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In 1968 Shirley Chisholm became the first Black woman elected to the United States Congress. In 1972 she launched a campaign for the Democratic presidential nomination, breaking another political barrier.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
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A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control and translation. Alternatively, reaction wheels can be used for attitude control, rather than RCS. Use of diverted engine thrust to provide stable attitude control of a short-or-vertical takeoff and landing aircraft below conventional winged flight speeds, such as with the Harrier "jump jet", may also be referred to as a reaction control system.[1]
Reaction control systems are capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control of rotation (roll, pitch, and yaw).[2]
Reaction control systems often use combinations of large and small (vernier) thrusters, to allow different levels of response.
Spacecraft reaction control systems are used for:
Because spacecraft only contain a finite amount of fuel and there is little chance to refill them, alternative reaction control systems have been developed so that fuel can be conserved. For stationkeeping, some spacecraft (particularly those in geosynchronous orbit) use high-specific impulse engines such as arcjets, ion thrusters, or Hall effect thrusters. To control orientation, a few spacecraft, including the ISS, use momentum wheels which spin to control rotational rates on the vehicle.
The Mercury space capsule and Gemini reentry module both used groupings of nozzles to provide attitude control. The thrusters were located off their center of mass, thus providing a torque to rotate the capsule. The Gemini capsule was also capable of adjusting its reentry course by rolling, which directed its off-center lifting force.[clarification needed] The Mercury thrusters used a hydrogen peroxide monopropellant which turned to steam when forced through a tungsten screen, and the Gemini thrusters used hypergolic mono-methyl hydrazine fuel oxidized with nitrogen tetroxide.
The Gemini spacecraft was also equipped with a hypergolic Orbit Attitude and Maneuvering System, which made it the first crewed spacecraft with translation as well as rotation capability. In-orbit attitude control was achieved by firing pairs of eight 25-pound-force (110 N) thrusters located around the circumference of its adapter module at the extreme aft end. Lateral translation control was provided by four 100-pound-force (440 N) thrusters around the circumference at the forward end of the adaptor module (close to the spacecraft's center of mass). Two forward-pointing 85-pound-force (380 N) thrusters at the same location, provided aft translation, and two 100-pound-force (440 N) thrusters located in the aft end of the adapter module provided forward thrust, which could be used to change the craft's orbit. The Gemini reentry module also had a separate Reentry Control System of sixteen thrusters located at the base of its nose, to provide rotational control during reentry.
The Apollo Command Module had a set of twelve hypergolic thrusters for attitude control, and directional reentry control similar to Gemini.
The Apollo Service Module and Lunar Module each had a set of sixteen R-4D hypergolic thrusters, grouped into external clusters of four, to provide both translation and attitude control. The clusters were located near the craft's average centers of mass, and were fired in pairs in opposite directions for attitude control.
A pair of translation thrusters are located at the rear of the Soyuz spacecraft; the counter-acting thrusters are similarly paired in the middle of the spacecraft (near the center of mass) pointing outwards and forward. These act in pairs to prevent the spacecraft from rotating. The thrusters for the lateral directions are mounted close to the center of mass of the spacecraft, in pairs as well.[citation needed]
The suborbital X-15 and a companion training aero-spacecraft, the NF-104 AST, both intended to travel to an altitude that rendered their aerodynamic control surfaces unusable, established a convention for locations for thrusters on winged vehicles not intended to dock in space; that is, those that only have attitude control thrusters. Those for pitch and yaw are located in the nose, forward of the cockpit, and replace a standard radar system. Those for roll are located at the wingtips. The X-20, which would have gone into orbit, continued this pattern.
Unlike these, the Space Shuttle Orbiter had many more thrusters, which were required to control vehicle attitude in both orbital flight and during the early part of atmospheric entry, as well as carry out rendezvous and docking maneuvers in orbit. Shuttle thrusters were grouped in the nose of the vehicle and on each of the two aft Orbital Maneuvering System pods. No nozzles interrupted the heat shield on the underside of the craft; instead, the nose RCS nozzles which control positive pitch were mounted on the side of the vehicle, and were canted downward. The downward-facing negative pitch thrusters were located in the OMS pods mounted in the tail/afterbody.
The International Space Station uses electrically powered control moment gyroscopes (CMG) for primary attitude control, with RCS thruster systems as backup and augmentation systems.[5][unreliable source?]
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A reaction control system (RCS) is a spacecraft system that uses thrusters to provide attitude control and translation. Alternatively, reaction wheels can be used for attitude control, rather than RCS. Use of diverted engine thrust to provide stable attitude control of a short-or-vertical takeoff and landing aircraft below conventional winged flight speeds, such as with the Harrier "jump jet", may also be referred to as a reaction control system. Reaction control systems are capable of providing small amounts of thrust in any desired direction or combination of directions. An RCS is also capable of providing torque to allow control of rotation (roll, pitch, and yaw). Reaction control systems often use combinations of large and small (vernier) thrusters, to allow different levels of response.
