Blasting Off To The ISS - Liquid Rocket Engines

Launch Rocket

The Karman Line, defined to be 100,000 kilometers (62 miles) above the earth, is generally regarded as the boundary point between earth's atmosphere and outer space. (However, there is not a legal agreement as far as regulatory measures go regarding space operations.) The International Space Station (ISS) is located 254 miles above the earth. For the purposes of this web page, "Deep Space" means space beyond the Moon which on average is about 240,000 miles from earth.

The rockets that currently take people and supplies to the ISS are chemical rockets that use liquid fuel as a propellant. Liquid rocket engines are used to place humans in orbit using the Space Shuttle and on many un-manned trips to place satellites in earth orbit. Rockets create thrust by expelling mass backwards at high velocity

In a liquid rocket, stored fuel and stored oxidizer are pumped into a combustion chamber where they are mixed and burned. See the NASA schematic to the left below. The combustion process produces great amounts of exhaust gas at high temperatures and pressures. The hot exhaust is passed through a nozzle which accelerates the flow. Forward thrust is produced according to Newton's third law of motion. The amount of thrust produced by the rocket depends on the mass flow rate through the engine, the exit velocity of the exhaust, and the pressure at the nozzle exit. All of these variables depend on the design of the nozzle.

Liquid Rocket Engine

The smallest cross-sectional area of the nozzle is called the throat. The hot exhaust flow is choked at the throat, and the mass flow rate m dot is determined by the throat area. The area ratio from the throat to the exit sets the exit velocity Ve and the exit pressure Pe.

Since the oxidizer is carried on board the rocket, rockets can generate thrust in a vacuum where there is no other source of oxygen. That's why a rocket will work in space, where there is no surrounding air, and a gas turbine (or propeller) will not work. To fly in the air around Earth, jet engines rely on the elements of the air to provide the working fuel and the oxygen in the air is the oxidizer for combustion.

The details of how to mix and burn the fuel and oxidizer, without blowing out the flame, are very complex. It does take a "rocket scientist" to figure it out!

Electric type rocket engines are far less powerful than chemical rockets, so they are not used to launch rockets into space. But once in orbit, electric rockets have some big advantages. They are far more efficient on a unit of propellant mass basis than chemical rockets. And because they rely on electricity, they can be powered by solar panels with very few other ingredients necessary.  Top

Electric Rockets

Deep Space 1

NASA plans to extend human presence across the solar system, including the Moon and Mars. A one-way trip to Mars using conventional chemical rockets could take up to nine months. It’s a long time for a human crew to spend in a spaceship exposed to radiation and other hazards. That’s one reason why NASA and private industry, are pursuing different types of rocket technologies beyond the SpaceX Falcon Heavy, a 230-foot-tall engineering marvel, with ability to ferry loads of up to 141,000 pounds into low-earth orbit.

NASA’s Solar Electric Propulsion (SEP) project is developing critical technologies that will enable us to extend the scope of new exploration missions and support a wide variety of journeys to destinations in deep space. See the artist's conception of NASA's Deep Space 1 to the left showing both the solar panels and ion engine (with the blue exhaust).

Energized by the electric power from on-board solar arrays, the electrically propelled system will use 10 times less propellant than a comparable, conventional chemical propulsion system. The system will boost robotic and crewed missions well beyond low earth orbit. Solar Electric Propulsion (SEP) will send exploration spacecraft to distant destinations and ferry cargo to and from points of interest. In 2015, NASA tagged three startups to develop solar electric propulsion (SEP) systems, providing each with a three-year grant as part of the agency’s Next Space Technologies program.

SEP solar panels provide free electricity on a spacecraft in combination with a powerful ion thruster. (Solar energy may also be temporarily stored in a battery inside the spacecraft.) Ion thrusters are being designed for a wide variety of missions - from keeping communications satellites in the proper position to propelling spacecraft throughout the solar system. Ion propulsion is considered to be mission enabling for cases where sufficient chemical propellant cannot be carried, because of its weight, to accomplish the desired mission. A human spacecraft would need 600 to 800 kilowatts of electrical power coupled with an ion engine with a specific impulse of 2000 to 2500 seconds. (The specific impulse is a measure of how effectively a rocket uses propellant or a jet engine uses fuel. By definition, it is the total change in momentum delivered per unit of propellant consumed. If the propellant's weight (pounds of force) is used, then the specific impulse has units of time - seconds). The most common propellant used in ion propulsion is xenon, which is an easily ionized gas, has a high atomic mass, thus generating the desirable level of thrust when ions are accelerated. It also is inert and has a high storage density; so it is well suited for storing on spacecraft.

