. The Cryogenic Rocket Engines

. Advanced Propulsion Technologies

. Some others Rockets Engines

Note:The LE-9 is a liquid cryogenic rocket engine burning liquid hydrogen and liquid oxygen in an expander bleed cycle. Two or three will be used to power the core stage of the H-3 launch vehicle. Credit: SPACE and ISRO news

 
 

 
 
 
 

 
 

MIT Science Reporter — "Landing on the Moon" (1966). Credit: From the Vault of MIT

HELIOS is an advanced solar sail concept being evaluated by NASA's Space Technology Mission Directorate (STMD).

NASA's STMD rapidly develops, demonstrates, and infuses revolutionary, high-payoff technologies through transparent, collaborative partnerships, expanding the boundaries of the aerospace enterprise.

http://www.nasa.gov/spacetech

NASA SLS Rocket Engine Test-Fired in Mississippi

The RS-25 rocket engine had a “500-second test on the A-1 Test Stand at NASA’s Stennis Space Center in Mississippi,” according to space agency. Four of these engines will be able to produce 2 million pounds of thrust for the Space Launch System (SLS) rocket. Credit: VideoFromSpace

The Space Technology and Exploration Directorate at NASA Langley Research Center presented a five-part lecture series on “The Future of America’s Space Exploration Program” on the campus of Christopher Newport University. Each 75-minute lecture took place at the Yoder Barn Theatre in Newport News, Virginia and explores the ins and outs of America’s vision for deep space exploration.

Lecture 1: Path to Mars and Asteroid Redirect Mission: The First Step

Lecture 2: The Next Human Spacecraft: Orion and the Launch Abort System

Lecture 3: Escaping Earth’s Gravity: Space Launch System

Lecture 4: Mars Entry, Descent and Landing with Humans

Lecture 5: Spacecraft, Habitats and Radiation Protection

NASA Langley researchers want to get a better idea about conditions on our nearest planetary neighbor, Venus, so they have come up with HAVOC or a High Altitude Venus Operational Concept – a lighter-than-air rocket ship that would help send two astronauts on a 30-day mission to explore the planet’s atmosphere. Exploration of Venus is a challenge not only because its smog-like sulfuric acid-laced atmosphere, but also its extremely hot surface temperature and extremely high air pressure on the surface.

Some day astronauts may live on other planets outside the Earth's protective magnetic field. But to do that engineers must reduce astronaut exposure to dangerous space radiation. NASA's Langley Research Center is working with other NASA centers to design better space habitats.

Engineers at NASA Langley Research Center are studying ways to shield future space explorers from radiation, especially on another planet. Radworks is an Advanced Exploration System (AES) project to address detection, monitoring and protection of radiation that will be needed for human exploration beyond low Earth orbit

Some others Rockets Engines

The RD-180 is built by RD AMROSS, a joint effort between Aerojet Rocketdyne and NPO Energomash. This Russian engine use a LOX-Kerosene propellant mixture that powers the Common Core Booster (CCB) of the Atlas V vehicle (and the Delta IV), with a thrust of about 3,830 kN (860,000 lbf).

However, world events and market driven competition has removed the RD-180 from the supply chain. In fact, the National Defense Authorization Act of 2015 limits the use of the RD-180 for national security missions and the government has directed a replacement engine be in operation by 2019.

The RD-181 engine is also developed by NPO Energomash for the Antares vehicle, built by Orbital ATK. The original Antares, which was used on four missions, used two AJ26 engines on its first stage. The AJ26 was essentially a significantly modified NK-33 engine. Aerojet Rocketdyne purchased 36 of the original 150 NK-33 engines, which were inspected, refurbished, and designated AJ26. Following the loss of the fourth Antares vehicle in October 2014 due to an engine failure, Orbital ATK moved to replace the engines on future Antares vehicles. In 2015, Orbital ATK contracted with NPO Energomash for 20 RD-181 units. The Antares will feature two LOX-kerosene RD-181 engines, each producing about 1,913 kN (430,000 lbf) of thrust. The first launch of the Antares using the new engines took place in 2016.

