Theory:

Like other chemical rocket engines, hybrid motors combine fuel and an oxidizer to produce combustion gases and thrust.

Liquid rockets achieve this using liquid fuel and liquid oxidizer stored in tanks. The propellants are either pressure fed or pumped from their tanks into a combustion chamber. They generally provide good thrust, can be throttled, and tend to be the most efficient. However, due to plumbing complexity, redundancies for reliability, and propellant storage issues, their cost & weight can be high.

In solid rocket motors, the fuel and oxidizer are chemically premixed and formed into a solid fuel grain. When ignited, the oxidizer and fuel react and produce thrust. The geometry of the grain's central core can be varied to produce a desired thrust profile. Solid motors are very simple. However, they are not as efficient as liquid motors, can not be throttled or stopped, and may present an explosion hazard.

Hybrid motors combine elements from both systems. Gaseous or liquid oxidizer - generally liquid oxygen (LOX) or nitrous oxide (NOX) - is stored in a tank, and a hollow fuel grain - generally plastic or rubber - lines the combustion chamber. A source of ignition is applied to the fuel grain, vaporizes some of the fuel, and the oxidizer is injected into the chamber. At about 1000 degrees, the 36% oxygen (by molecular weight) in NOX is released to combust with the fuel vapor in a thin boundary layer above the fuel grain. This creates both gases and more heat, which in turn continues to vaporize the fuel grain. The oxidizer flow can be increased or decreased for throttling, and if the ignition system is re-usable and onboard, the motor can be stopped and re-started in flight.

A reverse hybrid motor uses a solid oxidizer and liquid fuel; this combination does not provide the advantages of a regular hybrid motor, since either or both propellants may not be inert.

All certified high power model rocketry (HPR) motors use NOX. Since NOX is self-pressurizing at 650-750 pounds per square inch, no pumps are needed to run the combustion chamber up to ~550 psi. As the tank empties, it cools evaporatively, which lowers the tank pressure, and thus the combustion pressure, providing a regressive thrust curve. When the liquid NOX runs out, the gaseous NOX left in the tank provides reduced thrust for a short period known as "blow-down".

Since the chamber pressure of HPR motors must be kept below 550 psi, the grain and combustion chamber must have a large enough surface area to generate sufficient gases. This, combined with the tank, is why HPR nitrous motors are usually so long. The traditional raspy buzz or howl of a hybrid motor is due to combustion chamber pressure fluctuation. Nitrous enters the chamber raising the pressure; combustion & exhaust occurs which lowers the chamber pressure. This causes the combustion process to be slightly inefficient, but for HPR purposes is insignificant.

Hybrid vs. solid cost comparison:

Motor	Hardware	Reloads	Nitrous	Cost per flight	Ns/$
3 flights:
RATT H70	$60	$30	$3	$32	5.7
Aerotech H180	$62	$48	-	$37	6.4
3 flights:
Hypertek J250	$150	3 incl.	$16	$55	13.4
Aerotech J350	$50	$130	-	$60	11.6
3 flights:
RATT K240	$250	$100	$21	$124	14.5
Aerotech K550	$120	$270	-	$130	12.6
2 flights:
Hypertek M1000	$450	2 incl.	$70	$260	35.0
Aerotech M1419	$380	$800	-	$589	13.2
5 flights (ref):
Hypertek M1000, including GSE				$299	30.5
Aerotech M1419, excluding GSE				$541	14.3
Notes: 1) hybrid motors require electronics for recovery, which will add to flight costs, especially for lower power flights where a solid motor will have an ejection charge. 2) special ground support equipment (GSE) is expensive unless provided by a group or organization, and will add significantly to flight costs at lower power levels. 3) even with a GSE purchase, the cost savings in the M range are significant. 4) motors compared are of a similar impulse where possible. 5) Prices are from Pratt Hobbies, NowHybrids, and Magnum Rockets on 11/30/2001. 6) $3.50/lb for racing NOX for the total flights with some leakage & venting.

History:

The history of hybrid rocket motors can be divided into 3 phases covering 1933 to the present. Also included is an overview of hybrid model rocket history.

Pioneers, 1933-1950:

The first hybrid rocket was launched was launched August 18, 1933. The Russian GIRD-09, developed by Sergei Korolev and Mikhail Tikhonravov, used jellied gasoline suspended on a metal mesh and LOX under its own pressure. The semisolid fuel both eliminated the need for a cooling system and protected the combustion chamber walls, while the pressurized LOX eliminated any pumps. The 7 inch by 8 foot rocket generated a thrust of 500N for 15 sec. (M514) and reached 1500m.

GIRD-09 (Retro Rockets, Peter Alway)

In Germany from 1937-1939, I.G. Farben ran tests using coal and gaseous NOX, which developed 10,000N for 120 sec. Hans Oberth also tested a LOX and tar-wood-saltpetre mixture.

The first US tests were conducted from 1938 to 1941 by the Californian Rocket Society using coal and GOX. In 1947, the Pacific Rocket Society tested wood and LOX motors.

