Hybrid-propellant rocket

(${\displaystyle I_{sp}}$) performance of hybrids is generally higher than solid motors and lower than liquid engines. ${\displaystyle I_{sp}}$ as high as 400 s has been measured in a hybrid rocket using metalized fuels.

History

The first work on hybrid rockets was performed in the early 1930s at the

In 1953 Pacific Rocket Society (est. 1943) was developing the XDF-23, a 4 inches (10 cm) × 72 inches (180 cm) hybrid rocket, designed by Jim Nuding, using LOX and rubber polymer called "Thiokol". They had already tried other fuels in prior iterations including cotton, paraffin wax and wood. The XDF name itself comes from "experimental Douglas fir" from one of the first units.^[6]

In the 1960s, European organizations also began work on hybrid rockets.

In its simplest form, a hybrid rocket consists of a pressure vessel (tank) containing the liquid oxidizer, the combustion chamber containing the solid propellant, and a mechanical device separating the two. When thrust is desired, a suitable ignition source is introduced in the combustion chamber and the valve is opened. The liquid oxidiser (or gas) flows into the combustion chamber where it is vaporized and then reacted with the solid propellant. Combustion occurs in a boundary layer diffusion flame adjacent to the surface of the solid propellant.

Generally, the liquid propellant is the

Combustion

The governing equation for hybrid rocket combustion shows that the

where ${\displaystyle {\dot {r}}}$ is the regression rate, a_o is the regression rate coefficient (incorporating the grain length), G_o is the oxidizer mass flux rate, and n is the regression rate exponent.[5]

As the motor burns, the increase in diameter of the fuel port results in an increased fuel mass flow rate. This phenomenon makes the oxidizer to fuel ratio (O/F) shift during the burn. The increased fuel mass flow rate can be compensated for by also increasing the oxidizer mass flow rate. In addition to the O/F varying as a function of time, it also varies based on the position down the fuel grain. The closer the position is to the top of the fuel grain, the higher the O/F ratio. Since the O/F varies down the port, a point called the

• Mechanically simpler – requires only a single liquid propellant resulting in less plumbing, fewer valves, and simpler operations.
• Denser fuel – fuels in the solid phase generally have higher density than those in the liquid phase, reducing overall system volume.
• Metal additives – reactive metals such as aluminium, magnesium, lithium or beryllium can be easily included in the fuel grain increasing specific impulse (${\displaystyle I_{sp}}$), density, or both.
• Combustion instabilities – Hybrid rockets do not typically exhibit high frequency combustion instabilities that plague liquid rockets due to the solid fuel grain breaking up acoustic waves that would otherwise reflect in an open liquid engine combustion chamber.
• Propellant pressurization – One of the most difficult to design portions of a liquid rocket system are the
  turbopumps. Turbopump design is complex as it has to precisely and efficiently pump and keep separated two fluids of different properties in precise ratios at very high volumetric flow rates, often cryogenic temperatures, and highly volatile chemicals while combusting those same fluids in order to power itself. Hybrids have far less fluid to move and can often be pressurized by a blow-down system (which would be prohibitively heavy in a liquid rocket) or self-pressurized oxidizers (such as N₂O
  ).
• Cooling – Liquid rockets often depend on one of the propellants, typically the fuel, to cool the combustion chamber and nozzle due to the very high heat fluxes and vulnerability of the metal walls to oxidation and stress cracking. Hybrid rockets have combustion chambers that are lined with the solid propellant which shields it from the product gases. Their nozzles are often graphite or coated in ablative materials similarly to solid rocket motors. The design, construction, and testing of liquid cooling flows is complex, making the system more prone to failure.

Advantages compared with solid rockets

• Higher theoretical ${\displaystyle I_{sp}}$ – Possible due to limits of known solid oxidizers compared to often used liquid oxidizers.
• Less explosion hazard – Propellant grain is more tolerant of processing errors such as cracks since the burn rate is dependent on oxidizer mass flux rate. Propellant grain cannot be ignited by stray electrical charge and is very insensitive to auto-igniting due to heat. Hybrid rocket motors can be transported to the launch site with the oxidizer and fuel stored separately, improving safety.
• Fewer handling and storage issues – Ingredients in solid rockets are often incompatible chemically and thermally. Repeated changes in temperature can cause distortion of the grain. Antioxidants and coatings are used to keep the grain from breaking down or decomposing.
• More controllable – Stop/restart and throttling are all easily incorporated into most designs. Solid rockets rarely can be shut down easily and almost never have throttling or restart capabilities.

