Weightlessness and the Shuttle landing

Okay, I’m not a physicist, but I heard this morning that the Shuttle’s landing had to be delayed so they could secure the tons of trash and leftovers they’re bringing back from the space station.

Correct me if I’m wrong, but if you pile tons of material into the Shuttle, wouldn’t that affect how it lands – when to fire the rockets to bring it out of orbit, at what speed it comes down, how long it takes to stop once it lands, etc.?

But if the debris they’re bringing back is weightless in space, how do they know how much effect it will have on the landing? Everything can’t be coded and in a database if it’s space junk, can it?

Or is the Shuttle big enugh that a few tons of trash won’t make that much difference?

A bunch of trash rattling around in the payload bay wouldn’t make it impossible for the Shuttle to land, but it probably could cause some damage (to the junk, and to the bay) if it was unsecured, wouldn’t you think?

Also, just because things are weightless in orbit doesn’t mean they don’t have mass. That means that a few “tons” (hundreds of kilograms) plus or minus will have a strong effect on when and with what thrust they fire the rockets. And yes, I’ll just bet they do have a database of every component they take up and bring down, anyway. NASA’s anal about that kind of thing. I don’t know what you mean by “space junk.” We’re not talking about rusted cars and fast-food wrappers. This is stuff they brought into space for a reason, and they wouldn’t bother to bring it down again unless they have a good reason for that, too.

So I guess my question is, how can the Shuttle crew determine the mass of all the stuff they’re bringing back? Obviously they can’t weigh it, and I assume the stuff is made of many different materials, so they can’t simply figure the volume and multiply from there.

You can measure mass in space. Here is a crude way, as I am not knowledgeable about NASA’s specific technique. Estimate earth-weight of object. 20 pounds? OK, push the object and a known 20 pound “weight” with equal force (use a machine of some kind). If the object is going faster than the 20 lb weight, it “weighs” less than 20 pounds. If slower, than it weighs more. Repeat with different weights until you have your desired precision.

You could do it even better by measuring the exact speed resulting from an exact force, but I’m not sure if that’s needed.

If your measuring tools were sophisticated enough, you can take it into the payload, burn your engines for a bit and mesaure the Space Shuttle’s acceleration. Compare that to speed without the stuff in your payload, and compute. (Taking into account burned fuel or any other factors)

Mass can be seen here as resistance to inertial change.
Apologies for crudity of mass/weight and speed/acceleration terms, I’m being non-technical

As I said before, I don’t think your idea of a database of all the stuff is so crazy.

Have you seen the movie Apollo 13? Remember how they had to fix the carbon dioxide scrubbers, and they walked into the room with a pile of stuff including hoses and duct tape and all sort of assorted whoosits, and said, “That’s what they have up there.”? Well, they did know, down to the last roll of duct tape, what they had up there.

If the stuff the Shuttle is bringing down is construction equipment and hunks of ISS components, I’ll just bet that an engineer on the ground can consult a database or a manual tell them the mass of each piece down to the gram.

Maybe they’d do something along the line of muttrox’s suggestion: once everything’s on board, do a quick burn, and use telemetry to deduce the mass of the Shuttle and all its contents–but trying to figure out the mass of every component inertially sounds like too big a pain in the neck.

Yes, they have a database. Everything up there had to be lifted. Repeat: Everything up there had to be lifted. It had to be stowed (stored) and recorded location and how it fit and its dimensions including weight. This is a considerable effort, and is part of the considerable effort preparing for each flight. They did put barcodes on most of the items specifically to aid in tracking for these purposes. In the Shuttle only days it was easier, because everything that went up came down in the same vehicle - kinda nice that way. Where with ISS, they have to track taking things up on one flight and bringing down in another, sometimes using a Soyuz instead of the Shuttle. Yes, it is a lot of overhead.

Discrete objects (like metal boxes, tools, etc) are easy. Trickier things are things like feces, urine bags, clothing, food containers, etc. These often use things like known sized containers for the return trip. For instance, dirty clothing would be folded or rolled and packed into bags and stuffed full. The full bag can be estimated to weigh (or mass) a certain amount based on measurements on earth of clothing and compaction. I think they have some assumed value which is an average. (Not entirely certain on that part.) Garbage like used food containers are, I believe, put into a trash compactor and mushed flat into known sized containers. Again, an average value is probably used.

Please note that none of this is free-floating in the payload bay. All of it is strapped down into storage lockers, either in the Shuttle Middeck lockers or in the Leonardo Logistics Module (LM) racks or in bags in the airlock of the Shuttle. There is a whole section at NASA that is devoted to collecting mass and stowage information and tracking this stuff.

I’m not aware of any calibrated thruster burns or calculations to determine CM. My impression is that they prerun calculations over a range of possible configurations to establish a comfort zone of behavior. Realize they send up the LM full of racks and are bringing it home full of racks, so the change in mass properties may not be that significant. Also, they plan pretty well what they intend to bring down, so they make some good guesses what is being moved when. And then they rely on the the skill of the pilot to compensate for differences in wind, variations in the CM (as long as it is in a certain band, or rather not past a certain point, it is good), etc. I suppose a calibrated thruster burn could be used in combination with reading the effect on the gyros and use that to calculate center of mass, but I don’t think they do it and it might not have the sensitivity level needed. I think they just rely on a ton of preflight calculations and a wide comfort zone in loading conditions.

And no, they don’t really have any way of weighing things. Some methods could be devised, but so far they haven’t been needed. They rely either on preflight measurements (for static objects) or estimates based on density and dimensions (# full urine bags, clothing storage bags, etc).

There are undoubtedly scales for measuring mass in space. Its actually quite simple to devise a method and one can think of inumerable reasons why they would have such devices in a station intended to serve as a laboratory. It would, for instance, be an important part of medical monitoring of astronauts to determine the effects of weightlessness.

A simple method would be a sliding chair between equal strenght springs. Set up an oscillation, measure the period and you can easily calculate the mass.

A quick search even finds me a picture and descriptions of such a device. I understand skylab had one.