Ion thrusters have been used for years on satellites and even some deep space missions. In 2015, for instance, ion engines powered NASA’s Dawn probe into an orbit around the dwarf planet Ceres, which sits in an asteroid belt between the orbits of Mars and Jupiter.  Top

Solar Sails And Arrays

Solar Sail

Particles of light, called photons, are like ping pong balls bounding off a wall. The solar sail idea is to catch enough of them to pick up significant thrust. A spacecraft with a large enough sail could eventually reach incredible speeds. While photons have no mass, they have momentum.

Solar sails capture this momentum with sheets of large, reflective material such as Mylar. As photons bounce off the sail, most of their momentum is transferred, speeding up the sail in the direction opposite the bouncing light. The concept was proven in 2010 when Japan’s project Interplanetary Kite-craft Accelerated by Radiation Of the Sun (IKAROS) unfurled a 196-square-meter sail for propulsion on a mission to Venus.

The LightSail project showed this approach can work on a shoestring budget. LightSail is a crowd funded solar sail project from The Planetary Society. The LightSail 2 spacecraft launched on the 25th of June 2019 as a secondary payload on a Falcon Heavy rocket. See an Artist's depiction of the LightSail to the left.

Its solar sail was deployed on July 23, and successfully down linked photographs of the deployed sail on July 24. It is the first spacecraft in Earth orbit propelled solely by sunlight. On 31 July 2019, LightSail announced it had officially raised LightSail 2’s orbit by a measurable amount, demonstrating that pure solar sails are a viable means of propulsion.

The SEP project has been developing large, flexible, radiation-resistant solar "arrays" that can be stowed into small, lightweight, cost-effective packages for launch. After launch, they will be unfurled to capture enough solar energy to provide the high levels of electrical power needed to enable powerful solar "electric propulsion" as opposed to pure solar sail propulsion. See the solar thrust sections below.

However, as a solar sail or array moves away from the Sun, the sunlight becomes feebler, and the available thrust then decreases. More ambitious designs imagine travel to the nearest stars (not planets) by firing a massive laser to fill the sail, store the new energy in a battery, and then use it to power through the interstellar doldrums.  Top

The Hall Thruster

Hall Thruster

Hall Thrusters are electrostatic ion thrusters that utilize an inert gas, mostly xenon, as a propellant. They are named after Edwin Herbert Hall of John Hopkins University who discovered the Hall Effect in 1879. The initial work on Hall Effect thrusters was carried out independently both in the United States and the Soviet Union in the 1950s and '60s. Because of inefficiency in the early US designs, development of this type of thruster came to a halt in the US about 1970.

However, continuing research in the Soviet Union into the ion acceleration mechanisms led to the Hall Thruster becoming an efficient propulsion device. With the end of the Cold War, this technology became available in the West. Since Hall Thrusters use an inert gas such as xenon for the propellant, there is no risk of explosion as there is with chemical rockets.

When the propellant ions of xenon are generated, they experience the electric field produced between the channel (positive) and the ring of electrons (negative) and accelerate out of the thruster, creating an ion beam. The thrust is generated from the force that the ions impart to the electron cloud. The cathode electrons are highly mobile and are attracted to the ions in the beam, causing an equal amount of electrons and ions to leave the thruster at the same time. This keeps the electrical charge of the thruster neutral.

Hall thrusters have a specific impulse typically in the range 1,200 to 1,800 seconds - much higher than the 300 to 400 seconds of chemical rockets. However, they provide a much lower thrust. A modern Hall thruster can deliver up to 3 newtons (0.7 pounds) of thrust, which is equivalent to the force you would feel by holding 54 US quarters in your hand. The high specific impulse enables a spacecraft powered by a Hall thruster to reach a top speed of about 112,000 miles per hour (50,000 meters/sec.). The low thrust, on the other hand, means that weeks or months are needed to attain that speed.  Top

The Gridded Ion Thruster

Gridded Ion Thruster

The gridded ion thruster is a common US design for ion thrusters. They are a highly efficient low-thrust spacecraft propulsion system. The designs use high voltage grid electrodes to accelerate ions to create thrust. Instead of ejecting combustion gases that produce thrust in chemical rockets, ion thrusters apply force to move an object by ionizing an inert gas like xenon with an electric charge (usually from solar or nuclear materials). A spacecraft powered by several ion thrusters could theoretically streak through the cosmos at more than 200,000 mph, according to NASA.

