Designing a propulsion system for landing on the Moon

A look at the propulsion system of the TeamIndus lunar lander

The TeamIndus spacecraft will soft-land on the Moon in 2020. We have designed a robust and unique propulsion system capable of landing a craft on the Moon and operate in different modes meeting the needs of each phase of the mission. Let’s have a look at some of the factors that go into designing the propulsion system of the spacecraft.

Choosing a propulsion system for the journey

As discussed in our previous article on the variables governing rocket science, the destination decides the delta-v required which in effect determines the type of propulsion system that can be chosen.

The delta-v required to go from Low Earth Orbit (LEO) (at an altitude of 250 km) to the lunar surface is ~5.9 km/s. However, since the launch vehicle can put the spacecraft on a trajectory that intersects with the Moon’s orbit, the journey starts from a distance farther than LEO. The delta-v requirement is thus reduced to ~3 km/s.

The main engine on the spacecraft is a liquid rocket engine utilizing Hydrazine (N2H4) as the fuel and an oxidizer majorly composed of Nitrogen Tetroxide (N2O4). The resultant system has a thrust capability of 440 N for major maneuvers which gives us most of the required delta-v for the mission.

Bottom view of the spacecraft showing the main engine and the secondary thrusters

This is the first time a Moon mission will be attempted with a fixed thrust propulsion system as opposed to the commonly used variable thrust-throttleable ones. The variation in thrust is instead achieved here by firing the engine in definite short pulses. The main engine is accompanied by sixteen smaller 22 N thrusters for finer control.

ISRO’s GSAT-2 geostationary communication satellite used a fixed thrust propulsion system.

Maneuvering in zero-g

During its journey to the Moon, the spacecraft will be in a gravity free environment. With the propellant essentially floating in the tanks in spherical globules, not all of it will directly go into the feed lines and thus into the combustion chamber to produce thrust. To work around this, an interesting surface-tension based solution is used.

A surface tension based Propellant Management Device (PMD) rests inside each of the tanks which always holds onto a small amount of propellant. Therefore, even in zero-g, there is always some propellant to inject into the feed line. Once the engine fires, the resultant forces on spacecraft will drag the fuel in the direction of the feed line, allowing for a continuous propellant supply.

Maintaining precise attitude in space

Facing the Sun

As soon as the launch vehicle undocks our spacecraft, the attitude (orientation) of the spacecraft needs to be such that its solar panels face the Sun for power generation. This reorientation is enabled by the Attitude Control Thrusters (ACTs).

Canted (tilted) thrusters at the extreme left and right, used for attitude control

Eight of the sixteen 22 N thrusters are canted (tilted) with respect to the spacecraft. By firing a given combination of these thrusters in a series of short pulses, the spacecraft can be made to point in any direction. This enables the spacecraft to face the Sun throughout the journey to the Moon and remain power positive.

Minor attitude changes can impact the whole mission

Space missions are averse to even the smallest of deviations as they can prevent achieving mission objectives. This is most important for extremely long distance missions like NASA’s New Horizons probe to the Pluto system. Since Pluto is so far away, even a 1° change in attitude during the initial post-launch phase can completely miss the target. While corrections can be made at later phases, it would be taxing on the amount of fuel which is a limited onboard resource.

If the trajectory that the New Horizons was put in was off by even 1°, it would have missed the Pluto flyby. Source: Wikipedia

During our mission, the main engine will be fired for all major maneuvers, like getting captured into lunar orbit for one. If the spacecraft is pointing even slightly off from the desired orbital plane at any given point, it could affect the mission in critical ways. Having precise attitude control is therefore mandatory for such missions.

The above solutions work only if all the engines fire with the same amount of thrust — with minuscule error margins. The fineness of thruster firing is mission-critical.

How to ensure consistent thrust from all engines?

There are two major areas where inconsistent thrust delivery can cause problems.

The propellant pressure drops inconsistently while traversing through the feed lines and thus the final thrust in the thrusters can vary accordingly. This leads to the spacecraft not attaining a desired orientation. Getting desired attitude is a mission critical thing as seen in the above examples.

Any inconsistent pressure changes across different parts of the propulsion system is automatically conditioned via valves controlled by the Heater Propulsion Card (HPC) onboard the spacecraft.

A pressurized Helium tank is thus needed which acts as a pressuring gas (pressurant) for the propellants. The gas (in this case Helium) needs to be inert to avoid reacting with the rocket propellants.

Propellant tanks onboard the spacecraft

Avoiding fuel freeze in lunar orbits

While orbiting the Moon in its nighttime, the spacecraft will face extreme cold temperatures. For example, in the S2 orbit shown in the diagram below, the spacecraft will experience nighttime temperatures colder than -50° C for more than an hour.

Orbits of the TeamIndus Z-01 spacecraft around the Moon

The propellants need to be maintained within a set of temperature and pressure conditions for the system to work as desired. For example, Hydrazine freezes at 2°C and therefore a network of heaters is used to keep its temperature always above 10°C.

The landing

The last phase of the mission — i.e. the lunar descent and soft-landing — is arguably the most critical phase and can be divided into two major parts.

A. Killing the velocity

The first part requires maximum thrust (> 600 N). This is because the high velocity of the spacecraft achieved during orbit needs to be killed. During this phase, both the 440 N main engine fire along with the eight 22N thrusters which are directly facing downwards. The required thrust is thus achieved using such a thrust augmentation mechanism pushing the spacecraft to the maximum.

Timeline showing spacecraft velocity at different phases of descent. The red and yellow phases are where majority of the spacecraft’s velocity is killed.

B. Nearing touchdown

As the spacecraft nears the lunar surface (from “Turn” phase onward), most of the velocity is killed. The main engine is still being fired to reduce the velocity further since it needs to approach zero (magenta line). However, if the main engine is not cut-off well above the lunar surface before landing, the rocket plume exhaust can hit the ground causing hot gases to backflow towards the spacecraft and damage critical spacecraft parts like solar panels, rover or other sensors.

It can also create unwanted backwards thrust which is not desirable for a safe landing. Hence the engine is cut-off at about ~1.5 m from the surface. This is akin to what the Apollo Lunar Module did where the engine was cut-off at 1.3 m.

Another reason to cut-off the engine well above the surface is that the rocket exhaust can interact with the lunar regolith and change its physical and chemical properties. This should be avoided for preserving landing sites of high scientific value and the landing should be attempted nearby at a safe enough distance.

So at ~1m from the lunar surface, the spacecraft is essentially in a free fall. The spacecraft structure is designed to withstand the impact forces from the touchdown.

Fuel margins

While the delta-v and choice of the propulsion system determines the amount of propellant mass we need to carry, there are practical reasons to carry more. Some of the propellant will get trapped in parts of the tank and feed lines. Also, as mentioned in above examples, unwanted changes in thrust can happen leading to undesirable changes. As such, there can arise needs to perform correction maneuvers, requiring additional propellant to be carried.

Conclusion

That concludes a brief overview of how the TeamIndus propulsion system is designed to cater to different mission scenarios. Check out the rest of our blog for more such deep dives into space technologies.