Disadvantages of hybrid rockets

Hybrid rockets also exhibit some disadvantages when compared with liquid and solid rockets. These include:

• Oxidizer-to-fuel ratio shift ("O/F shift") – with a constant oxidizer flow-rate, the ratio of fuel production rate to oxidizer flow rate will change as a grain regresses. This leads to off-peak operation from a chemical performance point of view. However, for a well-designed hybrid, O/F shift has a very small impact on performance because ${\displaystyle I_{sp}}$ is insensitive to O/F shift near the peak.
• Poor
  regression characteristics often drive multi-port fuel grains. Multi-port fuel grains have poor volumetric efficiency and, often, structural deficiencies. High regression rate liquefying fuels developed in the late 1990s offer a potential solution to this problem.^[12]
• Compared with liquid-based propulsion, re-fueling a partially or totally depleted hybrid rocket would present significant challenges, as the solid propellant cannot simply be pumped into a fuel tank. This may or may not be an issue, depending upon how the rocket is planned to be used.

In general, much less development work has been completed with hybrids than liquids or solids and it is likely that some of these disadvantages could be rectified through further investment in research and development.

One problem in designing large hybrid orbital rockets is that turbopumps become necessary to achieve high flow rates and pressurization of the oxidizer. This turbopump must be powered by something. In a traditional liquid-propellant rocket, the turbopump uses the same fuel and oxidizer as the rocket, since they are both liquid and can be fed to the pre-burner. But in a hybrid, the fuel is solid and cannot be fed to a turbopump's engine. Some hybrids use an oxidizer that can also be used as a monopropellant, such as hydrogen peroxide, and so a turbopump can run on it alone. However, hydrogen peroxide is significantly less efficient than liquid oxygen, which cannot be used alone to run a turbopump. Another fuel would be needed, requiring its own tank and decreasing rocket performance.

Fuel

Common fuel choices

A reverse-hybrid rocket, which is not very common, is one where the engine uses a solid oxidizer and a liquid fuel. Some liquid fuel options are

Hybrid rocket fuel grains can be manufactured via casting techniques, since they are typically a plastic or a rubber. Complex geometries, which are driven by the need for higher fuel mass flow rates, makes casting fuel grains for hybrid rockets expensive and time-consuming due in part to equipment costs. On a larger scale, cast grains must be supported by internal webbing, so that large chunks of fuel do not impact or even potentially block the nozzle. Grain defects are also an issue in larger grains. Traditional fuels that are cast are hydroxyl-terminated polybutadiene (HTPB) and paraffin waxes.^[13]

Additive manufacturing

Additive manufacturing is currently being used to create grain structures that were otherwise not possible to manufacture. Helical ports have been shown to increase fuel regression rates while also increasing volumetric efficiency.^[14] An example of material used for a hybrid rocket fuel is acrylonitrile butadiene styrene (ABS). The printed material is also typically enhanced with additives to improve rocket performance.^[13] Recent work at the University of Tennessee Knoxville has shown that, due to the increased surface area, the use of powdered fuels (i.e. graphite, coal, aluminum) encased in a 3D printed, ABS matrix can significantly increase the fuel burn rate and thrust level as compared to traditional polymer grains.^[15][16]

Oxidizer

Common oxidizer choices

Common oxidizers include gaseous or liquid oxygen, nitrous oxide, and hydrogen peroxide. For a reverse hybrid, oxidizers such as frozen oxygen and ammonium perchlorate are used.^{[5]: 405–406}

Proper oxidizer vaporization is important for the rocket to perform efficiently. Improper vaporization can lead to very large regression rate differences at the head end of the motor when compared to the aft end. One method is to use a hot gas generator to heat the oxidizer in a pre-combustion chamber. Another method is to use an oxidizer that can also be used as a monopropellant. A good example is hydrogen peroxide, which can be catalytically decomposed over a silver bed into hot oxygen and steam. A third method is to inject a propellant that is hypergolic with the oxidizer into the flow. Some of the oxidizer will decompose, heating up the rest of the oxidizer in the flow.^{[5]: 406–407}

Hybrid safety

Generally, well designed and carefully constructed hybrids are very safe. The primary hazards associated with hybrids are:

• Pressure vessel failures – Chamber insulation failure may allow hot combustion gases near the chamber walls leading to a "burn-through" in which the vessel ruptures.
• Blow back – For oxidizers that decompose exothermically such as
  nitrogen tetroxide
  , unless fuel is present in the oxidizer tank.
• Hard starts – An excess of oxidizer in the combustion chamber prior to ignition, particularly for monopropellants such as nitrous oxide, can result in a temporary over-pressure or "spike" at ignition.

Because the fuel in a hybrid does not contain an oxidizer, it will not combust explosively on its own. For this reason, hybrids are classified as having no

Organizations working on hybrids

Commercial companies

In 1998

Orbital Technologies Corporation (Orbitec) has been involved in some U.S. government-funded research on hybrid rockets including the "Vortex Hybrid" concept.[23]

Environmental Aeroscience Corporation (eAc)^[24] was incorporated in 1994 to develop hybrid rocket propulsion systems. It was included in the design competition for the SpaceShipOne motor but lost the contract to SpaceDev. Environmental Aeroscience Corporation still supplied parts to SpaceDev for the oxidizer fill, vent, and dump system.^[25]

Rocket Lab formerly sold hybrid sounding rockets and related technology.

