TeamIndus Lunar Descent Strategy
The science and art of executing a Moon landing
To date, only 3 countries have successfully landed on the moon — USA, USSR and China — proving that it’s one of the most difficult of missions. These were achieved by the respective governmental organizations with budgets in the billions of dollars and many years of prior space technology expertise & past space mission experience to rely on. This ought to give you an idea of the complexity involved in such an attempt.
Through this short piece, we aim to give you some information in the descent strategy being developed at TeamIndus.
Understanding orbits before the descent
In preparation for the descent, the spacecraft’s orbit is changed from a 100 x 100 km circular orbit (we call it S3) to a 12 x 100 km (S4) orbit as shown in the diagram.
To land on the lunar surface in a fuel efficient manner, the spacecraft needs to start descending from the lowest point in the S4 orbit (periapsis), which is 12 km. The lowest point of the S4 orbit depends on the nature of the S3 orbit, which is where the physics advantage of the circular orbit comes in.
Had S3 been an elliptical orbit like S2, the closest point for starting the descent would lie where S4 is marked in the diagram. This would constrain the longitude for landing, leaving no flexibility in targeting various landing sites.
Instead, a circular orbit allows the engine burn to be executed anywhere in the circular S3 orbit and accordingly choose the lowest descent point in the S4 orbit. Circularizing the orbit thus allows flexibility in targeting various landing sites while requiring the least amount of energy expenditure.
Apart from the necessary subsystems pertaining to power, thermal, computation etc, the key hardware required for descent are the sensors. Our spacecraft will use the following during descent:
1. Laser Rangefinders (LRF) 2. Laser Altimeters (LALT) 3. Descent Cameras (LDS) 4. Inertial Measurement Unit (IMU)
Parameters measured during descent
In preparation for descent, the spacecraft’s orbit is changed from a 100 x 100 km circular orbit to 12 x 100 km. After a single orbit during which the team performs a sensor-checkout, the spacecraft descent sequence is initiated. Thereon, the spacecraft functions autonomously until touchdown; no telecommands are sent although it continues to relay telemetry data back to the TeamIndus Mission Operations Center.
The key parameters that feed into the on-board computer (OBC) and used by the descent algorithm are:
1. Altitude 2. Velocity 3. Position
The Laser Altimeters (LALT) provides data used to derive altitude. The LRFs on the other hand are used only during terminal descent — from 100 meters above the lunar surface.
The IMU plays a key role as the central sensor for navigation of the spacecraft. An IMU comprises a highly precise accelerometer and gyroscope. For easier understanding, think of modern day smartphones that allow you to motion-control or play motion-based mobile games. The IMU measures even the minutest of changes allowing our algorithms to determine orientation. During orbit, the Mission Planning team can predict position and velocity with a required degree of accuracy. Hence at the time of descent sequence being initiated, with this information available to the on-board computer OBC, acceleration is tracked and fed into a mathematical model that can propagate position and determine velocity as the spacecraft decelerates and approaches the landing site.
The knowledge of the spacecraft’s position, velocity and attitude is fed to the guidance program which calculates the reference attitude and thrust. The control system program uses the guidance and navigation commands to orchestrate firing of the main engine and the attitude and reaction control thrusters in synchronized firing patterns to land the spacecraft gently.
From 12 km above the lunar surface, at which the descent sequence is initiated, till touch-down takes 1000 seconds (~16 min), the main engine and all 16 attitude control thrusters (ACTs) are engaged either together or in a sequence determined by the phase.
In the initial braking sequence, the main engine and all 16 ACTs fire. The spacecraft which is traveling at ~1700 m/s and 900 km downrange (distance from landing site) has its velocity reduced to ~800 m/s.
Approach & Turn
During the approach phase, braking continues with main engine along with 16 attitude and reaction control thrusters firing. Velocity further reduces to just 5 m/s. At this point, the spacecraft is nearly above the desired landing site. The ACT fired in a manner that orients the spacecraft such that the thrusters point toward the lunar surface and a vertical terminal descent can begin.
The spacecraft is now about 100 meters from the lunar surface; the LRF takeover from the LA and begin feeding altitude data. It is interesting to note that while the IMU has been providing velocity data till now, it is necessary for the spacecraft to determine lateral velocity with higher accuracy.
The LDS (cameras) begins imaging the lunar surface. Images are clicked in intervals and compared through image processing algorithms to determine the velocity with relation to the ground.
With Main engine throttling down and ACT keeping spacecraft orientated while minimizing lateral velocity, the spacecraft descends to the surface. This is where our spacecraft’s structure and landing gear will be put to the test.