The trek of Artemis 2 has spawned multiple Sky Lights posts over the last few months. This week we’ll take a close look at NASA’s Space Launch System (SLS). The SLS carried the Orion Crew Module (named Integrity by the astronauts) to the Moon and back. We’ll start with the launch sequence.
The video starts at T-10 seconds. At this point the SLS is already disconnected from the tower and running on internal power. Several crucial events occur even before the rocket leaves the launch pad:
- Just before the engines ignite (this is visible if you look for it) some 450,000 gallons of water flood the launch pad under the rocket. This is the Sound Suppression System. It minimizes reflection of the 176 dB acoustic energy from the engines, protecting the rocket itself as well as the pad. It also insulates the pad from the 3,200°F heat of the rocket exhaust — hot enough to melt steel and turn sand into glass.
- Around that same time you’ll see what looks like “sparklers” igniting under the rocket. These are the Hydrogen Burn-Off Igniters. Even before the engines are fired up, highly flammable hydrogen fuel is being leaked from the nozzles. To prevent it from building up and potentially causing an explosion it’s continuously burned off by sparks showering the area.
- At T-5 seconds the core engines (four RS-25s) are ignited. They quickly ramp up to their full combined thrust of 2 million pounds. That’s not enough on its own to lift the 5.75 million pound (fully-fueled) SLS plus the Orion crew module, but the RS-25s must be confirmed operational before the boosters are ignited.
- If all goes as planned, at T-0 the two solid rocket boosters (SRB) will be ignited, and the hold-down bolts will be explosively released. It only takes 0.6 seconds for the SRBs to reach their full combined thrust of 7.2 million pounds. With thrust now exceeding its weight, the SLS begins to ascend. The initial acceleration is slow, but the SLS burns 30,000 pounds of fuel every second so acceleration increases as it burns off weight.
- Unlike the core RS-25 engines, the SRBs cannot be throttled. Once lit, they’ll burn until they’re out of fuel — a bit more than 2 minutes into the flight. At this point, the rocket has a speed of 3200 mph, an altitude of 30 miles, and is around 16 miles downrange from the launch site. You can see in the video how the rocket is not going straight up, but doing what they call a roll pitch maneuver so that when they reach orbit they’ll be moving in a horizontal direction.
- When the SRBs burn out they’re ejected by 16 small separation rockets that push them away from the SLS and from each other. The SRBs are designed to be expendable and splash down in the Atlantic Ocean. Here’s what that separation looked like as seen by a ground-based infrared tracking camera. In infrared, the sky looks dark and you can see through clouds. On the left is the SLS continuing to orbit:
- The four RS-25s continue burning for another 8.5 minutes to achieve orbit, reaching a speed of 18,000 mph at an altitude of 100 miles. That’s a stable orbital configuration. But they were headed for the Moon, so …
- After checking onboard systems, around 90 minutes after launch, they ignited the Interim Cryogenic Propulsion Stage (ICPS) RL10 engine for an 18 minute burn. That further increased their altitude and circularized their orbit. About an hour later, the RL10 was fired a second time to put them into a highly elliptical orbit with a speed of 22,600 mph at perigee. This exceeds Earth’s escape velocity, so the RL10 could be turned off. Their momentum allowed them to coast the rest of the way to the Moon.
- Because the mission used a free return trajectory, no further maneuvers were needed to complete the mission. So around 3 hours into the flight, the crew capsule separated from the ICPS to practice what are called Proximity Operations. Once they separated, the crew took manual control of the Orion capsule for about 75 minutes. They flew Orion around the spent ICPS stage to practice precise manual flight control operations, simulating the docking maneuvers they will use on future lunar missions.
Here’s a nice infographic from NASA showing how the SLS compares to the Saturn V Moon rocket and the Space Shuttle (both now retired):
There’s a lot of physics behind a rocket launch, even without getting into equations. Hopefully this post clarified some of that for you. It should also foster an appreciation of the level of engineering required for Artemis. Spaceflight is a challenging endeavor — it takes the brightest minds in science and engineering to pull it off.
Next Week in Sky Lights ⇒ Why Wind Gusts and Changes Direction