Question
If a 747 can carry a space shuttle, why are airlines so strict with baggage allowances?
Answer
The sincere answers are correct (I haven't checked the numbers, but they look right based on almost forgotten aerospace engineering classes), but I don't think they will satisfy the OP because I dont think that's the real question.
So I'll answer the question seriously. I am an aerospace engineer, but this is not an aerospace engineering question really. It is not even an economics question. It is a psychology question.
The only substantive interpretation I can assume here is that unconsciously, the OP feels like a couple of extra kilos is trivial/rounding error level weight. If you calibrate purely based on mass and do a wild guesstimate, you might intuitively guess that the marginal cost of carrying the extra weight will be less than the added transaction costs of doing something about it at check in. After all, our everyday experience of +2kg seems very innocuous. Surely policing it is not worth it?
Your intuition is misleading you for two reasons.
1. Your mass instincts are correct (2 kg IS trivial), but the reason you're guessing wrong is that you have no natural calibration for high accelerations and velocities. Think momentum, not mass. There is a reason a speeding bullet does more damage than a bowling ball dropped on your toe. You are not walking your +two kg, you are shooting it at near bullet speeds. Imagine a gun that can shoot a 2 kg cannonball from LA to NY. Getting it? Also we have NO intuitions about aerodynamic lift and drag, since we are not birds. Suffice it to say that 2 kg held up by solid structures is a VERY different beast from 2kg held up by an airflow pressure differential and pushed against a Category 5 hurricane level wind. Don't believe me? Try balancing a ping-pong ball by puffing out air from your mouth for a few seconds. Or try to move a toy car by blowing on it. Your drag instincts are even worse than your lift instincts. How much further do you think a typical artillery shell will travel in a vacuum instead of air? 10% more? 50%? How about TEN times further. That's right. Your high-velocity drag intuitions are off by an order of magnitude, not a few percent (particularly relevant since the piggy-back shuttle isn't just adding more weight, it is adding a crap-load more drag. It probably adds a little extra lift, biplane style, but not even enough to hold itself up (remember the shuttle is a rocket that glides back to earth, not a plane in its own right).
2. The other reason your instincts are misleading you is that you are missing aggregate human behavior. You probably assumed unconsciously that extra poundage is an outlier phenomenon on a typical flight. Maybe 2-3 people will bring too-heavy bags. Right? Wrong. If airlines were sloppy about enforcing weight limits, the MAJORITY would exploit it maximally. How do I know? Look at the number of people who bring max-size roll-ons. Look at the number of people who consistently go 5-8 mph over the speed limit because they know that's the margin of error for radar guns. In other words, they exploit ambiguity in their own favor. An honor code would just create a principal-agent problem. So 1-2 people going over would probably not be worth policing with respect to added transaction costs for the entire airline's operations, but the problem is that NOT policing would lead to 200 people abusing the ambiguity per flight, not 2. That's 400 extra kg. Or about 6 extra average humans. Or 4 extra Americans. And remember, that's near-bullet-speed, not walking speed. Definitely not in the transaction cost noise zone, given the razor thin margins in airlines.
That scratch your itch?
So I'll answer the question seriously. I am an aerospace engineer, but this is not an aerospace engineering question really. It is not even an economics question. It is a psychology question.
The only substantive interpretation I can assume here is that unconsciously, the OP feels like a couple of extra kilos is trivial/rounding error level weight. If you calibrate purely based on mass and do a wild guesstimate, you might intuitively guess that the marginal cost of carrying the extra weight will be less than the added transaction costs of doing something about it at check in. After all, our everyday experience of +2kg seems very innocuous. Surely policing it is not worth it?
Your intuition is misleading you for two reasons.
1. Your mass instincts are correct (2 kg IS trivial), but the reason you're guessing wrong is that you have no natural calibration for high accelerations and velocities. Think momentum, not mass. There is a reason a speeding bullet does more damage than a bowling ball dropped on your toe. You are not walking your +two kg, you are shooting it at near bullet speeds. Imagine a gun that can shoot a 2 kg cannonball from LA to NY. Getting it? Also we have NO intuitions about aerodynamic lift and drag, since we are not birds. Suffice it to say that 2 kg held up by solid structures is a VERY different beast from 2kg held up by an airflow pressure differential and pushed against a Category 5 hurricane level wind. Don't believe me? Try balancing a ping-pong ball by puffing out air from your mouth for a few seconds. Or try to move a toy car by blowing on it. Your drag instincts are even worse than your lift instincts. How much further do you think a typical artillery shell will travel in a vacuum instead of air? 10% more? 50%? How about TEN times further. That's right. Your high-velocity drag intuitions are off by an order of magnitude, not a few percent (particularly relevant since the piggy-back shuttle isn't just adding more weight, it is adding a crap-load more drag. It probably adds a little extra lift, biplane style, but not even enough to hold itself up (remember the shuttle is a rocket that glides back to earth, not a plane in its own right).
2. The other reason your instincts are misleading you is that you are missing aggregate human behavior. You probably assumed unconsciously that extra poundage is an outlier phenomenon on a typical flight. Maybe 2-3 people will bring too-heavy bags. Right? Wrong. If airlines were sloppy about enforcing weight limits, the MAJORITY would exploit it maximally. How do I know? Look at the number of people who bring max-size roll-ons. Look at the number of people who consistently go 5-8 mph over the speed limit because they know that's the margin of error for radar guns. In other words, they exploit ambiguity in their own favor. An honor code would just create a principal-agent problem. So 1-2 people going over would probably not be worth policing with respect to added transaction costs for the entire airline's operations, but the problem is that NOT policing would lead to 200 people abusing the ambiguity per flight, not 2. That's 400 extra kg. Or about 6 extra average humans. Or 4 extra Americans. And remember, that's near-bullet-speed, not walking speed. Definitely not in the transaction cost noise zone, given the razor thin margins in airlines.
That scratch your itch?