The Merlin 1D is the engine used to power both the first and second stages of SpaceX’s Falcon 9 and Falcon Heavy launch vehicles. This engine produces about 756 kN (185,500 lbf) of thrust and burns a LOX-kerosene mixture. Nine of these engines power the Falcon 9 first stage (for a total thrust of about 6,806 kN or 1,530,000 lbf) and one is used to power the second stage.

The Merlin 1D is a fourth generation SpaceX engine that traces its lineage to the Merlin 1A that powered the Falcon 1 vehicle. The Merlin 1A leveraged technology developed for NASA’s Fastrac engine, which used a pintle single-feed injector as opposed to the more typical arrangement of hundreds of injector holes. The Merlin 1C was used for the Falcon 9 v1.0 vehicle, whereas the Merlin 1D powers the Falcon 9 v1.1 vehicle. The more powerful Falcon 9 Full Thrust (Falcon 9 FT) will feature a higher thrust capability, giving the vehicle a 30 percent increase in performance from the Falcon 9 v1.1. This upgraded vehicle was introduced in late 2015.

The Newton series of engines being developed by Virgin Galactic will power the company’s air-launched LauncherOne vehicle. These engines use LOX and kerosene as propellants. The NewtonThree, which produces 327 kN (73,500 lbf) of thrust, will power the LauncherOne first stage. A NewtonFour engine, producing 22 kN (5,000 lbf) of thrust, will power the second stage to orbit. First flight of LauncherOne is expected in 2017.

Rocket Lab has designed the Rutherford engine for use in the first stage of the company’s Electron vehicle. The engine burns a mixture of LOX and kerosene, producing a thrust of about 22 kN (5,000 lbf). Rocket Lab is employing additive manufacturing (3D printing) in the construction of all primary components of the Rutherford, making it a unique example in the industry. 3D printing reduces costs by simplifying the manufacturing process. The first launch of the Electron is expected in 2017 from a site in New Zealand.

XCOR Aerospace has been developing engines since 2000, when the company fully integrated the XR-3A2 and XR-4A3 into an EZ-Rocket test aircraft. Currently, XCOR is developing the XR-5K18 engine for the company’s Lynx suborbital vehicle. The XR-5K18 burns a LOX and kerosene propellant mixture, producing a thrust of about 13 kN (2,900 lbf). The Lynx will be powered by four XR-5K18 engines. The company is a partner with ULA on the development of a LOX-liquid hydrogen upper stage engine, capable of producing up to 130 kN (30,000 lbf) of thrust. This effort leverages technologies developed for the XR-5K18.

FRE-1 and FRE-2: Firefly Space has developed the FRE line of engines to power the first and second stages of its Alpha launch vehicle. The FRE-2 is an aerospike engine that, if successful, may prove to be the first aerospike engine employed in an operational launch system. An aerospike does not feature a traditional bell-shaped nozzle, which reduces weight but also reduces exhaust pressure (specific impulse). Aerodynamic design is used to counter this loss of pressure and increase efficiency. The engine burns LOX and kerosene to produce a thrust of about 443 kN (99,600 lbf). The FRE-1 is a conventional nozzle engine burning the same propellant mixture to produce 28 kN (6,200 lbf) of thrust.

 
 

 Six countries have successfully developed and deployed the Cryogenic Rocket Engines

Cryogenic rocket engine for first stage

Some examples about Staged Combustion

The staged combustion cycle, or pre-burner cycle, is a thermodynamic cycle used in some bipropellant rocket engines. One propellant is sent through a pre-burner and is partially burned in using a small portion of the second propellant. The resulting hot gas is used to power the engine's turbines and pumps, then injected into the main combustion chamber along with the remainder of the second propellant to complete the combustion. There are two
main variants of the cycle depending on which propellant is sent through the pre-burner: the oxidizer-rich staged combustion (ORSC) and the fuel-rich staged combustion (FRSC).