The Reaction Research Society (RRS), although known primarily for their work with liquid rocket propulsion, has a long history of research and development with hybrid rocket propulsion.

Copenhagen Suborbitals, a Danish rocket group, has designed and test-fired several hybrids using N₂O at first and currently LOX. Their fuel is epoxy, paraffin wax, or polyurethane.^[26] The group eventually moved away from hybrids because of thrust instabilities, and now uses a motor similar to that of the V-2 rocket.

TiSPACE is a Taiwanese company which is developing a family of hybrid-propellant rockets.^[27]

bluShift Aerospace in Brunswick, Maine, won a NASA SBIR grant to develop a modular hybrid rocket engine for its proprietary bio-derived fuel in June 2019.^[28] Having completed the grant bluShift has launched its first sounding rocket using the technology.^[29]

Vaya Space based out of Cocoa, Florida, is expected to launch its hybrid fuel rocket Dauntless in 2023.^[30][31]

Reaction Dynamics based out Saint-Jean-sur-Richelieu, Quebec, began developing a hybrid rocket engine in 2017 capable of producing 21.6 kN of thrust. Their Aurora rocket will use nine engines on the first stage and one engine on the second stage and will be capable of delivering a payload of 50-150 kg to LEO.^[32] In May 2022, Reaction Dynamics announced they were partnering with Maritime Launch Services to launch the Aurora rocket from their launch site currently under construction in Canso, Nova Scotia, beginning with suborbital test flights in Summer, 2023 with a target of 2024 for the first orbital launch.^[33]

Universities

Space Propulsion Group was founded in 1999 by Arif Karabeyoglu, Brian Cantwell, and others from

The University of Tennessee Knoxville has carried out hybrid rocket research since 1999, working in collaboration with NASA Marshall Space Flight Center and private industry. This work has included the integration of a water-cooled calorimeter nozzle, one of the first 3D-printed, hot section components successfully used in a rocket motor.^[36] Other work at the university has focused on the use of helical oxidizer injection, bio-derived fuels^[37] and powdered fuels encased in a 3D-printed, ABS matrix, including the successful launch of a coal-fired hybrid at the 2019 Spaceport America Cup.^[15][16]

At the Delft University of Technology, the student team Delft Aerospace Rocket Engineering (DARE) is very active in the design and building of hybrid rockets. In October 2015, DARE broke the European student altitude record with the Stratos II+ sounding rocket. Stratos II+ was propelled by the DHX-200 hybrid rocket engine, using a nitrous oxide oxidizer and fuel blend of paraffin, sorbitol and aluminium powder. On July 26, 2018 DARE attempted to launch the Stratos III hybrid rocket. This rocket used the same fuel/oxidizer combination as its predecessor, but with an increased impulse of around 360 kNs.^[38] At the time of development, this was the most powerful hybrid rocket engine ever developed by a student team in terms of total impulse. Unfortunately, the Stratos III vehicle was lost 20 seconds into the flight.^[39]

Boston University's student-run "Rocket Propulsion Group",^[41] which in the past has launched only solid motor rockets, is attempting to design and build a single-stage hybrid sounding rocket to launch into sub-orbital space by July 2015.^[42]

The University of Brasilia's (UnB) Hybrid Rocket Team initiated their endeavors in 1999 within the Faculty of Technology, marking the pioneering institution in the Southern Hemisphere to engage with hybrid rockets. Over time, the team has achieved notable milestones, encompassing the creation of various sounding rockets and hybrid rocket engines. Presently, the team is known as the Chemical Propulsion Laboratory (CPL) and is situated at Campus UnB Gama. CPL has made significant strides in the advancement of critical hybrid engine technologies. This includes the development of a modular 1 kN hybrid rocket engine for the SARA platform, an innovative methane-oxygen gas-torch ignition system, an efficient oxidizer feed system, precision flow control valves, and thrust vector control mechanisms tailored for hybrid engines. Additionally, they've achieved a breakthrough with a 3D-printed, actively cooled hybrid rocket engine. Furthermore, the Laboratory is actively engaged in diverse areas of research and development, with current projects spanning the formulation of hybrid engine fuels using paraffin wax and N2O, numerical simulations, optimization techniques, and rocket design. CPL collaborates extensively with governmental agencies, private investors, and other educational institutions, including FAPDF, FAPESP, CNPq, and AEB. A notable collaborative effort includes the Capital Rocket Team (CRT), a group of students from UnB, who are currently partnering with CPL to develop hybrid sounding rockets. In a remarkable achievement, CRT clinched the top spot in the 2022 Latin American Space Challenge (LASC).

