Pressure Sensors (cont’d)

Pressure Sensors (cont’d) • Sensors measure pressure and temperature at different points in the system. • Even a simple system – such as a cold-gas thruster for a small satellite like FireSat requires tanks, valves, and sensors to measure and control the propellant flow. FireSat’s Propulsion Subsystem Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Thermodynamic Rockets • Thermodynamic rockets available or being considered fall into five groups, according to their source of energy: • Cold gas • Chemical • Solar thermal • Thermoelectric • Nuclear thermal Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Thermodynamic Rockets (cont’d) • Cold gas—use mechanical energy of a gas stored under pressure. • Chemical—rely on chemical energy (from breaking down or decomposing fuels or combustion of propellants) to produce heat. • Solar thermal—use concentrated solar energy to produce heat. • Thermoelectric—use the heat produced from electrical resistance. • Nuclear thermal—use the heat from a nuclear reaction. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Chemical Rockets • Most of the rockets in use today rely on chemical energy. • In chemical rockets, the propellants release energy from their chemical bonds during combustion. • The Space Shuttle relies on chemical rockets. In the Shuttle’s main engines, liquid hydrogen and liquid oxygen combine in the most basic of chemical reactions. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Chemical Rockets (cont’d) • All combustion reactions must have two things: • A fuel (such as hydrogen) • An oxidizer (such as oxygen) • These two combine, freeing a vast amount of heat and creating by-products that form the exhaust. • The heat transfers to the combustion products, raising their temperatures. • This chemical reaction and energy transfer take place in the combustion chamber. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Types of Chemical Rockets • Chemical rockets fall into one of three general categories: • Liquid • Solid • Hybrid Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Liquid-chemical Rockets • Liquid-chemical rockets usually are one of two types: • bipropellant • monopropellant Space Shuttle Main Engine – an example of a bipropellant rocket Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Bipropellant Rockets • Bipropellant rockets use two liquid propellants. • One is a fuel, such as liquid hydrogen and the other, an oxidizer, such as liquid oxygen. LEROS Bipropellant Engine Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Bipropellant Rockets (cont’d) • Brought together under pressure in the combustion chamber by the propellant-management subsystem, the two compounds chemically react (combust). • This releases huge amounts of heat and produces combustion products varying on the propellants. • To ensure complete, efficient combustion, the oxidizer and fuel must mix in the correct proportions. The oxidizer/fuel ratio (O/F) is the proportion by mass of oxidizer to fuel. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Bipropellant Rockets (cont’d) • Hypergolic propellants are combinations of propellants that react on contact and don’t use a separate means of ignition. They ignite on contact with each other. • Cryogenic propellants have a low storage temperature; however it is difficult to maintain the extremely low temperatures for long periods of time. • For long-term storage, storable propellants can remain stable at room temperature for a very long time. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Table 12-2 Bipropellant Rockets • Operating Principle A liquid oxidizer and a liquid fuel react in combustion, liberating heat and creating exhaust products that thermodynamically expand through a nozzle. • Typical Propellants Oxidizers: Liquid oxygen, high-test hydrogen peroxide, nitrogen tetroxide Fuels: Liquid hydrogen, kerosene, hydrazine • Advantages High specific impulse Can be throttled Can be re-started • Disadvantages Must manage two propellants Intense combustion heat creates thermal-control problems for chamber and nozzle Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Monopropellant Rocket • The most widely used monopropellant today is hydrazine. • The main disadvantage of hydrazine is that it’s very toxic. • Technicians need special handling procedures and equipment during testing and launch operations. • The biggest advantage is simplicity, however performance suffers. • For certain mission applications this trade-off is well worth it. Monopropellant Rocket Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Table 12-3 Monopropellant Rockets • Operating PrincipleA single propellant decomposes using a catalyst, releasing heat and creating by-products that thermodynamically expand through a nozzle. • Typical Propellants Hydrazine (N2H4), HTP = high-test hydrogen peroxide ( >85% H2O2) • Advantages Simple, reliable One propellant to manage Lower temperature reactions means fewer thermal problems in the chamber and nozzle. • Disadvantages Lower specific impulse than bipropellant Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Solid-chemical Rockets • Solid rockets date back thousands of years to the Chinese, who used them to confuse and frighten their enemies on the battlefield. In modern times, these rockets create thrust for intercontinental ballistic missiles, as well as space-launch vehicles. • A solid rocket contains a mixture of fuel, oxidizer, and binder, blended correctly and solidified into a single package called a motor. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Solid-chemical Rockets (cont’d) • A solid rocket contains fuel, oxidizer, and binder blended correctly and solidified into a single package called a motor. • A typical composite solid-rocket fuel is powdered aluminum. • A common composite solid-rocket fuel is powdered aluminum. • Together, the fuel and oxidizer comprise about 85%–90% of the rocket motor’s mass. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Solid-chemical Rockets (cont’d) • The motor’s remaining mass consists of a binder that holds together the other ingredients. The motor’s remaining mass consists of a binder that holds the other ingredients together. Binders usually are a hard, rubber-like compound. During combustion, the binder also acts as fuel. • Because combustion in solid-rocket motors depends on the exposed propellant’s surface area, manufacturers must carefully mold the propellant mixture to prevent cracks. • Burning occurs on any exposed surface, even along undetected cracks in the propellant grain. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Solid Rockets (cont’d) • Figure 12-25 shows solid-propellant grain designs. • By altering the grain design, engineers cause increasing or neutral burn rates. • Shaded areas show propellant and blank areas show empty space. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Solid Rockets (cont’d) • Investigators linked the Space Shuttle Challenger’s accident to an improperly sealed joint between solid-motor segments. This open seal exposed the motor case to hot gasses, burning it through and causing the accident. • The Challenger disaster highlighted another drawback of solid motors—once they start, they are very difficult to stop. • Various missions use solid motors despite their drawbacks, however, because they offer good, cost-effective performance in a simple, self-contained package that doesn’t require a separate subsystem to manage the propellant. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Solid Rockets (cont’d) • One important use of solid motors is to boost launch vehicles. • Several launch vehicles use various combinations of strap-on solid motors giving users a choice in payload-lifting capacity without needing to redesign the entire vehicle. • For example, the Delta II launch vehicle can add three, six or nine solid motors depending on the payload mass. NASA’s workhorse: the Delta Rocket Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Table 12-4: Solid Rockets • Operating Principle An oxidizer and fuel blend with a binder in a single,solid grain. Combustion takes place along any exposed surface producing heat and by-products that expand thermodynamically through a nozzle. • Typical Propellants Fuel: aluminum; oxidizer: Ammonium perchlorate; Binder: Hydroxyl-terminated polybutadien • Advantages Simple, reliable No propellant management needed High specific impulse compared to bipropellant No combustion chamber cooling issues • Disadvantages Can get cracks in the grain Can’t restart Difficult to stop Modest specific impulse Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Hybrid Rockets • Hybrid-propulsion systems combine aspects of liquid and solid systems. • A typical hybrid rocket uses a liquid oxidizer and a solid fuel. • The molded fuel grain forms the combustion chamber, into which we inject the oxidizer. • This approach offers the flexibility of a liquid system with the simplicity and density of a solid motor. Hybrid Rocket Motor Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Table 12-5 Hybrid Rockets • Operating Principle Hybrid rockets typically use a liquid oxidizer with a solid fuel. The oxidizer is injected into a hollow port (or ports) within the fuel grain where combustion takes place along the boundary with the surface. • Typical Propellants Oxidizers: Liquid oxygen (LOX), nitrous oxide (N2O), high- test hydrogen peroxide (> 85% H2O2) Fuels: HTPB = hydroxyl-terminated polybutadiene (rubber), PE = polyethylene (plastic) • Advantages Simpler than a bipropellant system with similar performance • Safer, more flexible than solids • No combustion-chamber cooling issues • Disadvantages Limited heritage • Modest specific impulse Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Solar-thermal Rockets • Solar-thermal rockets focus the Sun’s heat onto a combustion chamber, where the heat transfers to a propellant. • They use the Sun’s unlimited power to produce relatively high thrust with high specific impulse. • However, up to now, none has been tested in orbit. Solar-thermal Rocket Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Table 12-7 Solar-thermal Rockets • Operating Principle Lenses or mirrors concentrate solar energy onto a heat-transfer chamber. A propellant, such as liquid hydrogen, flows through the chamber, absorbs heat, and then expands through a nozzle. • Typical Propellants Can use just about any propellant, but hydrogen produces the best specific impulse • Advantages Limitless energy supply, can be refueled and re-used Potentially very high specific impulse Environmentally friendly products • Disadvantages Needs intense, direct sunlight Must carefully point a large mirror or lens No flight heritage Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Thermoelectric Rockets • Thermoelectric rockets transfer heat to the propellant by conduction and convection. • One of the simplest examples of a thermoelectric rocket is a resistojet. Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Thermoelectric Rockets (cont’d):How a Resistojet Works • Much like an electric tea kettle. • Electric current flows through a metal heating element inside a combustion chamber. Arcjet Thruster Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Thermoelectric Rockets (cont’d):How a Resistojet Works • The resistance (or electrical friction) in the metal heats it. As propellant flows around the heating element, heat transfers to it by convection and increases its temperature before it expands through a nozzle. • This simple principle can apply to nearly any propellant. (NASA considered urine as a propellant on the Space Station!) Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Thermoelectric Rockets (cont’d):How a Resistojet Works • The International Space Station uses resistojets, such as shown in Figure 12-32 to help maintain its orbit and attitude. Resistojets at Work Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Arcjets • An arcjet thruster works by passing a propellant through an electric arc. • This rapidly increases its temperature before expanding it out a nozzle. • Arcjet systems can achieve relatively high specific impulse (up to 1000 seconds) with small but significant thrust levels (up to 1 newton). Arcjet Thruster Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Arcjets (cont’d) • The Argos spacecraft shown in Figure 12-34 tested a powerful 26 kilowatt ammonia arcjet, setting a record for the most powerful electric-propulsion system ever tested in orbit. • Its specific impulse was 800 seconds, and its thrust was 2 newtons. Arcjets at Work Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Table 12-8 Thermoelectric Rockets • Operating PrincipleHeat comes from an electric resistance or spark discharge inside a heat-transfer chamber. A propellant flows through the chamber, absorbs heat, and then expands through a nozzle. • Typical Propellants Hydrazine, water, ammonia, or almost any other propellant • Advantages Simple, reliable Can use as an “add on” to conventional monopropellant rocket to boost specific Impulse High-power arcjets offer very high specific impulses • Disadvantages Requires large amounts of onboard electrical power Relatively low thrust (less than one newton) Unit 3, Chapter 12, Lesson 12: Rockets and Launch Vehicles

Pressure Sensors (cont’d)