The Aerojet Rocketdyne RS-25, or Space Shuttle Main Engine (SSME), is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is planned to be used on its successor, the Space Launch System (SLS). The American engine RS-25 burns cryogenic liquid hydrogen and liquid oxygen (LH2/LOX) propellants producing 1,859 kN (418,000 lbf) of thrust at liftoff for each one used in the Shuttle. Although the RS-25 can trace its heritage back to the 1960s, concerted development of the engine began in the 1970s, with the first flight occurring on April 12, 1981, with the STS-1.

The RS-25E, built by Aerojet Rocketdyne, is an expendable version of the RS-25, also called the Space Shuttle Main Engine (SSME). Four RS-25E engines will be used for each core stage of NASA’s upcoming SLS. Actually, sixteen of these engines are available for the SLS missions, which begin in late 2018. The RS-25E will be used on subsequent SLS vehicles. Each RS-25E will burn a Liquid Oxygen-Liquid Hydrogen (LOX-LH2) propellant mixture to produce about 2,277 kN (512,000 lbf) of thrust. Though the original SSMEs were expensive, NASA is working with Aerojet Rocketdyne to develop manufacturing methods to increase performance while at the same time reduce the per-unit cost.

The RS-27A is the engine used to power the core stage of the Delta II. Also developed by Aerojet Rocketdyne, the RS-27A burns LOX and kerosene, producing a thrust of about 890 kN (200,100 lbf).

About the Gas-Generator cycle

The J-2 underwent several minor upgrades over its operational history to  improve the engine's performance. This is be done with two major upgrade programs: the de Laval nozzle-type J-2S and theaerospike-type J-2T, which were cancelled after the conclusion of the Apollo program. 

The gas-generator cycle is a power cycle of a bipropellant rocket engine. Some of the propellant is burned in a gas-generator given a hot gas used to power the engine's pumps. The gas is then exhausted. Becausesomething is "thrown away" this type of engine is also known as open cycle.

The J-2 Engine

The J-2 was a liquid-fuel cryogenic rocket engine used on NASA's Saturn IB and Saturn V launch vehicles. Built in the U.S. by Rocketdyne, the J-2 burned cryogenic LH2/LOX propellants, with each engine producing 1,033.1 kN (232,250 lbf) of thrust in vacuum.  The engine's preliminary design dates back to recommendations of the 1959 Silverstein Committee.  Rocketdyne won approval to develop the J-2 in June 1960 and the first flight occurred in February 1966. 

The engine produced a specific impulse (Isp) of 421 seconds (s) in vacuum or 200 s at the sea level and had a mass of about 1,788 kilograms (kg). In the past, five J-2 engines were used on the Saturn V's S-II second stage, and one on the S-IVB upper stage of the Saturn IB and Saturn V. Some proposals to using various numbers of J-2 engines in the upper stages of an even larger rocket, the planned Nova, has been done.

The J-2 was the America's largest production LH2-fuelled rocket engine before the RS-25 SSME. A modernized version of the engine, the J-2X, is intended to be used on the Earth Departure Stage of NASA's Space Launch System.

The Rocket System 68

The Aerojet Rocketdyne RS-68 is a liquid-fuel rocket engine that uses LH2/LOX as propellants in a gas-generator power cycle. It is the largest hydrogen-fueled rocket engine.

Unlike most liquid-fuelled rocket engines in service at the time, the J-2 was designed to be restarted once after
shutdown when flown on the Saturn V S-IVB third stage. The first burn, lasting about two minutes, placed the Apollo spacecraft into a low Earth parking orbit. After the crew verified that the spacecraft was operating nominally, the J-2 was re-ignited for translunar injection, a 6.5-minute burn which accelerated the vehicle to a course for the Moon.

The RS-68A is an updated version of the RS-68, with changes to provide increased specific impulse and thrust to over 700,000 pounds-force (3,100 kN) at sea level. The first launch used three RS-68A engines mounted in a Delta IV Heavy occurred in June 2012 from the Cape Canaveral Air Force Station, Florida.