Electrons are fired from a gun at the neutral xenon atoms in a chamber creating positively charged ions. The resulting chamber gas consists of positive ions and negative electrons (a plasma). See the gridded ion thruster diagram at the left. The positively charged ions migrate towards the grids that contain thousands of very precisely aligned holes (apertures) at the end of the thruster. Each set of apertures acts as a lens that electrically focuses ions out of the thruster creating thousands of small ion jets.

NEXT Thruster

The first grid is a positively charged electrode. A very high voltage is applied to the positive grid to force the plasma to reside at a high voltage. As ions pass between the grids, they are accelerated toward a highly negatively charged electrode. The positively charged ions are accelerated to very high speeds out of the thruster. The stream of all the tiny ion jets together is called the ion beam which produces the forward thrust.

The neutralizer (at the top of the above diagram) is another hollow cathode that expels an equal amount of electrons to make the total charge of the exhaust beam neutral. (Without a neutralizer, the spacecraft would build up a negative charge and eventually ions would be drawn back to the spacecraft reducing thrust.)

NASA’s Evolutionary Xenon Thruster (NEXT - pictured to the left under test) is a gridded ion thruster. NEXT is projected to be three times as powerful as the Deep Space 1 thruster (pictured in the Electric Rockets section above). NEXT will deliver larger payloads, allow a smaller launch vehicle size, and make possible other mission enhancements.

The NEXT engine accelerates the xenon propellant to speeds of up to 90,000 mph. and has a specific impulse of 4,190 seconds which is the highest impulse ever demonstrated by an ion thruster. NEXT was developed by NASA and is being manufactured by Aerojet Rocketdyne. The NEXT-C will be the ion thruster used on a 2021 mission named DART. DART is a mission that will demonstrate the effects of crashing a spacecraft into an asteroid for planetary defense purposes. The mission is intended to test whether a spacecraft impact could successfully deflect an asteroid on a collision course with earth.

The mission's target is Didymos, a binary asteroid system in which one asteroid is orbited by a smaller one. The primary asteroid is about 2,600 feet in diameter; its small satellite is about 490 feet in diameter in an orbit about 3/4 of a mile from the primary. DART will target the smaller asteroid. Didymos is not an earth crossing asteroid, and there is no possibility that the deflection experiment could create an impact hazard. The project is strictly a test demonstration.  Top

Plasma Engines

Plasma Engine

Plasma engines are like high-octane versions of the ion drive. Magnetic currents and electrical potentials accelerate ions in a plasma to generate thrust. The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a plasma propulsion thruster under development for possible use in long range spacecraft. It uses radio waves to ionize and heat an inert propellant, then a magnetic field to accelerate the resulting plasma, generating thrust.

The VASIMR engine is being developed by Ad Astra Rocket Company, headquartered in Houston, TX. A Nova Scotia, Canada-based company Nautel, is producing the 200 kW RF generators required to ionize the propellant. Some component tests and "Plasma Shoot" experiments are performed in a Liberia, Costa Rica laboratory. This project is led by former NASA astronaut Dr. Franklin Chang-Díaz.

Advantages.  Plasma engines have a much higher specific impulse than most other types of rocket technology. The VASIMR engine is capable of reaching a specific impulse of over 12,000 seconds, while hall thrusters can reach about 2,000 seconds and girdded thrusters about 4,200. This is much higher than chemical propellants that reach a specific impulse of 450 seconds. With a very high specific impulse, these rockets are capable of reaching relatively high speeds. Ex-astronaut Franklin Chang-Diaz claims his VASIMR engine could send a payload to Mars in as little as 39 days while reaching a maximum velocity of about 120,000 miles per hour. (The speeds and time range are about the same for other proposed plasma rockets.)

Drawbacks.  For plasma thruster technologies, one of the biggest problems is generating enough electricity to turn the gases into plasma. This is also a problem for Diaz's VASIMR thruster. Diaz's device would need so much electricity, that the engine would need several nuclear reactors in order to generate enough power. Not only would the reactors add significant mass to the payload, but this has caused concern by some who fear a possible failure caused by an explosion of a reactor. Another common issue plasma rockets have run into is the possibility of the rocket breaking up. Over time, the plasma these rockets produce will damage the walls of the device, ultimately causing it to break. This means that on a mission to Mars, it is possible that the rocket could destroy itself. Lastly, due to their low thrust, plasma engines are not suitable for sending large payloads into space. On average this type of rocket provides about 2 pounds of thrust maximum. This is a problem since in order to be financially efficient, heavy payloads need to be sent up every time a mission is scheduled. While plasma engines could take over once in space, chemical rockets would still be needed to launch the vehicle. Although plasma rocket "ideas" are a half century old, no plasma rocket has ever made it to space.