Research, 1951-1971:

GE conducted the first US commercial tests between 1951 and 1956 using polyethylene and 90% hydrogen peroxide.

The first hypergolic (spontaneous ignition) hybrid was fired in June, 1951 by the Rocket Missile Research Society in Watsonville, CA using Mixed Acids for oxidizer and Asphalt/KClO3 as the fuel grain.

The French National Aerospace Research Establishment (ONERA) started researching hybrids in 1956. ONERA's first flight was April 25, 1964, with a thrust of 10,000N. Other flights through 1967 reached 100km.

In the US, Rocketdyne started testing Plexiglas and oxygen motors in 1960, and United Technologies Corp. (UTC) started research in 1961, with a 18.4 ton test on April 25, 1967, and a 50,000N test in 1970.

Tests were conducted in Germany from 1965 to 1970, and further testing occurred between 1974 and 1987.

Sweden also ran tests from 1965 to 1971, with a first flight in 1971 launching a 20kg payload to 80 km.

Development, 1979-2001:

Teledyne Ryan began design work for the AQM-81A "Firebolt" target drone in late 1979 and the vehicle, powered by an UTC hybrid motor, entered USAF service in 1983.

James C. Bennett, a principal in US hybrid rocket research and development, co-founded Space Enterprise Consultants in 1980, and Arc Technologies, Inc., later known as Starstruck, Inc. Starstruck successfully conducted a sea launch of its Dolphin rocket on August 3, 1984 with a thrust of 175,000N using HTPB and LOX. This Dolphin launch produced two notable firsts: the first flight of a privately developed large launch vehicle in the US, and the first flight of a large hybrid rocket. In 1985, Mr. Bennett co-founded American Rocket Company (AMROC), which tested engines up to 324,000N, and unsuccessfully launched the SET-1 sounding rocket on October 5, 1989, which failed for reasons unrelated to the hybrid motor. AMROC folded in 1995, but SpaceDev acquired rights to AMROC's hybrid technology in 1999, and continues to develop hybrid technology.

Dolphin, 1984 (astronautix.com)

250K hybrid, 8/13/99 (NASA/SSC)

In 1995, NASA and DARPA started the Hybrid Propulsion Demonstration Program (HPDP). Under this program, the world's largest hybrid engine was tested at NASA's Stennis Space Center August 13, 1999. It was 70 inches by 45 feet, and developed 250,000 pounds of thrust for 15 seconds.

Also under HPDP, in 1996 and 1997 Environmental Aeroscience Corporation (eAc) launched the 6" diameter Hyperion 1A four times from NASA Wallops Flight Facility, reaching an altitude of 120,000 feet burning N2O and HTPB. These were the first hybrid flights for NASA. In affiliation with Cesaroni Technology Incorporated (CTI), their 112,500N Hyperion 1C motor was successfully tested in February 2001. The Hyperion 1C is anticipated to reach an altitude in excess of 250,000 feet, while the proposed 12" diameter 890,000N Hyperion 2 will exceed 500,000 feet and Mach 5.

Hyperion (eAc)

Model Rocketry:

In the early 1990's, Korey Kline started development of HPR hybrid motors after a discussion with Bill Wood. Korey Kline and the group which founded eAc launched the first HPR hybrid August 16, 1994, and made them available in 1995 under the Hypertek brand. Hypertek is now manufactured by CTI in affiliation with eAc.

In December 1994, Aerotech launched their first "RMS/Hybrid" test, and in 1995, introduced its line of hybrid rocket motors to the market.

Also in 1994, Bob Fortune, John Urbanski & Bill Colburn were experimenting with nitrous motors. In 1995, John & Bill independently came up with the idea of substituting a plastic hose for the metal stem used in the Hypertek launch system. The hose acted both as a burst safety valve and the nitrous release valve when melted by a preheater grain.

R.A.T.T. Works combined this idea with a monocoque/floating bulkhead design and certified their motors with Tripoli in 2000. Propulsion Polymers is currently working with Tripoli to certify motors with a similar design.

Myths:

Myth #1: "Nitrous motors are cheaper."
From the comparison above, lower impulse motors are only a little less expensive than solids, without electronics costs. In the high power range, cost savings are quite significant.

Myth #2: "HPR nitrous flights were first."
Yes, HPR nitrous motors were available before any US government nitrous rockets. However, Russia, Germany, France, and Sweden all had successful flights prior to 1970. The USAF used a nitrous motor in the Firebolt in 1983, and Starstruck flew the Dolphin in 1984, ten years before HPR flights started.

Myth #3: "Nitrous is a drug."
There are 3 types of nitrous: medical, industrial & racing:

1. you need a prescription for medical/U.S.P. grade, since it is a controlled substance.
2. industrial nitrous (99-99.9998%) is only available from gas suppliers at $1.25-1.50/lb (99% "Atomic Absorbtion"), and they usually will only fill A & B size tanks; 40 or 20 lbs.
   
3. racing nitrous has sulphur dioxide added to prevent inhaling, is frequently available at auto stores, and costs ~$3.50/lb. Also, auto shops will fill the C (10 lb) tanks.