ELI40

Lunar Lander Instructions

Part 3: How to land on the moon

lander

Obviously this lunar lander is not a fully accurate recreation. You're probably sitting in a chair with Earth gravity, you're probably not wearing a spacesuit, and if you look out your window you'll probably see that you're not actually approaching the moon. You're not risking your life doing something nobody has done before while the entire world watches. You also haven't spent the past several days with three people crammed in a tiny vessel the size of a compact car with no beds, no bathroom, and no privacy. You'll have to use your imagination for those parts.

This lunar lander is not a complete simulation but neither is it a game, it is somewhere between the two. The control panel has been simplified, thank goodness. The parts critical to the final stages of landing are still there and have been reproduced as accurately as possible for this little emulator. For example, the astronauts really did have a joystick, though they didn't call it a joystick, they called it a controller assembly. There really was a master alarm button, an abort switch, a lunar contact light, and several checklists to follow. There was no boost button but the throttle did have a boost mode, sort of. More about that below.

The emulator's physics, while simplified to a 2D world, have been tuned to reproduce the Apollo 11 landing. It uses the actual Apollo 11 descent profile which is slightly different than the planned descent profile. Every Apollo mission had a descent profile that varied slightly from ideal. Your descent profile will most likely vary somewhat too. Slight variations are acceptable but there's no room for outright mistakes. The moon already has enough craters.

The speeds and distances are the same as the actual Apollo 11 mission, as is the thrust generated from the descent engine and the amount of fuel remaining. The timer, just like the timer on the real LM, is counting up from descent engine start time. With an engine time of ten minutes, that means there is only about two minutes of fuel remaining. That's more than enough for an experienced astronaut to land safely.[6]

profile
Apollo 11 Planned Descent Profile
Event Time Altitude Speed Sink ΔV Pitch Range
A) Ullage-0:07
B) Descent ignition0:004881455604092
C) Max. throttle0:2648725552933193
D) Rotate face-up2:5644934400050157279
F) Throttle recovery6:242463914561064239
H) High gate8:26751550614553755527342
I) Low gate10:0651268166176162000
J) Touchdown11:541203677500

Low Gate

EngTime
At Low Gate there is approximately two minutes of fuel remaining.

This Lunar Lander Emulator starts at Low Gate which is a fancy aerospace term for the end of the approach phase. With the automated portion of the powered descent nearly complete, Low Gate is where the astronauts got ready to take over the controls. There is approximately two minutes of fuel remaining. The LM is at 500 feet (altitude) above the lunar surface with a descent (sink rate) of 16 feet per second. The target landing site is still 1800 feet away (range) and the horizontal ground speed is 68 feet per second. The LM has a backward lean (pitch) of 15 degrees.

The goal is to get the sink rate and ground speed to approach zero at the same time the altitude and range approach zero. To make sure the LM doesn't tip over when it lands, the pitch should be near zero too. A negative range simply means you overshot the planned landing spot. A negative range is acceptable but a negative altitude would be bad. You should do everything you can to avoid a negative altitude.

To start the emulator, simply follow the Low Gate checklist. Verify the three toggle switches then press and hold the joystick boost button for two seconds and you'll be on your way. Things happen fast. It might be a good idea to complete your training first.

Pitch

pitch

Pitch is an important concept to understand. Basically, it's the steering wheel. Using some quick trigonometry with our starting pitch of 15 degrees, we can see that the horizontal thrust component is sin(15) = 0.26 and the vertical thrust component is cos(15) = 0.97. In other words, most of the thrust is pointed down, slowing the descent rate against the moon's gravity, with a much smaller amount of thrust pointed forward, acting like brakes to reduce the forward velocity.

A pitch of zero (vertical) means the thrust is pointing straight down. Moving the pitch out of vertical creates more horizontal thrust with correspondingly less vertical thrust. For most of the powered descent, the pitch was close to 90 degrees (i.e. feet first) in order to shed the huge orbital velocity of 5560 feet per second. This meant there was no vertical thrust to slow their fall but they were still so high and moving so fast, it didn't matter. At the end of the approach phase, with most of the horizontal velocity nullified, the autopilot pitched the LM forward. This slowed the descent rate and allowed the astronauts to see the ground out the window. When landing, keeping track of the ground is a good thing. This emulator doesn't have a window, instead you'll need to keep an eye on the Surface Hazards indicator.

Pitch is adjusted by tapping the joystick left to decrease the pitch or right to increase the pitch. The pitch gauge shows the current pitch.

Throttle

Increasing the pitch away from vertical will require more throttle to counteract gravity while a smaller pitch requires less throttle. With a pitch and sink rate of zero, a throttle setting of ~52.7% is required to hover. Though with so little fuel available, spending time hovering is not advised.

The boost button produces extra thrust but slowly destroys the engine.

Throttle is adjusted by tapping the joystick up to increase throttle or down to decrease throttle. This assumes the Thrust Control ("THR CONT") toggle is set to manual ("MAN"). For more details, see the Thrust Control section below.

Boost

The Apollo descent engine had two throttle settings: fixed and adjustable. Fixed Throttle Position (FTP) is full-blast and is the most efficient setting. It was used to slow down from orbit. The problem with with FTP is that the descent engine used ablative cooling. That means the engine could only operate at this full-blast setting for a limited time before it would tear itself apart. The astronauts were understandably relieved when it was time to throttle down out of FTP.

An adjustable throttle is necessary for approach and landing. The adjustable throttle operated at 10% - 60% of the maximum thrust because that was the safe range to avoid additional ablation. The only setting above 60% was back to FTP.

The astronauts never used FTP during the final approach and landing but it was there if they needed it. For the emulator, we call this Boost thrust and it is activated with the red button on the joystick. Pressing the Boost button will give you extra thrust at the cost of extra fuel and engine heat. Too much boost will cause the engine to overheat and fail. That's a good thing to avoid.