The Expander Cycle

However, the volume of fuel that must be heated increases as the cube of the radius. Thus, it exist a maximum engine size of approximately 300 kN of thrust beyond which there is no longer enough nozzle area to heat enough fuel to drive the turbines and hence the fuel pumps.

Higher thrust levels can be achieved using a bypass expander cycle, where a portion of the fuel bypasses the turbine and/or thethrust chamber cooling passages and goes directly to the main chamber injector. Non-toroidal aerospike engines do not suffer from the same limitations because the linear shape of the engine is not subject to the square-cube law. As the width of the engine increases, both the volume of fuel to be heated and the available thermal energy increase linearly, allowing arbitrarily wide engines to be constructed. All expander cycle enginesneed to use a cryogenic fuel such as hydrogen, methane, or propane that easily reach their boiling points.

The Rocket Engine RL10

The expander cycle is a power cycle of a bipropellant rocket engine. In this cycle, the fuel is used to cool the engine's combustion chamber, picking up heat and changing phase. The heated gaseous fuel then powers the turbine that drives the engine's fuel and oxidizer pumps before being injected into the combustion chamber and burned.

Because of the necessary phase change, the expander cycle is athrust limited by the square-cube rule. As the size of a bell-shaped nozzle increases with increasing thrust, the nozzle surface area, from which the heat can be extracted to expand the fuel, increases as the square of the radius.

The RL10 is a liquid-fuel cryogenic rocket engine used on the Centaur, S-IV and Delta Cryogenic Second Stage (DCSS) upper stages. Built again in the United State by Pratt & Whitney Rocketdyne, the RL10 burns cryogenic LH2/LOX propellants, with each engine producing 64.7 to 110 kN (14,545–24,729 lbf) of thrust in vacuum depending on the version in use. The RL10 was the first liquid hydrogen rocket engine to be built in the USA by Marshall Space Flight Center and Pratt & Whitney in 1950s - the first flight occurring in 1961. After that, several versions have been flown, as theRL10A-4-2 and the RL10B-2, while still being produced and flown on the Atlas V and Delta IV.

The engine produces a specific impulse (Isp) of 373 to 470 s (3.66–4.61 km/s) in a vacuum and has a mass ranging from 131 to 317 kg (289–699 lb), depending of the version. Six RL10A-3 engines were used in the S-IV second stage of the Saturn I rocket, one or two RL10 engines are used in the Centaur upper stages of Atlas and Titan rockets and one RL10B-2 is used in the upper stage of Delta IV rockets.

Some expander cycle engines may use a gas-generator to start the turbine and run the engine until the heat input from the thrust chamber and nozzle skirt increases as the chamber pressure builds up.

Called also Open cycle, only some of the  fuel is heated to drive the turbines, which is then vented to atmosphere to increase the turbine efficiency.  While this increases the power output, the dumped fuel leads to a decrease in propellant efficiency, i.e. a lower engine specific impulse.

A closed cycle expander engine sends the turbine exhaust to the combustion chamber. Some others examples of an expander cycle engine are the Pratt & Whitney RL60 and the Vinci engine for the future Ariane 5 ME.

The combustion tap-off cycle

The combustion tap-off cycle is a power cycle of a bipropellant rocket engine. The cycle takes hot gases from the main combustion chamber and routes them through engine turbo pump turbines to pump fuel, then is exhausted. Since not all fuel flows through the main combustion chamber, the tap-off cycle is considered an open-cycle engine. The cycle is comparable to a cycle engine with turbines driven by main combustion chamber exhaust rather than a gas-generator.

The Blue Engine 3 & 4

The Blue Engine 3 (BE-3) is a LH2/LOX rocket engine developed by Blue Origin. The engine beginning its  development in the early 2010s and has completed the acceptance testing task in early 2015. The engine has been used on the Blue Origin New Shepard suborbital rocket for test flights began in 2015. United Launch Alliance (ULA) considered the engine for use in a new second stage, the Advanced Cryogenic Evolved Stage, in ULA's Vulcan orbital launch vehicle with first flight in the 2020s. Placed at the Centaur upper stage, Blue Origin is also considering three engines from various manufacturers for the ACES stage which would begin flights in 2023. The selection is expected before 2019.

The BE-3 follows the earliest rocket engine development efforts in the 2000s. Blue's first engine was a "simple, single-propellant engine" called the BE-1 using peroxide propellant and generating only 8.9 kN (2,000 lbf) of thrust.  Their second engine, the BE-2, was a bipropellant engine using kerosene and peroxide producing 140 kN (31,000 lbf) ofthrust.

The BE-4 is an engine under development by Blue Origin. This 4th generation engine will burn a mixture of liquid oxygen (LOX) and liquefied natural gas (LNG), mostly composed of methane, and produce 2,447 kN (550,000 lbf) of thrust. This is the baseline engine for the company’s orbital launch vehicles and the first stage of ULA’s Vulcan. Blue Origin is planning to have the BE-4 available for operational flights in 2017. The BE-4 is derived from the LOX-liquid hydrogen BE-3, an engine being used for Blue Origin’s New Shepard suborbital launch vehicle.

While development of a sea-level version of the engine was completed and fully qualified in early 2015, Blue intend to develop a vacuum version of the engine to operate in space. That is, in January 2016. The US Air Force provided partial development funding to Orbital ATK to develop an extendable nozzle for the Blue BE-3U.

Recently, Blue Origin has successfully flight-tested the BE-3 engine using a tap-off cycle. According to Blue Origin, the cycle is particularly suited to human spaceflight due to its simplicity, with only one combustion chamber and a less stressful engine shutdown process. However, engine start up is more complicated, and due to its nature of feeding gases from the main combustion chamber into the turbo-pumps, the turbine must be built to withstand higher-than-normal temperatures. And, today...

Sources:

  • http://www.astronautix.com/n/n2o4udmh.html

  • 2015 NASA Technology Roadmaps TA 1: Launch Propulsion Systems

  • 2015 NASA Technology Roadmaps TA 2: In-Space Propulsion Technologies

  • 2015 NASA Technology Roadmaps TA 3: Space Power and Energy Storage

  • The Annual Compendium of Commercial Space Transportation: 2013 & 2017, Federal Aviation Administration, February 2014 & January 2017

  • American Institute of Engineers Symposium on 3D Printing and Additive Manufacturing for Defense and Government • October 21-22, 2014, NASA

MIT Science Reporter—"Computer for Apollo" (1965). Credit: From the Vault of MIT

Advanced Propulsion Technologies

Advanced Propulsion Technologies use chemical or non-chemical physics to produce thrust, but are generally considered to be of lower technical maturity.
These systems have the potential to provide new ways to reach beyond LEO, deliver more mass to destinations, provide ultra-high delta-V capability, process very high power levels, and enable rapid transit times to destinations deep in the solar system.

The Beamed-energy propulsion

The Beamed-energy propulsion uses laser or microwave energy from a ground- or space based energy source and beams it to an orbital vehicle, which uses it to heat a propellant. The advantage isbeing a high exit velocity of exhaust products over traditional chemical propulsion.

In the long-term, the objective is to achieve an orbit transfer in using a high Isp propulsion system that receives power from ground- or space-based stations.

The American challenges is the need for a very large laser with adaptive real-time optics, advanced ceramic composites, cooling, and optics technology.

