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Message started by phoenixshade on 01/25/11 at 06:12:46

Title: Preliminary: Apollo 8-like Test
Post by phoenixshade on 01/25/11 at 06:12:46

I'm working on a rather ambitions spacecraft simulation: An Apollo mission to the moon. This is a preliminary version. Much like the actual Apollo 8 mission of December 1968, this puts the spacecraft into lunar orbit and returns it home. All thrust events are scripted via the autopilot, and all are realistic acceleration rates comparable to those in the actual missions.

Mission Overview

Trans-Lunar Injection
This 8-minute burn at 0.82g puts the spacecraft into approximate a Hohmann transfer:

Furthermore, this is a free-return trajectory. In the absence of any other burns, the spacecraft will return to earth:

Lunar Orbit Insertion
Once it reaches the vicinity of the moon, a retrograde burn for 4 minutes at 0.36g puts the craft into a retrograde elliptical orbit. A second 17-second burn after the first orbit circularizes the orbit at an altitude of about 200km.

Trans-Earth Injection
The last burn of the mission lasts 5 minutes at 0.41g to put the spacecraft on a return trajectory back to earth.

A perfect splashdown:

Total mission time: 6 days, 12 hours, 3 minutes, 29 seconds.

I will be simulating the following additional events to the next Apollo simulation:
• S-IVB separation and firing onto an impact trajectory with the moon
• Lunar landing vehicle that will separate from the command module in lunar orbit and touch down with a velocity of less than 1 m/s
• Ascent from lunar surface and rendezvous with the command module
• Final separtion from the lander, leaving it in lunar orbit
• Service module separation

Ideal settings: stepsize: 0.25 seconds; graphics interval: 250.

Title: Re: Preliminary: Apollo 8-like Test
Post by Tony on 01/25/11 at 09:03:01

You're really putting together some top-notch simulations!  If you dig deep enough in the forum, you'll see a similar moon mission.  But I never bothered to use real Apollo numbers.

A neat way to view this sim is in rotating frame to keep the moon stationary.

I guess you'll be using "Creating Objects" in the autopilot to create the different stages as you need them.  One problem you'll run into when landing on the moon is that your spacecraft merges with the moon on contact.  There's no more craft left to return back to lunar orbit and dock with the Command Module.  But one thing you could do is use the fact that "Open" is one of Autopilot's commands.  So at the moment you want to blast off from the moon, save your scenerio under a new name (for example Apollo11part2.gsim).  Pause the sim, then create the LEM 1 meter above the lunar surface and give it lift-off velocity.  Save it, then go back to your first sim (for example Apollo11part1.gsim), and add an "Open" command to to the autopilot to open the 2nd part.

You seem to be using some of the seldom-used features of Gravity Simulator.  So if you encounter any bugs, let me know!

Title: UPDATE – Lunar Mission Simulation
Post by phoenixshade on 01/31/11 at 09:36:22

As some of you may have noticed, my projects tend to advance rapidly in conception. This is one of them, and my goal has changed with this simulation into something far more interesting than just a repeat of something that's already been done. Instead, this is a conceptual future lunar mission.

This mission abandons the traditional Lunar Orbit Rendezvous for something I have never seen, but the rudiments are already in place. In overview, the mission is this:

1. Launch an unmanned Lunar Landing and Exploration Module (LLEM) into lunar orbit on a slow, efficient trajectory1
2. Launch a Crew Transport Module (CTM) to the moon on a much faster, traditional trajectory
3. Transfer to the LLEM and descend to the moon for a surface mission.2
4. Return to the CTM via an ascent stage of the LLEM.
5. Return to Earth in the CTM.

1 This is the primary difference between my approach and NASA's Constellation Program. This is also its biggest advantage, because the majority of the Lunar Payload is launched on a fuel-efficient trajectory, enabling much larger mission payloads.
2 Surface mission design is beyond the scope of this project.

Conventions regarding the spacecraft: The primary divisions are termed "Modules". Each module is delivered separately to the moon, and have already been introduced. Each module itself consists of units known as "Platforms," each of which is designed to separate from the spacecraft at some point after they have fulfilled their purpose. These will be described in more detail below.

1. This mission would start with placing a Lunar Landing and Exploration Module (LLEM) into orbit around the moon, taking advantage of Weak Stability Boundary Theory ( Such a trajectory requires approximately 1/4 – 1/3 less [ch8710]v for lunar orbit insertion, when compared to a traditional Hohmann Transfer ( However, such an approach takes approximately 5 months to reach lunar orbit, so it is impractical for use with a manned spaceflight. For this reason, the LLEM is unmanned at this point.