Engine Arm/Off

The astronauts were trained to shut the engine off a couple feet above the lunar surface. This was to avoid possible nozzle contact. Despite this training, a couple astronauts left the engine running until they were all the way on the surface. Of course turning off the engine too soon could result in an extra large bump that breaks things. With no tow truck available, breaking things could be a problem.

The mission is not complete until the lander settles safely on the moon and the "ENGINE" toggle is set to "OFF". Don't forget this step or else you might accidentally bump the throttle up, lift off the surface, and need to land again.

Likewise, the emulator won't start unless the "ENGINE" toggle is set to "ARM". If the displays say "EnG OFF" simply flip the toggle to "ARM". Also make sure the Abort toggle is down.

Thrust Control Auto/Manual

This is essentially a cruise control for sink rate. In manual mode, the pilot has direct control of the throttle. Bumping the joystick up/down will increase/decrease the throttle setting. This is nice at first but can be difficult to handle when close to the surface.

Setting the "THR CONT" toggle to "AUTO" will engage the sink rate controller. Now, instead of controlling the throttle directly, the autopilot controls the throttle in order to maintain the specified sink rate. Bumping the joystick up will decrease the sink rate (slower descent), bumping the joystick down will increase the sink rate (faster descent).

Automatic thrust allows the pilot to concentrate on adjusting the pitch to control the ground speed without having to fiddle with the throttle to maintain a safe sink rate. Of course it's still necessary to adjust the sink rate but that's easier than adjusting the throttle. Also, if the pitch gets too large, the autopilot won't be able to hold the sink rate steady. The autopilot doesn't care about fuel, altitude, range or anything but sink rate. The autopilot will happily maintain a big sink rate all the way into the ground. Don't let the autopilot do that.

Abort

abort

The Apollo astronauts never had to use the abort switch but it was there just in case. Neil Armstrong got close on Apollo 11. They were getting close to the "Bingo Fuel" call which meant land in 20 seconds or abort. It turns out they had more fuel on board than indicated but nobody knew that until afterwards.

The tricky part about an abort is that it's not guaranteed. If anything too dramatic is happening, like an inverted pitch, huge descent rate, or tumbling out of control, then the abort doesn't have much chance of success. Once the astronauts initiated the abort, it took a few seconds to jettison the descent stage and fire up the ascent engine. Once the stages separated, the autopilot would attempt to right the LM to vertical and get enough altitude to avoid terrain. If all that went well, the autopilot would then pitch over to about 45 degrees for the long push to orbit.

Obviously, abort is a last resort, but it's better than having a new crater named after you.

Master Alarm

If the computer catches any major problems, it lights up the Master Alarm. It's up to the astronauts to figure out what the problem is. Common warnings include excessive pitch, ground speed, sink rate, or engine temperature. If the condition is corrected, the master alarm will turn off by itself. Alternatively, pressing the Master Alarm button will extinguish the light and silence the alarm until another problem happens.

The master alarm is your friend, don't ignore it.

Lunar Contact

This is the best light on the console because it means you're almost there. Hanging off the landing pads are sensors, 5.6 feet long. When one of those sensors feels the ground, it turns on the lunar contact light. Procedure is to then cut the engine and fall the remaining few feet. Some of the astronauts did this, others did not. Either way, assuming your ground speed, sink rate and pitch are all close to zero, then you should soon be safely on the moon.

Surface Hazards

surface
Red = Dangerous obstacle
Yellow = Marginal terrain
Green = Flat and obstacle free

Another inaccuracy you may have noticed about this Apollo Lunar Landing Emulator is that there are no windows. The astronauts had windows but they also practiced landing without them, just in case. While a realistic virtual-reality rendering of the lunar surface would be fascinating, it is beyond the scope of this emulator. Instead, the lunar surface is represented by a scrolling LED bar graph. Each LED represents 33.3 feet, the approximate width of the LM. The zero mark is the current position. The bar graph shows 560 feet ahead and 200 feet behind.

Green is a safe landing area. Green areas are relatively flat and free of obstacles. It should be safe to land in a green area.

Yellow is marginal, it's probably safe but may result in excessive pitch. If you're going to land in the yellow, it's best to get the pitch and ground speed close to zero first. If you're coming in too fast, the landing struts might catch on something and flip the LM over.

Red is a dangerous hazard such as a large rock or deep crater. Landing in a red area will result in a fatal crash so don't land in the red areas.

Hints

The emulator starts at low gate with approximately two minutes of fuel remaining. On Apollo 11, that's when Neil Armstrong stopped being distracted by the unexpected program alarms and looked out the window to realize that the planned landing spot was not safe. He flipped on the Thrust Control to quickly stop the sink rate. He then pitched up to vertical to stop shedding horizontal ground speed. This let him float past the craters and boulders to a safer landing spot on the other side. Once there, he pitched back to reduce the ground speed while gently increasing the sink rate to ease to a landing.

At low gate, the autopilot has everything arranged for a good landing. Leaving the controls as they are for a bit longer will maintain the proper trajectory. When the speeds get low enough, it's time to pitch forward and decrease the throttle. The astronauts were trained to target a sink rate of 3 fps as 150 ft. Ideally, that would correspond with a range of 100 ft and a ground speed of less than 10 fps. That's gentle enough to find a good landing spot but still aggressive enough to land before running out of fuel.

Remember, the astronauts were talented pilots before becoming astronauts. They also received many hours of training before going to the moon. That said, not a single one of them made any serious mistakes. In fact, they made it look easy. With enough practice, maybe you can do the same.

landing
Profile guidelines from Low Gate (emulator start) to the lunar surface.