Electric sails

Electric sails operate through the exchange of momentum between an array of long, electrically-biased wires and solar wind protons, which flow radially away from the sun at speeds ranging from 300 to 700 kilometers per second (km/s). A high-voltage, positive bias on the wires, which are oriented normal to the solar wind flow, deflects the streaming protons and results in a reaction force on the wires that is also directed radially away from the sun. Over a period of months, this small force can accelerate the spacecraft to enormous speeds—on the order of 100 to 150 km/s (~ 20 to 30 AU/year).
Electric sails allow for rapid transit times, estimated to be less than 15 years, to the outer solar system, the heliopause, and beyond (> 250 AU). The main principle that allows this quick
trip time to be achieved is that the electric sail continually accelerates to distances of 30 AU from the sun, which greatly enhances the final velocity achieved versus current or
planned future chemical propulsion or solar sail propulsion.

Fusion propulsion

The Fusion propulsion use fusion reactions to produce the energy required for the primary propulsion of the spacecraft. This can be accomplished indirectly, with a fusion
reactor producing electrical power that is in turn utilized in an electric thruster. It can be achieved also directly in using the thermal/kinetic energy resulting from the fusion reactions to accelerate a propellant.

A variety of fusion propulsion concepts exist:

. In the Inertial Confinement Fusion (ICF) approach, high-intensity laser strike the fusion target from multiple directions to rapidly compress the target to near degenerate densities. This concept can achieve specific impulse of more than 100,000 seconds.
. In the Magnetic Confinement Fusion (MCF) approach, the plasma is contained in the magnetic field and can provide a specific impulse in the 30,000 to 100,000 seconds’ range.
. In the Magneto-Inertial Fusion (MIF) approach, the plasma is confined magnetically while being impacted inertially with external materials from a z-pinch, dense plasma focus, or
plasma gun. MIF fusion can achieve specific impulse in the 30,000 to 70,000 seconds’ range.
. The Inertial Electrostatic Confinement (IEC) concept creates an electrostatic potential between a center and outer shell, which confines and accelerates the plasma.

Some of the fusion approaches may be preceded by a fission-fusion hybrid approach. Contrary to the full fusion propulsion using existing, proven technologies, the fission-fusion hybrid offer a potential of development.

The most popular concept for a steady-state fission-fusion hybrid is applied to power generation and nuclear waste disposal, rather than propulsion applications. Here, the fusion plasma is brought to a condition where it produces neutrons (but not to a gain of unity) to bombard the fission shell and promote more complete burn up of the fission fuel.

In the pulsed of the antimatter catalyzed approach, the initial energy release is driven by a matter-antimatter explosion, releasing enough energy to compress and drive fission reactions. The steady state, antimatter catalyzed approach uses a confined fusion plasma that has injected antimatter and fissionable fuel.

The antimatter catalyzes the fission of the fission fuel, boosting the energy of the fusion fuel. Performance for this hybrid system can offer specific impulse between 5,000 and 100,000 seconds and thrust to weight ratio from 0.1 to well above unity.

The objective for this technology is to demonstrate a fusion propulsion approach in a laboratory environment and measure performance parameters. Challenges include achieving high energy gains, materials withstanding high temperature and radiation, and efficient conversion to directed jet power for thrust. Isp can range from 10,000 to 100,000 seconds.


Fusion reactions require reaching a certain temperature level so that a fraction of the fusion fuel reaches the kinetic energy to crash into another fusing molecule with enough force to overcome their electrostatic repulsion from their respective electron clouds and their nuclei. At that point, the nuclei can fuse, releasing an amount of energy an order of magnitude greater than fission.

Each fusion concept approach this challenge in a different manner. Some achieve very high temperatures and densities for a fraction of a second, while others hold the plasma at lower temperatures for a longer period of time.

Fusion propulsion has the potential to provide low-mass, long-life, very high delta-V capabilities for future missions with power levels in the gigawatts. It has the potential to enable human exploration missions in the outer solar system and robotic exploration in the near-interstellar space. 

High-Energy-Density Materials

High-energy-density materials are characterized by atoms trapped in solid cryogens (e.g., neon). Atomic hydrogen, boron, and carbon fuels are very high-energy-density, free-radical propellants.
Atomic hydrogen may deliver an Isp of 600 to 1,500 seconds. Atom storage density has improved greatly over the last several decades. Lab studies have demonstrated 0.2 and 2  weight percent atomic hydrogen in a solid hydrogen matrix. If the atom storage were to reach 10 to 15 percent, that amount would produce an Isp of 600 to 750 seconds.

