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Apollo 11 CSM on the trip back from the moon

Landing on the Moon

The Orbiter Space Simulator: An Appreciation

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Introduction

The first decade of the twenty-first century is an exciting time to be a space nut. Not only is the International Space Station whizzing overhead constantly, but China has launched men into orbit, and private spaceflight is finally getting off the ground. But even this frenzy of activity does not begin to approach the excitement of the 1960s, when the Apollo program launched men to the moon. Fortunately, we can relive and reenact the heady days of the Space Race through another technological wonder -- the computer.

In 1995, director-producer Ron Howard brought the Apollo program to a generation born after its conclusion with his box-office smash Apollo 13, casting self-avowed space nut Tom Hanks as Commander Jim Lovell. Shunning the use of NASA stock footage, the effects crew at Digital Domain used nascent digital techniques, such as Computer-Generated Imagery and computer compositing, to enhance traditional miniature photography of spacecraft models. Three years later, at Tom Hanks' behest, HBO spent even more money than Apollo 13 budgeted, getting an excellent 12-part hourly miniseries on the Apollo program. Digital effects were used more extensively on From the Earth to the Moon, not only to provide footage to supplement the larger amount of live action, but also because four times as much computer power had become availble, thanks to Moore's Law.

Well, time continues marching on, and the expensive Silicon Graphics workstations of yesteryear now pale in comparison with the computers sitting on your desk. In 2000, Dr. Martin Schweiger of University College London began work on a 3D space simulator called Orbiter. Over the years, he has maintained it, added features, and most importantly, developed an architecture that allows a plethora of community-contributed add-ons. In fact, even the core feature of sound effects — launch rumbles, spacecraft ambient noise, communications chatter — is implemented as an add-on, written by Orbiter enthusiast Dan Steph.


Orbiter screenshot. T+0:09:20. The expended Saturn V second stage separates. The small jets of flame are the ullage thrusters.

The beauty of the simulator is that it provides a much deeper understanding and appreciation of the technology that got Apollo to the moon. I've been a space nut since I was six years old, when my father bought me a copy of The Omni Space Almanac. I've read about the technology in numerous books and articles, but exciting as these descriptions were, they were just words. Orbiter, in contrast, provides the visceral thrill of being able to flip the switches and push the keys on the DSKY, and actually see their effect on the spacecraft. Spaceflight concepts that are usually difficult to understand become easy after they've been tried out in the simulator. Perplexing orbital mechanics which formerly raised difficult questions of 3D geometry now become intuitively obvious.

Microsoft Flight Simulator virtually buckles you in the pilot seat without having to shell out $100 an hour to rent a Cessna. Orbiter has taken the trail blazed by the long-ago discontinued Microsoft Space Simulator, and paved it into a multi-lane expressway. With Orbiter and its addons, you can virtually strap yourself into the astronaut's couch for $20 million less than it costs to fly along on a Soyuz mission to the International Space Station. In some ways, it's better. You can get a flying license and actually control a plane, but even the billionaire space tourists are just sightseeing. It's a geeky thrill that won't be available for decades.

Rocket Science

To fly to the moon in Orbiter, you first take a crash course in orbital mechanics. Fortunately, Bruce Irving's superb PDF tutorial Go Play in Space walks you through the basics and guides you through your first baby steps in a spacecraft. It walks you through downloading, setting up, and configuring the simulator, and then takes you through a series of gently-sloped lessons. With every command spelled out by the keystroke, you can learn the simulator's keyboard shortcuts at the same time that you get the geometry drilled into your head.

The tutorial is 136 pages long, and needs every page. There's a reason why "rocket science" is American English colloquial for "extremely hard problem" (see montage of rockets blowing up in the first great film about the Space Race, The Right Stuff). These days, "brain surgery" has replaced "rocket science" in the lexicon as the latter has passed its pioneering days, but it is still very techical and far from simple. You must know your ellipses, apoapses, periapses, delta-vees, progrades, retrogrades, orbit normals, orbit anti-normals, inclinations, and more.

The tutorial is done in the Delta Glider and the Shuttle-A, imaginary spacecraft from our bright future in space. These hypothetical ships carry an abundance of fuel, allowing you to make huge mistakes and still make it back to base. Real spacecraft, even large ones like the Space Shuttle, are engineered to performance specifications, with very little margin for error. Begin with pitch, roll, and yaw in space. Then progress to maneuvers to change orbits — raise your apoapsis, change your inclination, etc. Get handy in non-atmospheric planetary maneuvers (no wings, hover engine provides lift) by taking a spin around a moon base. Finally, fly to the moon and back, using an add-on to calculate the proper injection burns and capture orbits.

