide in the tanks held at ?
At room temperature, the pressure of the Nitrous Oxide in a pressurised oxidiser tank is around 750 psi or 54 bar. The pressure in the tank will increase in warmer temperatures, and the tank pressure will decrease in colder temperatures. The result of this, is that in warmer temperatures, a hybrid motor will burn for a shorter amount of time at a higher level of thrust, and in colder temperatures, a hybrid rocket motor will burn for a longer amount of time at a lower amount of thrust. The following table provides an approximate guideline:
| Vapour Pressure (psi) |
Temperature (° C) |
|---|---|
| 460 | 0 |
| 520 | 5 |
| 600 | 10 |
| 680 | 15 |
| 760 | 21 |
| 860 | 27 |
16. How dangerous are hybrid rocket motors ?
Hybrid rocket motors are very safe. Certainly far safer than solid rocket motors. There was a report conducted by the USAF that gave hybrid motors a TNT rating of zero. This basically means that they are non-explosive. Even with a mis-ignition where the oxidiser is released, but there is no oxidiser ignition, a hybrid rocket will fail safely, since the oxidiser will just blow out of the nozzle like any other pressurised gas that has been released. To really try and break a hybrid rocket motor, a nozzle blockage (caused by igniter grain fragments jamming in the nozzle throat) would be needed, since this would cause a pressure buildup, and an overpressure in the motor. Generally, in an instance such as this, the hybrid rocket motor will be designed to fail axially (as with a solid rocket motor), so that any catastrophic failure and resultant debris would be directed in one direction.
17. Is a pressure gauge neccesary on a Nitrous Oxide tank ?
Not strictly, no. A pressure gauge is a nice optional extra. In order to determine how much Nitrous Oxide is in a tank, the other way of measuring it (somewhat cheaper), is to weigh the tank when it is empty, then weigh it again when it has been filled with Nitrous Oxide. Each time Nitrous Oxide is used, then weigh the tank for an approximate measure of how much Nitrous Oxide has been used.
HYBRID FLIGHT OPERATIONS.
18. Does it take longer to prep a hybrid rocket than a solid rocket ?
Yes and no. It takes longer to set up the ground support equipment for a hybrid rocket motor, simply because there is no separate oxidiser involved with a solid rocket motor (a solid rocket motor contains both the fuel and the oxidiser bound into the solid propellant). If all the equipment is already available and is largely pre-assembled, then initial hybrid motor ground support setup can take as little as 5 minutes.
To prep a typical Hypertek or R.A.T.TWorks hybrid rocket motor takes less time than an equivalent solid rocket motor, simply because there is less to prep. However, hybrid rocket motors do not have ejection charges like solid rocket motors do, so time needs to be allowed to prep an ejection charge fired from an electronic recovery deployment system such as an altimeter, accelerometer, altimeter/accelerometer or timer.
So, to summarise, with practice, prepping a hybrid rocket motor can take less time than a solid rocket motor, but more often than not, it will take at least the same amount of time, if not more time, to prep a hybrid motor than a solid motor.
19. How does a hybrid motor differ from a solid motor during flight ?
As the oxidiser in the oxidiser tank is used up, there is a loss of mass that acts axially along the length of the rocket. This differs from a standard core burning solid rocket motor, where the mass loss occurs radially. The result of the axial mass loss with a hybrid rocket, is that the Centre of Gravity moves backward, causing the vehicle to become more unstable. This frequently results in weathercocking, or a tendency for the vehicle to fly into the wind. The more unstable the vehicle becomes, the more it weathercocks, and the more its trajectory becomes less vertical. This is why passive, fin stabilised hybrid rockets (i.e. most HPR and amateur rockets), tend to arc over, especially towards the end of their motor burn.
20. Does a rocket have to be built differently to accomodate a hybrid rocket motor ?
Yes. Generally, a rocket built to accomodate a hybrid rocket motor will need to have a longer motor section, sometimes 2-3 times the length of a comparable solid motor section.
Additionally, hybrid rocket motors tend to be lower thrust, longer burn motors than equivalent solid rocket motors. This neccessitates building rocket airframes lighter and with careful thought to payload and recovery system placement to account for the shift of mass during flight as the oxidiser in the oxidiser tank is used up.
21. How do I build a 2-stage hybrid rocket ?
Carefully.
If you are contemplating building a 2-stage hybrid rocket it is exceedingly advisable that you have plenty of flight experience with single stage hybrid vehicles, and have a good understanding of how they fly. If you have a low thrust to weight ratio on your proposed 2-stage hybrid rocket, you would be better advised not to even try it - unless you have an active guidance system.
N.B. Information presented on these web pages is for information purposes only. The author of these web pages does not take any responsibility for people harmed or injured, or for damage caused by hardware resulting from information contained within these web pages.
See Also
Hybrid Rocket Science, Hybrid Rocket Help Clinic, Amateur Hybrid Motors, Amateur Liquid Rockets, Guidance, Gimballed Motors, Launch Controller, UK Rocketry Vendors, UK Rocket Groups, UK Space Organisations