Antimatter Propulsion

Antimatter propulsion uses the high energy from antimatter annihilation to increase propulsion performance. The amount of performance gain depends on the amount of antimatter used. Particle accelerators produce nanogram quantities of antiprotons
worldwide each year for science. Small quantities have been stored for short durations by different concepts. The formation of anti-hydrogen has been demonstrated. No propulsion proof-of-concept has been made. The near-term primary challenge is to mature a propulsion concept that only needs small quantities of antimatter (micrograms or less).
The preliminary objective for this category of advanced propulsion is to design and conduct a proof-of-principle experiment to demonstrate a propulsive application (i.e.,
antimatter on target to produce energy for propulsion). The target geometry would  depend on concept approach and could range from a catalyzed pellet to a formed, sailtype material. The experiment would provide data on thrust magnitude, impulse, and overall efficiency. There are two concerns to address. First, bulk annihilation experiments require more experimental data because they are less understood and verified than single-particle annihilations. The generation and storage of the antimatteris a second system concern to address

Advanced Fission

Advanced fission propulsion has three different concepts: gas and liquid core nuclear thermal, fission-fragment, and external-pulsed plasma propulsion. These propulsion uses the fission-produced energy in different ways to get greater Isp than the one achieved by a solid core of a Nuclear Thermal Propulsion (NTP).


Gas and liquid core nuclear thermal concepts have a gaseous or liquid reactor and are similar to solid core NTP. Both gas and liquid core nuclear thermal propulsion concepts heat a propellant, which undergoes thermal expansion and is released through a traditional nozzle. Two gas core concepts have been investigated. The open cycle concept relies on fluid dynamics or electromagnetics
to contain the reactor core and minimize fuel loss. The closed cycle concept relies on a transparent wall to contain the nuclear fuel and uses a seeded propellant. Liquid core concepts rotate the molten reactor fuel.
Fission fragment propulsion uses naturally-occurring fission fragments (FFs) to directly or indirectly produce thrust. Direct thrust utilizing high exit velocity (~3 to 5  percent the speed of light) fission fragments could provide Isp at ~500,000 seconds and thrust of ~10 lbf. Indirect thrust uses the fission fragments in a magnetic beam to heat a propellant and provide higher thrust of ~1,000 lbf, with a reduced Isp at ~30,000 seconds.

External-pulse plasma propulsion operates with small nuclear pulse units ejected out the aft end of the spacecraft, which are then detonated to produce a force on the aft end pusher plate for high thrust and high Isp for large spacecraft. Another pulsed system is the pulsed fission-fusion concept, which strives for much smaller and more rapid pulses, triggered by an external z-pinch. Steady state concepts exist, including the toroidal
hybrid reactor and the antiproton gas dynamic mirror. External-pulsed plasma propulsion has the potential for high thrust of approximately 500,000 lbf and high Isp of greater than 5,000 seconds.
Gas and liquid core NTP provide high thrust and Isp two to three times that of solid core NTP, allowing faster trip times. The FF propulsion has the ability to vary the mass flow of an added neutral propellant so that the engine thrust and Isp can
be optimized for faster missions beyond Mars without altering the engine. Externalpulse plasma propulsion provides a simple and efficient direct use of nuclear energy to generate high thrust and Isp for large spacecraft with destinations to Mars and beyond.

Breakthrough Propulsion

Breakthrough propulsion is an area of technology development that seeks to explore and develop a deeper understanding of the nature of space-time, gravitation, inertial frames, quantum vacuum, and other fundamental physical phenomena with the overall objective of developing advanced propulsionapplications and systems that will revolutionize space exploration.

Source: NASA Technology Roadmaps TA 2: In-Space Propulsion Technologies

Cryogenic Rocket Engine for Upper Stage