This may sound like it's duplicating the Apollo flights, but it's just giving the broad outlines. You'll need these concepts for any flight to the moon. The Apollo flights, on the other hand, will be realistic to within minutes of the actual missions. There are a bewildering array of switches on photorealistic instrument panels, and you'll have enough worries trying to find the right switch. Intricate maneuvers must be done very precisely to be successful — LM extraction, TLI, lunar orbit insertion, rendezvous and docking, reentry alignment.

Orientation and Moodsetting

But before stepping (virtual) foot inside the Apollo command module, it is helpful to get an overview of where things are. Unless you live near a space center where you can sit inside a mockup, stock up on DVD rentals from Netflix or Blockbuster.

If you've read this far in this article, you're probably enough of a space enthusiast to have seen Apollo 13. As a film targeted at general audiences, it focused more on human relationships than on technology, and contains some notable technical inaccuracies (as Commander Jim Lovell points out in the DVD commentary track). But the panel displays are quite informative. Watch the circular engine lights during launch, and note what happens when one of them shuts down earlier than planned. Observe the actors as they enter commands into the Apollo Guidance Computer, and read the display. Take a glance at all the gauges as they change values — they'll help you to imagine gauges in your head as you look at the less dimensional ray-traced gauges in the Orbiter simulator.

But Apollo 13 only takes you so far. The mission never actually touched down on the moon, so we never see the landing sequence. For that, turn to the series From the Earth to the Moon. Not only is it longer, but it's also much geekier, getting into lots of technical detail. Undoubtedly this is due to Tom Hanks' influence as producer, and also because the audience for a 12-part miniseries is more technically oriented than for a 2-hour mass-release film.

In the Apollo 8 episode, you'll see the complete sequence of events for a lunar orbital flight — note how the lunar orbit insertion and trans-earth injection occur on the far side of the moon. Having worked through the Orbiter tutorial, you'll now know why it has to be done while the crew is out of communications range with Mission Control. The Spider episode (tangentially, Apollo 9 and 10) lays out the principles upon which the Lunar Module was designed. Pay attention to the segment where Tom Kelly, Grumman's head of the LM program, accepts a weight-saving but counterintuitive control layout by reasoning, "Astronauts are smart." So are you, because you too will have to make spatial transformations in your mind, maneuvering the LM while all the controls are rotated by 90 degrees.

In the Apollo 11 episode, the hair-raising first landing is reenacted in great detail, with many closeups of switches, dials, and gauges. Neil takes manual control as he sees the LM heading for a boulder. Note how the lunar module maneuvers forward and back: not with thrusters, but by pitching and rolling the whole craft. Buzz is shown looking at the surface motion indicator (Oh, so that's why he kept calling out those numbers in those lunar landing snippets that we saw in Grolier's Multimedia Encyclopedia), and calling out "Contact light" as the camera shoots a closeup of that lovely blue light that signals the completion of one of mankind's earliest dreams.

Further episodes focus more on the human element of the Apollo Program, exploring the news frenzy of Apollo 13, Alan Shephard's ear illness, the geological training given to the astronauts, and the tragic effects of the Program's stress on the astronaut's wives. There's also a nifty tie-in between space and film with reenactments of the filming of George Méliès' pioneering La Voyage dans la Lune (The Trip to the Moon). The first six episodes focus more on the technology, and provide an overview of the spacecraft you'll be "flying." You don't need to know how to trick the Apollo Guidance Computer when it hits an error condition, as happened during the Apollo 14 landing. Nor do you need to identify rocks, or set up equipment on the surface, or hit a golf ball "miles and miles and miles" — there's only a very limited set of activities in the simulation when you're on the surface. (Orbiter simulates the spacecraft; after all, Flight Simulator doesn't recreate the airport experience either.)

Simulating Apollo

The NASSP Project Apollo add-on is an open-source project which has folded in many other projects to produce an astoundingly comprehensive and high-quality Orbiter simulation of the Apollo missions. These include highly-detailed 3D models for the space craft which look great from the outside, as well as control panels for the Command and Lunar Modules with hundreds of switches, dials, and gauges. Plus, several functions of the Apollo Guidance Computer are available, including fully automatic launch, and precise control of Trans-Lunar Injection. Sounds from the real missions add ambiance, though they do not serve much of a simulation function since the Apollo program used feet and miles while Orbiter displays units in the metric system.