The LLEM spacecraft itself would consist of two platforms: The Science Platform (SP) and the Habitable Ascent Platform (HAP).

The SP is the far more massive of the two. The majority of the mission payload is included in this stage, and might include scientific equipment and supplies required for the surface stay but not needed on return to earth. This stage also includes the descent engine used during landing.

The HAP is the ascent stage of the LLEM, which doubles as sleeping quarters during the surface mission. It is capable of sleeping four, but can transfer as many as six astronauts. In later missions up to that number would be sent to the moon (sleeping in shifts); but early missions would be limited to four astronauts.

2. About five or six months later, after the LLEM is delivered to lunar orbit, a second launch occurs, carrying the manned Crew Transport Module (CTM) to the moon using a traditional Hohmann Transfer, reaching the moon in about three days. They rendezvous with the LLEM in Lunar Orbit. The crew transfers to the LLEM, leaving the CTM alone in lunar orbit.

The CTM itself is probably a blunt-body design, much like Orion of the defunct Constellation Program ( (This isn't why I voted for you, Obama.) It includes the Crew Platform (CP) and the Return Platform (RP). The CP is where the crew resides during transfer and reentry. The Return Platform consists primarily of the engine and fuel for the return to Earth.

3. The LLEM descends to the lunar surface. Surface missions may last two weeks, and perhaps as long as three for a reduced crew.

4. The HAP ascends to lunar orbit and docks with the CTM, leaving the SP behind on the lunar surface. The crew transfers back to the CTM, and the HAP remains in lunar orbit, perhaps carrying out an unmanned orbital mission controlled from Earth.

5. The CTM fires its engines to send the craft on a Hohmann Transfer back to Earth. Three days later, the RP is jettisoned and the CP reenters the Earth's atmosphere.

The biggest hurdle to clear in terms of the simulation is, of course, the Low-Energy Transfer for the LLEM. Low-Energy transfers take advantage of the chaotic nature of L1 and L2, and are equally complex in either direction, so starting at the end and running time backwards is not a practical approach. I hope to be able to start instead at the Lagrange point and work each direction from there, because otherwise I fear this is beyond my mathematical abilities. The flight of the CTM is essentially an Apollo mission, so the existing simulation can easily be adapted.

Here's a rough "napkin sketch" of the mission profile:

I'd really like it if when this was done, someone would model it in Celestia...

Title: Re: Preliminary: Apollo 8-like Test
Post by Bob on 02/01/11 at 04:39:25

So how does Celestia come into this? Does it take your grav sim orbit model and add decent graphics of the Moon and Earth? Can it also do the actual rockets  and landers?  

Title: Re: Preliminary: Apollo 8-like Test
Post by Tony on 02/01/11 at 10:51:45

I think Orbiter or Universe Sandbox would be the better choice.  Celestia doesn't model gravity.  EDG has a lot of experience with Celestia.  Perhaps he can give us more insight.

Title: Re: Preliminary: Apollo 8-like Test
Post by EDG on 02/01/11 at 10:56:26

Yeah, Celestia doesn't model gravity (or even mass) at all. You'd have to set up the paths by hand (i.e. by importing the trajectory data as a text file somehow) - unfortunately I have no idea how to do that.

AFAIK Orbiter does model gravity, but I've no experience with that program.

Title: Re: Preliminary: Apollo 8-like Test
Post by Tony on 02/01/11 at 16:12:41

Celestia has a forum where people can help you.  I can probably help you with Orbiter.  I've translated back and forth between Orbiter and Gravity Simulator.  Dan Dixon of Universe Sandbox occassionally is a member of the Gravity Simulator forum, and occasionally visits.  He can help you set it up in his program.  There is an option in Gravity Simulator under the File menu to create a Universe Sandbox file from a Gravity Simulator file.  But it doesn't work with the autopilot.

Title: Re: Preliminary: Apollo 8-like Test
Post by Bob on 02/03/11 at 03:29:36

I took a look at the sandbox app and at orbiter. Sandbox is great fun for 45 minutes, bit of a swizz that demo cut off  :( Orbiter looks very promising though. I put up links to both apps on the thread on the planet generator thread, in my favourite c4d site and I'm now reading up on the import of spreadsheet data into c4d (Cinema 4d) I think you may have got a few new members from that thread.

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