Monomethylhydrazine (MMH) is a highly toxic, volatile hydrazine derivative with the chemical formula CH6N2. It is used as a rocket propellant in bipropellant rocket engines because it is hypergolic with various oxidizers such as nitrogen tetroxide (N2O4) and nitric acid (HNO3). As a propellant, it is described in specification MIL-PRF-27404. MMH is a hydrazine derivative that was once used in the orbital maneuvering system (OMS) and reaction control system (RCS) engines of NASA's Space Shuttle, which used MMH and MON-3 (a mixture of nitrogen tetroxide with approximately 3% nitric oxide). This chemical is toxic and carcinogenic, but it is easily stored in orbit, providing moderate performance for very low fuel tank system weight. MMH and its chemical relative unsymmetrical dimethylhydrazine (UDMH) have a key advantage that they are stable enough to be used in regeneratively cooled rocket engines. The European Space Agency (ESA) has attempted to seek new options in terms of bipropellant rocket combinations to avoid using toxic chemicals such as MMH and its relatives. MMH is believed to be the primary active mycotoxin found in the false morel (Gyromitra esculenta). In these cases, MMH is formed by the hydrolysis of gyromitrin. Monomethylhydrazine is a possible occupational carcinogen, and the occupational exposure limits to MMH are set at protective levels to account for the possible carcinogenicity. MMH is used in the synthesis of suritozole. MMH is also assumed to be the active metabolite of temozolomide.
A vernier thruster is a rocket engine used on a spacecraft or launch vehicle for fine adjustments to the attitude or velocity. Depending on the design of a craft's maneuvering and stability systems, it may simply be a smaller thruster complementing the main propulsion system, or it may complement larger attitude control thrusters, or may be a part of the reaction control system. The name is derived from vernier calipers (named after Pierre Vernier) which have a primary scale for gross measurements, and a secondary scale for fine measurements. Vernier thrusters are used when a heavy spacecraft requires a wide range of different thrust levels for attitude or velocity control, as for maneuvering during docking with other spacecraft. On space vehicles with two sizes of attitude control thrusters, the main ACS (Attitude Control System) thrusters are used for larger movements, while the verniers are reserved for smaller adjustments. Due to their weight and the extra plumbing required for their operation, vernier rockets are seldom used in new designs. Instead, as modern rocket engines gained better control, larger thrusters could also be fired for very short pulses, resulting in the same change of momentum as a longer thrust from a smaller thruster. Vernier thrusters are used in rockets such as the R-7 for vehicle maneuvering because the main engine is fixed in place. For earlier versions of the Atlas rocket family (prior to the Atlas III), in addition to maneuvering, the verniers were used for roll control, although the booster engines could also perform this function. After main engine cutoff, the verniers would execute solo mode and fire for several seconds to make fine adjustments to the vehicle attitude. The Thor/Delta family also used verniers for roll control but were mounted on the base of the thrust section flanking the main engine.
The Space Shuttle orbiter was the spaceplane component of the Space Shuttle, a partially reusable orbital spacecraft system that was part of the discontinued Space Shuttle program. Operated from 1981 to 2011 by NASA, the U.S. space agency, this vehicle could carry astronauts and payloads into low Earth orbit, perform in-space operations, then re-enter the atmosphere and land as a glider, returning its crew and any on-board payload to the Earth. Six orbiters were built for flight: Enterprise, Columbia, Challenger, Discovery, Atlantis, and Endeavour. All were built in Palmdale, California, by the Pittsburgh, Pennsylvania-based Rockwell International company's North American Aircraft Operations branch. The first orbiter, Enterprise, made its maiden flight in 1977. An unpowered glider, it was carried by a modified Boeing 747 airliner called the Shuttle Carrier Aircraft and released for a series of atmospheric test flights and landings. Enterprise was partially disassembled and retired after completion of critical testing. The remaining orbiters were fully operational spacecraft, and were launched vertically as part of the Space Shuttle stack. Columbia was the first space-worthy orbiter; it made its inaugural flight in 1981. Challenger, Discovery, and Atlantis followed in 1983, 1984, and 1985, respectively. In 1986, Challenger was destroyed in a disaster shortly after its 10th launch, killing all seven crew members. Endeavour was built as Challenger's successor, and was first launched in 1992. In 2003, Columbia was destroyed during re-entry, leaving just three remaining orbiters. Discovery completed its final flight on March 9, 2011, and Endeavour completed its final flight on June 1, 2011. Atlantis completed the final Shuttle flight, STS-135, on July 21, 2011. In addition to their crews and payloads, the reusable orbiter carried most of the Space Shuttle's liquid-propellant rocket system, but both the liquid hydrogen fuel and the liquid oxygen oxidizer for its three main rocket engines were fed from an external cryogenic propellant tank. Two reusable solid rocket boosters (SRBs) provided additional thrust for approximately the first two minutes of launch. The orbiters themselves also carried hypergolic propellant for the onboard Reaction Control System (RCS) thrusters and Orbital Maneuvering System (OMS) engines.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →The Greenwood District.