University of Toronto's student-run "University of Toronto Aerospace Team", designs and builds hybrid engine powered rockets. They are currently constructing a new engine testing facility at the University of Toronto Institute for Aerospace Studies, and are working towards breaking the Canadian amateur rocketry altitude record with their new rocket, Defiance MKIII, currently under rigorous testing. Defiance MK III's engine, QUASAR, is a Nitrous-Paraffin hybrid engine, capable of producing 7 kN of thrust for a period of 9 seconds.^{[citation needed]}

In 2016, Pakistan's DHA Suffa University successfully developed^[44] Raheel-1, hybrid rocket engines in 1 kN class, using paraffin wax and liquid oxygen, thereby becoming the first university run rocket research program in the country.^[45] In India, Birla Institute of Technology, Mesra Space engineering and rocketry department has been working on Hybrid Projects with various fuels and oxidizers.

Pars Rocketry Group from Istanbul Technical University has designed and built the first hybrid rocket engine of Turkey, the rocket engine extensively tested in May 2015.^[46]

A United Kingdom-based team (laffin-gas) is using four N₂O hybrid rockets in a drag-racing style car. Each rocket has an outer diameter of 150 mm and is 1.4 m long. They use a fuel grain of high-density wound paper soaked in cooking oil. The N₂O supply is provided by Nitrogen-pressurised piston accumulators which provide a higher rate of delivery than N₂O gas alone and also provide damping of any reverse shock.^{[citation needed]}

In Italy one of the leading centers for research in hybrid propellants rockets is CISAS (Center of Studies and Activities for Space) "G. Colombo",

In (Germany) the University of Stuttgart's Student team HyEnd is the current world record holder for the highest-flying student-built hybrid rocket with their HEROS rockets.^[48]

In Bangladesh, Amateur Experimental Rocketry Dhaka supported by the American International University Bangladesh has also tested the country's first hybrid rocket engine, and are now working towards larger paraffin/nitrous oxide based prototypes. ^[49]

The Aerospace Team of the

The Polish Student team PWr in Space at Wrocław University of Science and Technology has developed three hybrid rockets: R2 "Setka", R3 "Dziewięćdziesiątka dziewiątka" and the most powerful of all - R4 "Lynx" with a successful test at their test stand ^[51]

Many other universities, such as

There are a number of hybrid rocket motor systems available for amateur/hobbyist use in high-powered model rocketry. These include the popular HyperTek systems[52] and a number of 'Urbanski-Colburn Valved' (U/C) systems such as RATTWorks,^[53] Contrail Rockets,^[54] and Propulsion Polymers.^[55] All of these systems use

In popular culture

This article may contain irrelevant references to popular culture. Please remove the content or add citations to reliable and independent sources. (May 2020)

An October 26, 2005 episode of the television show MythBusters entitled "Confederate Rocket" ^[56] featured a hybrid rocket motor using liquid nitrous oxide and paraffin wax. The myth purported that during the American Civil War, the Confederate Army was able to construct a rocket of this type. The myth was revisited in a later episode entitled Salami Rocket, using hollowed out dry salami as the solid fuel.

In the February 18, 2007, episode of Top Gear, a Reliant Robin was used by Richard Hammond and James May in an attempt to modify a normal K-reg Robin into a reusable Space Shuttle. Steve Holland, a professional radio-controlled aircraft pilot, helped Hammond to work out how to land a Robin safely. The craft was built by senior members of the United Kingdom Rocketry Association (UKRA) and achieved a successful launch, flew for several seconds into the air and managed to successfully jettison the solid-fuel rocket boosters on time. This was the largest rocket launched by a non-government organisation in Europe. It used 6 × 40960 NS O motors by Contrail Rockets giving a maximum thrust of 8 tonnes. However, the car failed to separate from the large external fuel tank due to faulty explosive bolts between the Robin and the external tank, and the Robin subsequently crashed into the ground and seemed to have exploded soon after. This explosion was added for dramatic effect as neither Reliant Robins nor hybrid rocket motors explode in the way depicted.

See also

• Spacecraft propulsion

References

Further reading

• Costa, Fernando de Souza; Vieira, Ricardo (2010). "Preliminary analysis of hybrid rockets for launching nanosats into LEO". Journal of the Brazilian Society of Mechanical Sciences and Engineering. 32 (4): 502–509.
  doi:10.1590/S1678-58782010000400012
  .

External links

Wikimedia Commons has media related to Hybrid rocket engines.

• "Developing and testing of a 2 kN hybrid rocket engine". hybrid-engine-development.de (in German).
• "Paraffin hybrid links". Portland State Aerospace Society. Archived from the original on September 1, 2006.
• "Hybridrocket". c-turbines.ch (private web page) (in German).