2

Why did the SLS only reach a TLI speed of 17500 mph when the Saturn V reached a TLI speed of 24500? The SLS was considerably lighter and had 15% more thrust and should have been able to achieve a higher speed.

Nunya Bidness
  • 21
  • 3
  • 4
    Because Apollo didn't take the most efficient route to the Moon. The Apollo missions burned harder on LEO departure to get to the Moon in about 3 days, whereas the Artemis missions are taking a Hohmann transfer course, which takes 5-6. Thrust isn't the issue here, it's delta-V. – notovny Dec 24 '23 at 11:20
  • 2
    Also, your numbers are wrong. You've listed orbital velocity for LEO instead of the TLI speed for SLS. – notovny Dec 24 '23 at 14:26
  • 1
    @notovny it's not even doing a Hohmann transfer to the moon, it's transferring to NRHO, a L2 halo orbit...specifically because it's easier to reach. – Christopher James Huff Dec 24 '23 at 15:14
  • @ChristopherJamesHuff: Artemis I (which I assume is what the OP is talking about, since I don’t know of another launch of SLS so far) was a distant retrograde orbit, not NRHO. Artemis II will be a free return trajectory. – Jörg W Mittag Dec 25 '23 at 02:06
  • @JörgWMittag that's right...I'd disregarded that because it's never going to be repeated, but the question does say "did" and "was", so seems to be about the first launch, not typical operations. The distant retrograde orbit was a similarly high, relatively accessible orbit, though, entered and departed via lunar powered flybys to reduce the propulsive delta-v needed. – Christopher James Huff Dec 25 '23 at 04:08

1 Answers1

5

An hour and twenty minutes after being inserted into low Earth orbit at 17,175 mph (27,640 km/h), the Artemis I SLS Interim Cryogenic Propulsion Stage (ICPS) performed the Trans-Lunar Injection (TLI) burn which accelerated the Orion spacecraft towards the Moon at a velocity of about 22,600 mph (36,371 km/h).

According to Apollo by the Numbers the velocity after the TLI burn performed by the Saturn S-IVB stage ranged from 24,129 mph (38,832 km/h) for Apollo 12, to 24,258 mph (39,039 km/h) for Apollo 15.

The Apollo and Artemis I missions used different trajectories to reach the Moon. The Saturn V TLI burns placed the Apollo spacecraft on a less efficient three-day trajectory, which was also a free-return trajectory that would have looped around the Moon and returned to Earth if a Lunar Orbit Insertion (LOI) burn was not performed by the Apollo spacecraft. This provided a safety margin in case a problem occurred during the trip to the Moon (as actually happened on Apollo 13).

Apollo 8 trajectory
Apollo 8, (NASA)

A slightly modified trajectory was used starting with Apollo 12 and all subsequent lunar missions. Besides the LOI burn a mid-course correction burn was also required to "cancel" the free-return trajectory. This method opened up a larger area of potential landing sites, while still allowing time to check out the spacecraft systems prior to committing to a non-return trajectory.

Artemis I was a non-crewed mission which used a very different trajectory than was used on the Apollo lunar missions, as well as being different from what will be used on the crewed Artemis II and Artemis III flights. Artemis I used a more efficient five-day trajectory to reach the Moon, and was not free-return. Two post TLI burns performed by the Orion spacecraft (not counting course corrections) placed Orion into a Distant Retrograde Orbit.

Artemis I trajectory
Artemis I (NASA)

Artemis II will be a crewed non-landing, non-orbiting mission and will use a free-return trajectory. However like Artemis I it will take more than three days for Artemis II to reach the Moon using a more efficient trajectory than Apollo. After separating from the ICPS the Orion spacecraft will make several burns on the way to the Moon as part of a Multi-Trans-Lunar Injection (MTLI) method.

Artemis II trajectory
Artemis II (NASA)

For Artemis III Orion will be placed into a Near-Rectilinear Halo Orbit (NRHO) where the the crew will transfer to the Starship HLS lunar lander.

Artemis III trajectory
Artemis III (NASA)

Additional Info:

The Artemis I 17,175 mph LEO number is from the NASA TV live broadcast commentary (timestamp 3:36:16) on November 16, 2022

The Artemis I 22,600 mph TLI number is based on the following sources:

NASA pre-launch document - 24,500 mph
NASA TV live broadcast commentary (timestamp 5:00:22) November 16, 2022 - 22,500 mph
CNN Interactive November 16, 2022 - 22,600
CollectSpace November 16, 2022 - 22,670

Apollo TLI (from Apollo by the Numbers)

Mission mph km/h
Apollo 8 24,208 38,959
Apollo 10 24,247 39,022
Apollo 11 24,236 39,004
Apollo 12 24,129 38,832
Apollo 13 24,231 38,996
Apollo 14 24,212 38,965
Apollo 15 24,258 39,039
Apollo 16 24,250 39,027
Apollo 17 24,242 39,014
Steve Pemberton
  • 3,399
  • 1
  • 9
  • 31