Orbiter is an interesting development model. One man controls the core code, but a highly modular architecture allows for a multitude of plugins. Of course, much of the core code involves numerical analysis, which must be done accurately to avoid introducing errors into spacecraft motions. Add-ons perform many basic functions, such as sound, flight and orbit planning, and computer guidance.


Frame capture from Apollo 13 DVD. This scene comes just after the accident, as the command module is being powered down. The display on the AGC DSKY mockup is accurate. Program 6 is G&N Power Down, verb 50 is "Please perform" and noun 25 is "checklist."

Project Apollo is among the most polished of the available spacecraft addons, though it still has many quirks. For example, you must click on the text of buttons to activate them; clicking on the side of the button will not work. The hot zone for switches is similarly limited. But the NASSP people aren't standing still. Version after version rolls out, each better than before (check out some of the archives — the panels used to have half as many instruments as they do now). The sheer amount of detail greatly increases the realism of the simulation. Just like it is impossible to properly fly an A380 in Flight Simulator when using a Cessna control panel, it is jarring to fly Yuri Gagarin's Vostok One flight with modern glass cockpit controls. Many of the other Orbiter add-ons are designed for splashy screenshots, and only look good from the outside. NASSP looks good from the outside and the inside.


The AGC DSKY in Orbiter. Program 11, verb 16 monitors the launch with a 2 Hz update rate, and noun 62 displays delta-v, downrange distance, and elapsed time respectively. In Orbiter these values are in metric units, though the real AGC used feet and nautical miles.

There's even a side project to run the real Apollo Guidance Computer programs inside a virtual machine that emulates the AGC hardware. (Currently NASSP simulates the functions of the computer, but in modern programming languages.) The programs are compiled from source code made available by the MIT Instrumentation Laboratory (now Draper Labs), where the AGC software was developed. [MIT AGC History Project] Such is the power of Moore's Law. In Apollo 13, Tom Hanks proudly describes a computer that fits in a single room and has a megabyte of memory. Film critic Roger Ebert remarked in his review that he was typing on a more powerful computer than the one that got us to the moon. Well, now, we have so much computer power that we can calculate trajectories, render realistic 1280x1024 images of the spacecraft at over 25 fps, and emulate every hardware function of the Apollo Guidance Computer, fast enough for the original software to run on it in real time. That's progress.

The problem with NASSP, as with many other open-source projects, is that very few people want to write documentation. As a result, the documentation is very poorly organized and sometimes quite skeletal, as in the case where a screenshot is overlaid with two arrows showing where the jettison switches for the SIV-B booster stage are located. Diagrams of the control panel, checklists for operation, and detailed instructions for operating the guidance computer are all available, but each is in a separate PDF.

Fortunately, people have written tutorials for getting to the moon and back in NASSP. At this level of proficiency, you've done the trip in fictional spacecraft and are trying it for the first time in realistic spacecraft with severe propellant limitations. The best tutorial I've found for this level is Matthias Holzer's. Unfortunately, that tutorial has not been updated for a couple of versions and makes use of a Lunar Landing MFD add-on that I could not find on the Internet no matter how much searching I did. Probably the author dropped out of the community, leaving behind forum postings with tantalizing clues but a bunch of dead links. Fortunately, a different MFD addon allows for a successful landing, albeit with a less authentic experience.

As I worked through the tutorial, I kept a list of inconsistencies I bumped into. To get through the mission successfully required saving early and often (though Orbiter tends to be unstable when loading up a saved state, it'll usually be successful and stable after two or three tries). Since the documentation was so dispersed, and since Matthias Holzer's tutorial does not give all the keystrokes (and some that he does give have changed in newer versions of addons), I kept almost a dozen PDF documents, diagrammed screenshots, and web pages open on another computer, pausing the sim and flipping through them whenever I bumped my head against something that didn't work. Unfortunately, Mr. Holzer's email address doesn't seem to work, so it appears the tutorial will remain unupdated for some time to come. Maybe it'll eventually go the way of the Lunar Landing MFD. I have every sympathy for Mr. Holzer, though, since it takes many hours to get a lunar landing right, and even more to take screenshots and document the process. Now that I've succeeded once, I may not get around to doing it again for a year or more, by which time new versions will have come out.

Lift-Off and the Clock is Running!

At this point, having worked through the Go Play in Space tutorial, you should have a working Orbiter install. As noted above, the best tutorial is the Matthias Holzer's:

The tutorial gives a list of components needed to fly Orbiter, but point to old versions:

If you find any of these links don't work, send me email! (Email address at bottom of page.) Now, do a whole bunch of unzipping and file overwriting, and you'll be all set to fly to the moon. Extracted, it's around 500 MiB in size.

