Why is the James Webb Space Telescope's data storage space so small?

Question

In a previous question, I learned that the JWST's solid state recorder (SSR) can hold at least 58.8 Gbytes of recorded science data. Which each day requires two, 4 hour downlinks with earth to empty its data buffers. In another question, I learn that the spacecraft's storage could fill up in as little as 160 minutes.

I'm curious to why the designers chose to give this $10 billion state-of-the-art satellite such a small amount of storage space?

I'm asking because I'm assuming there might be times where a transmission doesn't complete in its scheduled timeframe or where it might miss a day due to ground control issues. Or even a time where it has to stop collecting data and wait for its downlink window b/c the disk is full.

Wouldn't the spacecraft be more future-proof if it had a larger data buffer, so it also isn't affected by any "disk" sectors that have gone bad during its operation?

Or is there something I'm not considering, like a design constraint perhaps?

Answer 1

Best Technology Available

By late 2002, SSDs had just reached ~80 GB capacities. Of course, JWST is not going to take a product which has never seen enterprise deployment, and plonk it into \$2 billion platform with fingers crossed, hoping the vendor did a really good job. The bleeding edge referred above is for terrestrial, consumer-grade drives. Once we factor in radiation hardening for space usage, existing product history for confidence, and whatever additional redundancy mission designers chose to hedge their $2+ billion toy for its 5-10 year lifetime, 68 GB starts to look pretty reasonable.

Timeline

Yes, it took 10 years to go from MB to GB, and another 10 to go from GB to TB. So by 2010, we already had TB standalone drives (not arrays) in a COTS 3.5" SATA form factor. But by 2007, the JWST had already passed most of the core reviews, so technology had to have been chosen years prior, using whatever was available then. By 2006, NASA had already shelled out $1 billion on the project. The prime contract was awarded in 2003. It is not hard to imagine that the data storage technology was selected within a year or two of this point in time, given that they had already burned through a billion dollars in development just 3 years later.

If we were to redesign JWST right now, we could choose something like Mercury's 440 GB space-qualified RH3440. This is bleeding-edge technology, and it's not even 1 TB. Also note that it uses more "primitive" Single-Level Cell NAND technology, rather than high-density Quad-Level Cells. Obviously, this is for robustness and part of what makes it space-qualified. This is why you cannot compare consumer-grade and space-grade products on a level basis. Obviously, we could put a few of these in the JWST, and get over 1 TB of storage, but I would imagine that putting a more than dozen of them on board would push size and power constraints. So let's say we could get up to 6 TB of storage for JWST 2.0, 2021 edition.

The exponential growth of SSD density has followed an approximate ratio of 1000x per 10 years. Rewind 10 years to 2011, and we would expect to plop a mere 6 GB on board using the cutting-edge space tech available at the time. Go back 17 years to 2003-4, and the fact that it has 60+ GB of SSD storage is actually looking pretty remarkable, given that we would extrapolate it to maybe 60 MB (space-grade!!!). There may be well more than a dozen discrete drives on board JWST (can't find design details at that level of granularity).

The real question is not: "Why is there so little storage?" but rather: "How did they get so much on board?" Perhaps they were allowed to cheat and update the storage later in the design process to newer but still mature technology. The Critical Design Review, which I imagine cemented a lot of decisions into stone, occurred in 2010. If they got to harvest 6-7 years of advances, that could explain a 100x improvement over expected capacities.

Answer 2

Space hardware almost always is pretty archaic technology. The problem is the long lead time coupled with the need to certify it for flight. It wasn't archaic when it was engineered. The JWT was redesigned in 2005 so look at 2005 tech, not 2020 tech.

Answer 3

The other answers are good. I'll add that, usually due to misplaced budget constraints, project plans don't tend to expect enough iteration. A project plan is like a battle plan -- rarely survives contact with reality. Plan to throw away the first several (cheaper, partial) versions, and you'll have the flexibility of updating technology based on new information.

A great example of a project plan that had iteration built in was the Mercury/Gemini/Apollo series -- that effort would have failed horribly if we had planned to put boots on the lunar surface on the first launch.

Shuttle went the other direction -- with Enterprise the only prototype orbiter funded, we were never able to evolve that system into one that was truly safe, reliable, or cheap.

In recent years, we've finally started to see newer launch vehicles and satellite constellations that do follow more of an iterative development path -- the contrasts in schedule, cost, and capabilities are striking.

If iteration isn't built in, then early decisions get exhaustively analyzed and errors made on the side of caution, because everyone making those decisions knows there's going to be little ability to test hardware until it's too late to change. The resulting limitations have to be accepted as constraints as the project proceeds over subsequent years. These internally-generated constraints in turn tend to lead to projects running overtime, over budget, or failing completely.

It's bizarre to think that, while science is all about change based on new information, "big science" projects rarely can have that feature built into their own plans and contracts -- partners and vendors tend to be contracted to deliver complete, finished goods once, fully specified beforehand and built correctly the first time. The universe just doesn't work that way.

Answer 4

Looking backwards at tech, it necessarily looks primitive. One quote is probably just a legend, Bill Gates allegedly said back in 1981: "640K ought to be enough for anybody." And for people back then, this probably was reasonable.

Notice that in 1991,

SanDisk ships its first solid state storage drive or SSD. The SSD had a capacity of 20MB and sold for approximately $1,000.00. It was used by IBM in the ThinkPad pen computer.

The $1,000 won't be an issue here. But 20MB was the bomb. Some years ago, I bought the smallest pen drives on the market for giving away some docs, and they were much bigger than that.

In 1999, BiTMICRO made a number of introductions and announcements about flash-based SSDs, including an 18 GB 3.5-inch SSD.

For people designing the satellite around the end of XX century, the 60GB looked like 'wow, I wish I had this at home' kind of volume.

Answer 5

Because it is expected to be able to download its data between targets, so doesn't need vast amounts of space and any data it does accumulate will take time to download, so there isn't much point building up a huge buffer.