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After seeing this in Security SE

Magnetic core memory isn't quite extinct yet. The two Voyager spacecraft used it and are still functional. Core was/is certainly non-volatile.

I did some interwebbing which led me to Wired.com's 2013 article Interstellar 8-Track: How Voyager's Vintage Tech Keeps Running which contained the NASA image below captioned "Voyager's internal components during assembly."

This well-engineered aluminum structure looks like something that a large group of people could jump up and down on all day without structural issues, though they'd better turn of the attitude control system and the eight track tape player first perhaps.

This made me wonder, after a traumatizing launch with multi gee static and dynamic loads, just what kind of forces a deep space spacecraft actually has to deal with, and is much of the structural mass instantly unwanted dead weight as soon as the spacecraft is launched and off to work on a positive geocentric C3.

Questions:

  1. How much of a deep space spacecraft's structural mass is useless dead weight after launch and just "along for the ride" and soaking up precious delta-v?
  2. Are there any designs or plans for a spacecraft to shed some of it as soon as it's inserted into an interplanetary trajectory?
  3. Has this already been tried in the past?

Voyager's internal components during assembly - NASA from Wired's "Interstellar 8-Track: How Voyager's Vintage Tech Keeps Running"

uhoh
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    If it is needed during launch, it is necessary and therefore not a dead weight. Having it along with you cost you no delta-V as, you have already accelerated it. (Well, it still makes correction maneuvers and re-orienteering require more force, but I do not think this is what you meant). To clarify, could you describe what benefits do you expect to be achieved by point 2)? – Suma Feb 05 '21 at 07:41
  • Structural items you can jettison safely later are going to be some combination of more massive, more expensive, more complex, and less reliable than structural items that stay put, so there generally needs to be a pretty good reason to bring them along just to jettison them after the spacecraft has already spent most of its delta-V. – notovny Feb 05 '21 at 18:16
  • @Suma I've asked how much is dead weight after launch. Imagine going on backpacking vacation for a month, but leave 1 hour before finishing a report. So one carries around a laptop and books in ones backpack for the month because they were needed for the first hour. – uhoh Feb 06 '21 at 02:16
  • @notovny the spacecraft is usually a payload during the time that it is injected into its interplanetary trajectory and has spent none of its delta-v yet. The entire time that the spacecraft uses its own propulsion excess structural mass that was required for loading during launch is dead weight. – uhoh Feb 06 '21 at 02:18
  • To put things into some perspective, taking New Horizons as an example, after the 3rd stage has injected it into the interplanetary trajectory (cf. Star 48 Use on New Horizons), it seems its total delta v for correction maneuvers was 290 m/s - (see NASA) and the total mass about 400 kg. – Suma Feb 06 '21 at 07:52
  • @Suma Indeed New Horizons' dry mass is 400 kg, but the payload mass was only 30.4 kg. Every kilogram counts; 5 less kilograms in structure could be a new science instrument, or more fuel for maneuvering to intercept yet another KBO... I think arguing that mass isn't always a critical, limiting parameter for deep space missions isn't the way to go here. NH is incredibly light and they've taken pains to reduce the structural mass compared to the Voyager days. I Think that's probably where the answer to the question lies. – uhoh Feb 06 '21 at 08:08
  • random factoids: Dry masses of some other deep space spacecraft are a lot larger: Cassini: 2,523 kg, Juno: 1,593 kg, OSIRIS-REx: 880 kg, Dawn: 747.1 kg – uhoh Feb 06 '21 at 08:10
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    One note about NH mass: often mentioned 30 kg of scientific payload seems to be the mass of the instruments only (http://pluto.jhuapl.edu/Mission/Spacecraft.php). It does not include the RTG (with 11 kilograms of plutonium dioxide - https://www.planetary.org/space-missions/new-horizons) or communications. – Suma Feb 06 '21 at 08:21
  • @Suma I've just asked Are New Horizons' thrusters a bit on the weak side? (~157 sec) – uhoh Feb 06 '21 at 08:41
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    Even if a structural element is no longer structurally needed in deep space, it can retain its usefulness in relation to the position of the center of mass, so it is no longer a strictly useless mass. (this beam is there to carry the hole that's been drilled in it in order to balance the whole thing) – user721108 Feb 06 '21 at 16:52
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    However, it is pleasant to imagine a payload adapter that's maybe heavier and more complicated, but that would act as a protective exoskeleton during launch, releasing a frail and light probe designed to withstand only the stresses encountered in deep space, and no more. – user721108 Feb 06 '21 at 17:45
  • @qqjkztd thanks, yes the only thing necessary to answer #2 would be evidence of some plans or designs of something like this. It could be a "design alternative" that never got off the ground. As a hat tip to your excellent point on balance post-deployment deployed: Balancing Perseverance Mars Rover; couldn't they just “move stuff around a little” rather than add 6+ kg of dead weight? and Why did New Horizons have to be spin-balanced to grams-level precision? (With quarters!) – uhoh Feb 06 '21 at 22:46
  • @qqjkztd oh, and this too! :-) https://i.stack.imgur.com/RWcIT.jpg cool question btw. – uhoh Feb 07 '21 at 01:04

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