# Steering rockets in space by shifting CM

• RubinLicht
In summary, the conversation discusses different methods of steering rockets in space, with a focus on the use of movable mass to control the center of mass and thruster torque. Other methods mentioned include jet vanes and spin stabilization. The conversation also raises questions about the effectiveness of using movable mass and suggests alternative approaches, such as gimbaled thrust.

#### RubinLicht

So, I'm investigating a certain way of steering a rocket in space for my first undergraduate research project. Essentially, the idea is to control the location of some mass located on a horizontal track perpendicular to a rocket, so that when the mass is moved, the center of mass of the rocket is moved, and hence the thruster also has a torque on the rocket. I'll be writing a model for the rocket, and a controller, if I get far enough, I will be able to test it on a model rocket to demonstrate functionality.

I'm more interested in all the ways rockets are steered in space currently. I'm aware that some satellites use reaction wheels, very small satellites can use magnets to align with the north pole. SpaceX uses thrust vectoring, cold gas thrusters, and fins (these obv can't work in space due to the lack of air).

are there any other clever/lightweight methods that people have come up with to steer spacecraft ?

I personally feel like in order to achieve quick turning, the mass being moved would have to comparible to the rocket's center of mass, by which point the mass is much too large compared to the mass of the rocket. Alternatively, you'd want a very large track, but then again, that would have bad effects on the aerodynamics. I'll be thinking more about this, but if anyone has a good idea, I'd love to hear it.

berkeman
For short duration burns of a rocket with a single, short nozzle (like amateur rockets), jet vanes may be a fairly cheap alternative to gimballing. For example, Copenhagen Suborbitals have used them for some of their rockets [1]. However, jet vanes do not scale well (or at all) to bigger rockets.

If the question is how to control attitude stability and thrust direction during a burn in vacuum, then another simple approach is to set attitude before burn (using gyros for example) and then spin up the rocket or vehicle so that any off-axis thrust is averaged out. I believe this method is still very much in use for obit insertions and in-orbit maneuvers of satellites and probes. Of course, the vehicle must be designed to operate and maneuver while spinning.

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If the centre of mass is not on the axis of thrust, a continuous spin will be induced in the rocket body about the centre of mass. That spin must be removed, starting when the rocket has turned half way towards the required direction.
Your system requires there be additional movable mass in the rocket. Might you use movable fuel or batteries as the mass? Could you do better by changing the aim of the rocket motor very slightly to provide the same axial misalignment. That could be controlled automatically by a gyroscope which might provide better control during launch.
Offsetting the CofM or aiming the motor thrust are similar. Is there any difference in the dynamics and control requirement of either technique.
Are there situations where moving the mass will have a positive or negative gravity component. To fly upwards, will you not always be pushing that weight uphill. Can you aim the rocket motor thrust without having a similar gravity component? Does that require gimbal mount for the motor.

RubinLicht said:
are there any other clever/lightweight methods that people have come up with to steer spacecraft ?
Gimbaled thrust. The engines are mounted so that they can swivel. Same basic idea as yours, but done by changing the line of thrust relative to the center of mass vs. shifting the center of mass relative to the line of thrust.

## 1. How does shifting the center of mass (CM) affect the steering of rockets in space?

Shifting the center of mass (CM) of a rocket affects the steering by changing the distribution of weight and thrust. This alters the direction and stability of the rocket, allowing it to change its trajectory.

## 2. Why is it necessary to shift the CM when steering rockets in space?

Shifting the CM is necessary for steering rockets in space because it allows for control and stabilization of the rocket's movement. By shifting the CM, the rocket can adjust its direction and maintain stability, which is crucial for successful navigation in space.

## 3. How is the CM shifted in a rocket?

The CM is typically shifted in a rocket by adjusting the position of the payload or using movable fins or thrusters. This can be done manually by astronauts or automatically by onboard computer systems.

## 4. What are the potential risks of shifting the CM in space?

Shifting the CM in space can be risky as it can cause the rocket to lose stability or control. This can lead to a change in trajectory or even complete failure of the mission. It is essential to carefully calculate and monitor the shifting of the CM to minimize these risks.

## 5. Can the CM be shifted multiple times during a rocket's flight in space?

Yes, the CM can be shifted multiple times during a rocket's flight in space. This allows for continuous adjustments and corrections to the rocket's trajectory and stability. However, frequent and sudden shifts can also increase the risk of destabilizing the rocket, so it is essential to carefully plan and execute these changes.