You should have another computer available to cross-reference against the following documentation files — good use for a laptop, think of it as a checklist binder. Orbiter is processor-intensive and tends to go unstable when you flip out of full-screen mode, and running in windowed mode removes some of the immersive effect of being inside an Apollo capsule. Also, the sound cuts in and out with task switches, which is rather annoying.
Orbiter screenshot. T+3 hours, racing outward at 25000 miles/hour. The CSM/LM stack undocks from the S-IVB third stage. As you can see, it's around noon in Florida, since nearly the whole disc is illuminated. Apollo 11 launched at 9:32a Eastern Daylight Time.

Of course, the tutorial should be open. Now, with these files open, you will no longer be puzzled when the tutorial refers to something in an unclear way, or when you don't remember how to do something. Set the sim to 0.1x time, go to the other computer, and hit Ctrl-F to search the PDF files in Adobe Reader. But there are some other issues that forced me to spend significant amounts of time searching the manuals for answers. Minor typographical and filename issues aside (e.g. flying-saturn-v.pdf is now called Flying Saturn V.pdf), here are the main issues that I puzzled over:
Frame capture from Apollo 14 episode of From the Earth to the Moon. Alan Shephard is punching instructions into the DSKY at bottom center.

Good luck, save often, and have fun! For some reason I felt a great sense of accomplishment when the parachutes came out and the command module splashed into the Pacific Ocean. Maybe it's because of all the manuals I had to flip through — a tiny bit like the real astronauts must've experienced during the mission.

Lessons Learned

A lot of sources: the manual, the tutorial, the patchy NASSP documentation, the Apollo walkthrough, the random webpages and forum posts. When put together, they taught me a great deal about the science and engineering of the Apollo program. Often, I knew these facts and principles already, or should've known them, but never took them completely to heart until I went through the whole trip to the moon and back in Orbiter. For example:

Further Exploration

Orbiter follows the Microsoft Flight Simulator model of development — main program from one authoritative source, plus a jumble of third-party add-ons of inconsistent quality. There is a correspondingly very active Orbiter community. The Orbiter home page contains links to some very popular related sites: http://orbit.medphys.ucl.ac.uk/sites.html.

For more of the early NASA programs, the Project Mercury addon is approaching Project Apollo in quality and comprehensiveness. In fact, it is a more complete simulation than Apollo, if only because Mercury is much simpler than Apollo — you can fly a mission fully on automatic, for example. Mercury requires a scenery package to be installed, overwriting Cape Canaveral with its 1962 configuration.

The Project Gemini addon, though, is the neglected middle-child and lags both Mercury and Apollo. Many aspects of the missions are present, including an Agena that can be controlled with computer commands. But the add-on package is hampered by a simplistic control panel, with only a few active instruments layered on top of a static photograph. A far cry from the complex and detailed Apollo control panel.

The Space Shuttle and International Space Station are in the default installation of Orbiter, but most people recommend using the Shuttle Fleet addons with the autopilot. There are many Russian spacecraft available [OrbitMods], from the Vostok to the Soyuz to the Buran space shuttle, but many of these lack good panels. Panels are very difficult to get right, since they must not only look good but work well. Spacecraft can be designed to eye, with some technical data fudged in, since very few people know how they really handle in flight.

And, of course, there are a variety of futuristic spacecraft. Some are from the movies, 2001: A Space Odyssey, Star Wars, and Star Trek being quite popular. Since Orbiter uses Newtonian mechanics and doesn't model relativistic effects, you can actually exceed the speed of light in these purportedly faster-than-light ships. Of course, there are also more physically-realistic spacecraft, like the Delta Glider.

Conclusion

Bruce Irving's blog, Music of the Spheres, contains various musings on Orbiter and virtual spaceflight. Read his blogger profile for an idea of what type of person is interested in this sort of stuff. Good company to be in. Of course, there are some people who look at this as just a game, like playing a combat flight simulator with unlimited ammunition and unlimited fuel. That's fine too, but it only uses Orbiter for entertainment. Orbiter has great powers to teach and inform.

What I appreciated most from playing Orbiter was being able to understand all the engineering constraints and trade-offs involved in designing and executing the Apollo moon missions. It is not possible to appreciate flight merely by reading about it or watching videos, nor is it possible to fully grasp the complexities of space flight by looking at diagrams or Flash animations of orbital mechanisms. You've got to try it yourself and see.

taoyue@alum.mit.edu
Copyright © 1997-2008 Tao Yue. All rights reserved.
This page last updated 16 February 2008.