21
$\begingroup$

Back in the day, it was standard for commercial airlines to calculate the mass of an airliner before takeoff, using assumptions of average passenger weight along with accounts of fuel loaded or burned in previous flights..

After a crash in 2003 (which was largely due to erroneous assumptions of passenger weight) Onboard Weight and Balance Systems (OWBS) were developed. These systems use compression of the landing gear suspension to measure the actual weight of the loaded aircraft before take-off.

The idea was to switch from calculating mass to actually measuring mass in order to avoid errors (and accidents).

Presumably re-entry mass of the Shuttle was an important parameter. The mass of the space shuttle would be lower at the time of deorbit than at take-off. Was the deorbit mass of the space shuttle measured or calculated?

Obviously landing gear compression could not be used in orbit. But was there an alternate method of measuring mass? F=ma, so a thruster firing accompanied by accurate measurement of thrust and acceleration would do the job. Inertial navigation system accelerometers have 0.001-0.01milli-g accuracy and load cells are available with 0.02% accuracy.

Improve this question
$\endgroup$
9
  • 1
    $\begingroup$ You can easily measure mass by trying to accelerate it. That's what the sensors in the landing gears are measuring anyway with the acceleration provided for free by Earth's gravity. When you can't use gravity use rockets. $\endgroup$
    – slebetman
    Commented Aug 15 at 5:45
  • 1
    $\begingroup$ @asdfex Not in a free-fall. But you can on the ground. Earth's gravity will continue accelerating you even if you are sitting still on the ground. $\endgroup$
    – slebetman
    Commented Aug 15 at 8:24
  • 1
    $\begingroup$ Where do you draw the line between "measurement" and "calculation"? Is using the fuel gauge to determine the remaining propellant volume, and converting that to mass measuring or calculating? I would think almost any method of measuring mass other than a balance scale will require calculation. Even a common bathroom scale in reality "measures" an electrical current and "calculates" your weight from that. $\endgroup$
    – Nuclear Hoagie
    Commented Aug 15 at 16:54
  • 3
    $\begingroup$ "So aircraft switched from calculating mass to actually measuring mass in order to avoid errors" No, one or two airliner self-weighing systems were offered but rarely used. They were apparently maintenance nightmares. The first gen systems (A300 basically) were for verification only, you still needed to do the calculations. The second gen systems (777F) could be used operationally. $\endgroup$
    – user71659
    Commented Aug 15 at 17:49
  • 1
    $\begingroup$ @NuclearHoagie ... good point. Calculations can be 100% accurate. It is the inputted assumptions and measurements which have errors. In the accident calculation, the assumed average passenger weight was 185lb. Actual weights taken from medical records showed the average weight was well over 200lb (working from memory here). The error was in the assumption, not the calculation. $\endgroup$
    – Woody
    Commented Aug 15 at 20:37

1 Answer 1

Reset to default
29
$\begingroup$

Not only re-entry mass, but center of gravity (CG) locations, touchdown weight, etc.

Mass properties were calculated on the ground and uplinked. The responsible flight control position was FDO "Fido" the Flight Dynamics Officer.

They had ground software to assist in the task. The FDO Console Handbook Volume IV Chapter 4.3 Maneuver Generation spends pages and pages on the process. Here's a short quote

The purpose of the mass properties task for deorbit prep and entry phase operations is to compute predicted orbiter mass properties at the following points; before and after the deorbit burn, EI, TAEM, and landing. These include weight, CG’s, and engine trims. The predicted weights and CG’s at EI and Mach 3.5 (M3) are also of interest to the Guidance, Navigation, and Control (GNC) Flight Controller for glideslope, elevon schedule, and body bending filter selection requirements during entry.

Here's a high level view of the calculation process from Volume III of the console handbook:

Procedures entail configuring PMP displays to represent groups of payload masses, together with Orbiter consumables, using either telemetry data (for current values) or predicted data. These groups can be summarized as cargo, cryogenics, and non-propulsive consumables. Once mass property information for individual groups is determined, each is added together and combined with data for the inert Orbiter and propulsive consumables to determine the entire Orbiter mass properties.

Much of the inputs came from other flight controller positions as stated back in Volume IV:

...the flight controller positions of Environmental and Consumables Management (EECOM), Electrical Generation and Integrated Loading (EGIL), and MMACS. These positions are responsible for providing much of the predicted consumables information required for EOM mass properties predictions.

Here's a quote from further along in Volume IV just to confirm that they got uplinked:

If the expected post deorbit burn weight is very close to the weight switchpoint (within 200 lb), it may be desirable to bias the weight that is uplinked with the PEG 4 targets, which would allow some margin between the burnout weight and the switchpoint weight.

Since it would be nice to have a linkable reference, I'll point to my friend and former FDO Roger Balettie's website. He mentions this process in passing where he talks about his last shift as a FDO:

TOTAL WT: 224971

This was noting that we had done a routine update of the current Orbiter weight (in pounds), based on various things – including fuel expended so far, cargo/payloads transferred, consumables used, etc. It was important to know the vehicle weight for not only on-orbit state vector propagation, but also in the event of an unplanned or emergency deorbit situation to know if we needed to burn off extra fuel to get down below a certain maximum down-weight level.

The weight was visible to the crew on the Deorbit Maneuver computer display and could be changed by them there.

picture of spec with weight highlighted It was also read up to the crew during Deorbit Preparation to be entered on the Burn Pads.

picture of burn pad with weight highlighted

References:

Acronymology:

  • EI - Entry Interface, 400,000 feet altitude
  • EOM - End of Mission
  • MMACS - Mechanical, Maintenance, Arm and Crew Systems flight control position
  • PEG - Powered Explicit Guidance
  • PMP - Paperless Mass Properties calculation software
  • TAEM - Terminal Area Energy Management, ~ 85,000 feet altitude
Improve this answer
$\endgroup$
10
  • 5
    $\begingroup$ If I’m reading this correctly, it sounds like they only calculated the weight, rather than directly measuring it. Is that accurate, or did they do something w/ a known thrust level & orbital parameter change/etc to calculate the mass? $\endgroup$
    – fyrepenguin
    Commented Aug 15 at 0:21
  • 3
    $\begingroup$ @fyrepenguin I edited the answer to add some info. Briefly, they took the prelaunch mass properties for the inert orbiter and got consumables masses from the other flight control teams, then calculated the current mass properties using their "Paperless Mass Properties" software. I did not see anything about propulsive maneuvers to validate the results. $\endgroup$
    – Organic Marble
    Commented Aug 15 at 1:18
  • 5
    $\begingroup$ @fyrepenguin the document does mention that errors in the mass calculation can cause burn times to differ from what is predicted. "If TGO counts down at a different rate, one of several anomalies may exist. The Orbiter weight used by guidance may be in error, causing measured acceleration levels to differ from those expected by guidance." (TGO is time to go in the burn) So it looks like they were cognizant of that potential source of error during burns but didn't do special validation burns. $\endgroup$
    – Organic Marble
    Commented Aug 15 at 1:29
  • 9
    $\begingroup$ @OrganicMarble so is it fair to answer, "No, the space shuttle did not weigh itself. Mass was calculated instead of measured. But during the deorbit burn, acceleration was used to double check the calculated mass and the burn adjusted accordingly in real time" ? $\endgroup$
    – Woody
    Commented Aug 15 at 1:43
  • 13
    $\begingroup$ @Woody The burn kind of "adjusted" itself, it would burn until it reached the burn targets or ran out of prop; no realtime mass error correction. But otherwise yeah. $\endgroup$
    – Organic Marble
    Commented Aug 15 at 2:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.