Introduction

Question 1

[Rocket type] wherein the energy of a nuclear reactor is used to heat a propellant to high temperatures, which is then expelled through a nozzle to produce thrust.

Answer 1

Nuclear Thermal Rocket Engine

Question 2

[Rocket type] wherein the force of externally detonated nuclear explosions is used to accelerate a spacecraft.

Answer 2

Nuclear Pulse Rocket

Question 3

[Engine type] that uses a nuclear reactor to heat a gas (generally hydrogen) to high temperatures before expelling it through a nozzle to produce thrust.

Answer 3

Nuclear Thermal Rocket (NRT)

Question 4

[Engine type] wherein fuel ignites to form a gas which is subsequently discharged through a nozzle to produce thrust. The fuel in this case is also the propellant; that is, the substance that is used to generate thrust.

Answer 4

Chemical Engines

Question 5

[WRT chemical engines] When the temperature of the exhaust gases is limited by the amount of energy which may be extracted from the fuel as it reacts.

Answer 5

“Energy limited”

Question 6

[WRT NTRs] When the main limitation results from restrictions on the rate at which energy can be exracted from the nuclear fuel and transferred to the propellant. The rate of energy transfer is limited by the maximum temperature the nuclear fuel can withstand, thereby putting an upper limit on the maximum efficiency attainable

Answer 6

“Power density limited”

Question 7

[R&D] Nuclear Engine for Rocket Vehicle Applications began in the late 1950s. Last tests conducted in the 1970s.

Answer 7

“NERVA”

Question 8

[R&D] Uranium carbides and cermets (ceramic metals)

Answer 8

Two new promising fuel types post-NERVA

Question 9

[R&D] The Particle Bed Reactor was an Air Force NTR engine concept, canceled in early 1990s after fall of the Soviet Union. Had a very high thrust-to-weight ratio and was used in a ballistic missile interceptor / Timberwind program.

Answer 9

PBR

Question 10

[R&D] Soviet Union program from 1965-1980s, NTR was relatively smaller than NERVA engines, fuel elements made of uranium/tungsten carbide material (allowed for higher temps than NERVA). AAR, engines were slightly more efficient than NERVA.

Answer 10

RD-410

Question 11

[Components] Support elements in reactor core that supported six adjacent fuel elements in single grouping

Answer 11

Cluster

Question 12

[Components] Used to reflect back into the core neutrons emanating from the fuel which would normally escape the reactor. Composed of Beryllium. Conserves neutrons and leads to smaller more compact designs.

Answer 12

Reflector

Question 13

[Components] These serve as a control mechanism by which the reactor power can be varied. They operate by varying the number of neutrons which escape the core. Composed of beryllium cylinders with a sheet of material that strongly absorbs neutrons on one side (usually boron carbide)

Answer 13

Control drums

Question 14

[Engine design] Two concentric porous pipes in between which is supported a bed of tiny fuel particles. Hydrogen propellant flows through the walls of the outer cold ____, through the fuel particle bed where it is heated to high temperatures, and finally exits through the walls of the inner hot ____.

Answer 14

Frits

Question 15

[Engine type] Operates by expelling a high-temperature gas through a nozzle to produce thrust. This thrust acts to accelerate a spacecraft in the direction opposite to that of the expelled gas.

Answer 15

Rocket engines (chemical or nuclear)

Question 16

[Components] Converts the thermal energy in the hot propellant to kinetic energy in the form of a directed high-speed exhaust flow parallel to the line of flight, but in the opposite direction.

Answer 16

Nozzle

Question 17

The force produced by the rocket engine as a result of the time rate of change of momentum of the exhaust gas which is accelerated through the rocket engine nozzle.

Answer 17

Thrust

Question 18

[Measurement] Represents length of time one pound of propellant can produce one pound of thrust (or produce 1 N of thrust from 1 kg of propellant). Most useful parameter in determining efficiency of a rocket engine. Has units of seconds.

Answer 18

Specific Impulse (Isp)

Question 19

[Equation] Its solution yields the maximum velocity attainable by a space vehicle for a given engine-specific impulse and vehicle mass fraction .

Answer 19

The Rocket Equation

Question 20

[Measurement] The ratio of the fluid velocity to the speed of sound in the fluid.

Answer 20

Mach number

Question 21

When the Mach number is less than 1 and the fluid flow is traveling at a velocity less than the speed of sound.

Answer 21

Subsonic

Question 22

When the Mach number is greater than 1 and the fluid flow is traveling at a velocity greater than the speed of sound.

Answer 22

Supersonic

Question 23

[Equation] The relationship that allows the area ratio at any point in the nozzle as referenced to the area at the nozzle sonic point to be determined.

Answer 23

Mach-area relationship

Question 24

[Pros & Cons] Where do NTR engines achieve their specific impulse advantages from versus Space Shuttle Main Engines for e.g.?

Answer 24

Lower molecular weight of hydrogen propellant. Not from greater chamber temperatures (actually somewhat lower than those produced in SSMEs).

Question 25

[Pros & Cons]:
1. High cycle efficiencies resulting from the low bleed flow required to drive the turbopump and the relative simplicity of the engine.

Answer 25

Hot Bleed Cycle’s Main Advantages

Question 26

[Pros & Cons]:
1. The portion of the bleed flow which is diverted from the core exit plenum will be quite hot and hard on any valves and piping in contact with it prior to its being mixed with the bleed flow shunted off before it would have entered the reactor core.

Answer 26

Hot Bleed Cycle’s Main Disadvantages

Question 27

[Pros & Cons]:

1. High turbopump reliability that results from the low turbine inlet temperatures;
2. Relative simplicity of the engine.

Answer 27

Cold Bleed Cycle’s Main Advantages:

Question 28

{Pros & Cons]:

1. The chamber pressures tend to be low because of the limited amounts of power available to the turbopump from the nozzle and chamber regenerative cooling flow;
2. Relative inefficiency of the cycle due to the waste of significant amounts of propellant from the bleed dump discharge from the turbopump.

Answer 28

Cold Bleed Cycle’s Main Disadvantages:

Question 29

[Pros & Cons]:
1. High turbopump reliability that results from the low turbine inlet temperatures and the efficient use of propellant resulting from the fact that no propellant bleed dump is required as in the bleed cycles.

Answer 29

Expander Cycle Main Advantage:

Question 30

[Pros & Cons]:

1. Chamber pressures tend to be low because of the limited amounts of power available to the turbopump from the nozzle and chamber regenerative cooling flow;
2. Relative complexity of the engine itself resulting from the additional flow paths required.

Answer 30

Expander Cycle Main Disadvantages: