Escaping the grip of Mother Earth

As we saw in our last article on spacecraft propulsion, getting your craft off the ground and into space – let alone into orbit – can be a difficult and expensive affair. But if you intend for it to leave the Earth altogether, launch only gets you half the way: in orbit, the outwards motion of the spacecraft is, popularly speaking, held in check by the inwards gravitational force exerted upon it by the Earth, keeping it on a elliptical trajectory around the planet. To break free of the Earth’s grasp entirely, the spacecraft must do more than just repel the planet’s gravity – it has to overcome it. This threshold is known as the planet’s escape velocity.

The particular escape maneuver to send a spacecraft on a trajectory from Earth orbit to an orbit around the Moon is called a Trans Lunar Injection (TLI). During the Space Race, TLI’s were typically performed by the launch vehicle itself. On Apollo 8, the first manned mission to leave Earth orbit, for instance, the third stage of the rocket assisted in driving the craft into Earth orbit but remained attached to later perform the TLI.

NASA)

The S-IVB third stage of the Apollo 8 Saturn V, shortly after separation from the Command/Service Module. (Source: NASA)

But since the days of Apollo, interest in the Earth’s distant natural satellite has waned, and few rockets built today have upper stages powerful enough to perform TLI: the telecommunications and television broadcast satellites, which constitute the majority of payloads launched into space today and act as a relay for radio communication between sites on the ground, have no need to leave Earth orbit. Thus any probes headed towards the Moon will generally be required to carry kick motors and fuel to perform TLI themselves. Conventional kick motors rely on the same chemical propulsive principles as rockets – obtaining thrust from the expulsion of gases produced by the combustion of fuel – but recent years has seen an increase in the development of so-called electric propulsion technologies.

Electric propulsion (EP) engines generate thrust by expelling electrically charged particles (ions) accelerated to very high speeds by electrostatic or electromagnetic fields. Typically, electric thrusters have much better fuel efficiency, or specific impulse, than comparable conventional chemical thrusters. This make them ideal for interplanetary flights; on the enormous distances between astronomical bodies, every gallon of propellant counts.

EP engines has been used on a number of probes and satellites since the 1970’s, including the European Space Agency‘s first Moon probe, SMART-1. During its 3 years long mission, SMART-1’s Hall effect thruster – an engine in which ions are accelerated by an electric field – produced a total increase in speed of about 4 km/s (14,000 km/h or 8,948 mph). That’s 45 m/s (162 km/h or 101 mph) per kilogram of propellant.

Early mission plans from ‘open source’ Google Lunar X Prize contestant Team FREDNET, which intend to land a robotic rover on the surface of the Moon, also considered the feasibility of using electric thrusters for their mission’s lunar transfer, but the team eventually decided against it:

“It not that it’s infeasible,” says Team FREDNET’s Lunar Bus Subsystem Manager, Michael Barrucco, who works with astrodynamics at the Naval Research Laboratory in the daytime. “It can certainly be done. In the spacecraft industry, however, we look for methods with heritage, meaning a system that has shown it can handle all needs of space travel,” he explains. “Electric propulsion has been used, but not often. Chemical engines have been taking us to the Moon for 50 years. I argue – especially with this mission being a race to the finish – that we stick with what we know works well.”

Apart from being a relatively unproven technology, there is also the issue of thrust. Although electric thrusters are very efficient per unit of propellant, their propulsive performance per unit of time is several orders of magnitudes poorer than that of conventional chemical thrusters. This limitation is also the reason that electric thrusters can not be used for launching the spacecraft – the thrust of the EP engine is far too small to make up for Earth’s gravity and atmospheric drag at the ground.

“Space travel is very different from what we see in the text books as homework problems,” Barrucco continues. “Electric propulsion works – we can calculate burn times given engine characteristics, scale fuel needs and such. However, it is less practical in the real world.”

By ‘burn times’  Barrucco refers to the durations of single firings of the engine. Orbital engineers like Barrucco dream of the ‘impulsive burn’ – a firing of the engine that change the velocity of the spacecraft instantly. According to the book Orbital Mechanics for Engineering Students by Howard D. Curtis, an “impulsive maneuver is an idealization by means of which we can avoid having to solve the equations of motion with the rocket thrust included” – or, in short, a simplification of the rocket science involved in changing a spacecraft’s trajectory. Being an idealization, no rocket engine can in fact perform an impulsive maneuver, but it can be approximated:

NASA)

Ion engine in operation. (Source: NASA)

“If the burn is not impulsive you lose efficiency,” Barrucco explains. “To keep the efficiency as high as possible, we can chop up the necessary burns into smaller burns starting a little before the periapsis point and ending a little after. The more times you chop up the burn, the closer to an impulsive maneuver. The higher the thrust, the less we need to chop up the burns.”

This ‘chopping up’ can be done for electric thruster burns as well as it can be done for chemical thruster burns, but in the case of electric thrusters it requires many more maneuvers in the mission plan, along with high-precision tracking of the position and orientation of the spacecraft to time the individual burns correctly, which typically can only be accomplished with support from several ground stations on the surface of the Earth – capabilities which a smaller private enterprise like Team FREDNET do not possess. Hence, Team FREDNET’s best option, from an operational point-of-view, is a higher-thrust, fewer-burns chemical engine.

Additionally, the electric engine requires a supply of electric power. Space probes like SMART-1, using electric thrusters for propulsion, rely on solar energy as their power source. This combination of solar power and electric propulsion, known as solar electric propulsion, has been studied by NASA for several lunar transfer missions, but for a private, non-profit mission like Team FREDNET’s, solar power is simply not affordable. “Solar electric propulsion requires large solar arrays, producing between 2 to 6 kW of electrical energy,” says Ryan Weed, team lead of Team FREDNET’s Propulsion Systems group. “At more than 1 million US dollars per kW of high efficiency multi-junction solar cells, the cost of these components, in addition to their mass, would be a major barrier for a project of this size.”

Michael Barrucco stresses that “the difference is not only in the price for the solar array, but the extra cost of mass as well. The big reason for using electric propulsion is saving fuel mass, but electric propulsion requires other hardware that adds to the mass,” he explains, and says that although he haven’t looked at the exact numbers in terms of mass, he is “betting that the mass difference, because of the high specific impulse, is all but negated by the extra cost of hardware and mass of the bigger solar arrays.”

Electric propulsion is not suited for the so-called retrothrusters that have to be employed to slow the descending spacecraft down before landing on the lunar surface either. At the low thrust the EP engine delivers, the craft would plunge into the ground. “While electric thrusters are very efficient, the thrust to weight ratios are just too small, and usually less than one,” says Ryan Weed. “For example a typical thruster may weigh 5 kg, but would only give a thrust of 50 mN, which on the lunar surface could only hold up 100 grams.”

Thus, with a lander weighing several kilograms, Team FREDNET has no other options than using chemical retrothrusters for landing. And using separate propulsion methods for TLI and landing is not a good idea either, Barrucco explains: “If you use two types of fuel, you add the mass of the tanks and extra valves and electronics to control the new fuel tank.” If the team instead opts to use chemical propulsion for TLI, the “propellant can be used for not only the attitude control system but also for landing,” says Barrucco. Depending on the mission requirements, the same chemical engine may even be shared for TLI and landing which, as opposed to having two separate engines, “would certainly save mass,” according to Barrucco.

Overall, Michael Barrucco is not in doubt as to which method of propulsion he would recommend his team: “In short, with the well-known heritage of chemical propellants, the fact that most hydrazine combinations in a bipropellant system produce a hypergolic reaction needing no spark for ignition, the decreased operations costs, and the fact that the chemical propellant can be used for the attitude control system and for landing as well, chemical propulsion seems like the best choice,” he says. “I will, however, conduct a trade study once the astrodynamics calculations are done to see what the best propulsion system choice will be”.

US Navy)

A SM-3 missile launches from the USS Lake Erie, impacting a non-functioning spy satellite approximately 247 kilometers over the Pacific Ocean to prevent it from releasing toxic hydrazine fuel. (Source: US Navy)

Hydrazine fueled engines have been used on such high-profile missions as the Viking program Mars landers in the late 1970’s as well as the Phoenix lander which is operating on the northern polar cap of Mars right now, after landing on the planet in May. Hydrazine is also sometimes used for thrusters on board satellites orbiting the Earth – most notably the defunct spy satellite USA 193 which caught some press earlier this year when the American government destroyed it with a sea-launched missile, purportedly due to the potential danger of a hydrazine release if the satellite re-entered the Earth’s atmosphere intact. Hydrazine is dangerously unstable and highly toxic – so toxic that the workers fueling hydrazine propelled spacecraft are required to wear big, protective suits when handling the substance. Can such a dangerous chemical even be considered for use in a spacecraft built by amateurs?

“The fueling will most likely need to be contracted out,” says Michael Barrucco. “I have not looked at prices or possible contractors as of yet, but it is definitely possible for a team of our caliber to design a craft to meet all the specifications required for a hydrazine propelled spacecraft,” he affirmed.

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