Slingshot Effect

Robert brand 2013Also Free Return Trajectory

I mentioned in a recent post on another website (Facebook) about the real issues about “slingshot Effect” and what it really was all about.

The following is a short bit of discussion on that. It has relevance to my current work as Tim Blaxland and I are working on Stellar’s Google Lunar X Prize documentation. Tim (below) is Stellar’s navigator or to use a more colourful word, our “Astrogator”. He joined the discussion.

Tim BlaxlandThere was some small confusion as I was aware of the Apollo free return trajectory. Note that my comments refer to the fact that even with free return, there are rockets and thrusters to be fired / used for a number of different reasons such as speed and improving the target of the flight. Apollo 13 yhad added dificulty as the lunar module’s thrusters were not arond the centre of gravity of the mass and those steering the three joined vehicles were at the rear end of the vessel.

I will add more detail if there are more posts on the subject.

I was asked about “slingshot effect” and found the wiki article is reasonable for a beginner. It is hard to understand the finer points of the poorly named effect.

This gives the basic info. Obviously it is never as simple as the basic equation as you can pretty much never change direction 180 degrees. If you tried to do this with the moon to slingshot back to earth, the gravity and speed would be wrong. No matter what you would need engines to get the equation right for an earth return. The further away you are, the less the effect.

Gravity assist – Wikipedia, the free encyclopedia

In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the sun) and gravity of a planet or other celestial body to alter the path and speed of a spacecraft, typically in order to save prop…
Cassini_interplanet_trajectoryThis from Wikipedia about the above diagram:The Cassini probe – multiple gravity assistsThe Cassini probe passed by Venus twice, then Earth, and finally Jupiter on the way to Saturn. The 6.7-year transit was slightly longer than the six years needed for a Hohmann transfer, but cut the extra velocity (delta-v) needed to about 2 km/s, so that the large and heavy Cassini probe was able to reach Saturn, which would not have been possible in a direct transfer even with the Titan IV, the largest launch vehicle available at the time. A Hohmann transfer to Saturn would require a total of 15.7 km/s delta-v (disregarding Earth’s and Saturn’s own gravity wells, and disregarding aerobraking), which is not within the capabilities of current launch vehicles and spacecraft propulsion systems.

  • Hitesh ॐ Mohan “Although the lunar landing was aborted (Apollo 13), the crew continued toward the moon and circled it so its gravity could provide a slingshot effect for the return to Earth.”…/

    Apollo 13 Commander Remembers the Aborted Moon MissionApollo 13 crew arrive on the prime recovery ship U.S.S. Iwo Jima following the ocean landing and rescue in the South Pacific. Exiting the helicopter are (from left) Fred Haise, mission Commander James Lovell and John Swigert. Click to Enlarge. Cr…

  • Kurt Kammeyer Technically, it was called a “free return trajectory”.
  • Robert Brand you still have a lot of maneuvering and firing of rockets / thrusters to make that happen. Things have to be very precise.
  • Robert Brand “After circling the Moon once and creating a speedy free-lunar return trajectory, the LM descent engine was fired twice to establish an even faster return path. The descent engine was fired twice during the return flight to correct the spacecraft’s trajectory.”
  • Hitesh ॐ Mohan What was the escape velocity from the the moment the lunar module, escaped the lunar atmosphere and at what speed did the craft sail at to return home?
  • Robert Brand Firstly there is no lunar atmosphere (well, nothing to write home about). I do not know the speeds.Maybe someone can advise.
  • Royce Jones This effect could be used to help power a Starship.
  • Tim Blaxland Robert, you can do a free return, literally free. Apollo 8, 10 & 11 used this trajectory. I have simulated these trajectories to verify. Later missions didn’t use true free return trajectories because they were trying to squeeze more payload onboard and open up a wider range of landing sites. True free return trajectories we just too restrictive. The hybrid trajectories were similar but required some thrust to get back to Earth. The deviation of the hybrid trajectory from the free return trajectory was limited so that a return could be achieved using either the SPS, DPS or SM RCS (but not the LM RCS). There is a very good essay on the subject here:

    Launch Windows Essay
    Let’s go to the moon. When shall we go? Right away. Where shall we go? Copernicus, Gassendi, Marius Hills? Let choose along the way.
  • Robert Brand Great to know Tim. There must have been a very lucky relationship with the size of the moon, the orbital speed of the moon, etc. I understand that once the mass is big enough, it is all a matter of distance from the object to get the free return trajectory right, but I was unaware that the moon was right. None the less, as I said, if the free return was possible, it was going to take some firing of rockets / thrusters, etc to get her home because the trajectory would not have been accurate enough. I read that it was a free return, but there was no way they were escaping the need to burn the LEM descent engine – in this case – twice! The burn coming out of the moon was two fold – much of it was to give them more speed to hasten their return trip to get back to earth. The final one was to course correct or end up cinders. They still had to orient the capsule after jettisoning everything so that they did not skip off the earths atmosphere and into space. Plenty to go wrong. My point was that they had to use thrusters and engines to get home. It was not quite hybrid, but it was a real problem. With the thrusters gone on the service module and I believe the command module (could someone in the know verify), it is like driving a car from the trunk / boot. Very hard indeed. The thrusters would not have been in the centre of mass.
  • Tim Blaxland Yes, the CM RCS were disabled to keep enough energy in the CM batteries for reentry, along with everything else they could possibly turn off.
  • Robert Brand Yes, I believed that the thrusters were non operational on the CM.

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