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In this video overview of NASA's proposed EM-1 mission, the solar panels on the Orion spacecraft are initially shown extending perpendicular to the spacecraft (at around 3:46).

Orion spacecraft with ICPS and perpendicular panels

Shortly after that, as the perigee raise maneuver begins, the panels are shown dramatically swept back (at 3:52):

Orion spacecraft with ICPS and swept panels

At 5:24 in the video, while flying by the moon, the panels once again extend perpendicularly, but are rotated to face the sun.

Orion spacecraft seen against the moon with perpendicular rotated solar panels

At 6:18, during a propulsive maneuver using the service module engines, the panels are now swept far forward.

Orion spacecraft firing service module engine with solar panels swept forward

What's going on here? Why do the panels sweep both forward and back?

Obviously the sweep isn't for aerodynamic purposes.

The only explanation that makes any sense to me is:

  • The forward sweep helps get the panels out of the way of the expanding exhaust plume from the service module's engine (an AJ-10-190 repurposed from space shuttle OMS);
  • Having paid the weight and complexity cost of a forward sweep mechanism, sweeping backward using the same mechanism reduces mechanical stress on the mountings during ICPS burns.

However, the RL10 engine on the ICPS develops about 110kN; against the 40 tons of stage and spacecraft at the end of the burn, this is only about 1/3g acceleration. It seems wrong that the extended solar panels wouldn't be able to support their own weight in Earth's gravity, if only for convenience in assembly of the spacecraft.

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    $\begingroup$ It uses the panels to swim through space like an octopus. $\endgroup$ – 0xDBFB7 Sep 29 '17 at 20:29
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    $\begingroup$ FWIW, this picture blogs.esa.int/orion/files/2016/03/… from here blogs.esa.int/orion/2016/03/01/testing-solar-array does show a hinge and an actuator that could move them like that. And states "The stresses of flying to the Moon and beyond – and back again – mean the array is designed to bend up to 60º forward and backward, much like a bird in flight." $\endgroup$ – Organic Marble Sep 30 '17 at 1:32
  • $\begingroup$ The latter link does confirm that they move “up to 60º forward and backward”, though it doesn’t quite satisfy my question why. $\endgroup$ – Russell Borogove Sep 30 '17 at 1:37
  • $\begingroup$ Yeah, that's exactly why I didn't write an answer. $\endgroup$ – Organic Marble Sep 30 '17 at 2:10
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It appears that the Solar Array Drive Mechanism (SADM) serves two primary purposes:

  1. Orient the solar panels toward the sun for maximum power generation. From Spaceflight 101:

    The wing is connected to the spacecraft structure via a two-degree of freedom Solar Array Drive Assembly that allows the array to be oriented in two independent axes by the Solar Array Drive Mechanism that is actuated by the Solar Array Drive Electronics (two units, each with two channels) which use inputs from sun and attitude sensors to optimize solar array pointing for maximum power generation.

  2. Reduce stress during acceleration maneuvers. Even though aerodynamic stress is non-existent, accelerating the vehicle causes the long solar arrays to exert torque on the drive mechanism. During Trans-Lunar Injection, the acceleration from the SLS upper stage requires the panels to be canted to the rear.

  3. Protect the solar panels. The lower acceleration burns for Lunar Orbit Insertion and Trans-Earth Injection of the ESM engine require forward canting to keep the panels out of the ESM engine plume. This is not necessary for TLI since the engine is farther away at the end of the SLS upper stage.

From an ESA resource:

For the trans-lunar injection performed with the upper stage of the Space Launch System rocket, actually the iCPS (interim Cryogenic Propulsion Stage), the arrays are canted backwards to sustain in deployed configuration the 1 g acceleration load. For the trans-earth injection performed with the ESM main engine, the acceleration is less severe and the arrays have to be canted forwards to prevent damages from the OMS-E engine plume while minimizing the load on the SAWs.

This information can be corroborated from an informational video published by RUAG, the SADM manufacturer. This article from EDN also gives the same information.

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    $\begingroup$ The acceleration on the spacecraft, and thus the torques on the panels, is in the same direction in TLI, LOI, and TEI -- that is, the engines on the service module point the same way as those on the transfer stage. Thus the Spaceflight101 explanation makes no sense, and there must be some other reason to have both the +55 position and the -60 position. $\endgroup$ – Russell Borogove Jul 30 '18 at 2:31
  • $\begingroup$ I asked a former colleague of mine who works on the service module and while he didn't give me a lot of details, I think it corroborates Matthew R.'s answer. The difference between the two burns is that for the first one mentioned in the quote, the burn is done by the booster upper stage. The 2nd one is done with the revamped OMS engine on the SM, and contamination is a huge problem apparently. He didn't explain why the arrays couldn't be tilted forward for the first burn as well as the 2nd though, so I'm not completely satisfied. Enough for a +1 though. $\endgroup$ – Organic Marble Aug 2 '18 at 17:27
  • $\begingroup$ If I understand your comment correctly, the backward canting in the 1st burn is necessary because of the larger g load than is experienced during the 2nd burn. $\endgroup$ – Matthew R. Aug 2 '18 at 18:48
  • $\begingroup$ Sweeping it forward or backward the same angle would result in the same loads. I haven't seen a good explanation of why it needs to sweep both ways. $\endgroup$ – Organic Marble Aug 2 '18 at 19:06
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    $\begingroup$ I agree that the torques would be the same, but the forward canting would cause the translational force to be into the joint, while the backward canting would cause the force to pull out on the joint. Not sure how relevant that is; I'll have to see if there's any information to support it as part of the decision making process. $\endgroup$ – Matthew R. Aug 2 '18 at 19:37

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