# Thread: why are there weight limits when sending things into space?

1. ## why are there weight limits when sending things into space?

when NASA sends either people or rovers into space there always seems to be the pressure of the weight limit, why is this?

surely an extra 10, 50, or 100 pounds won't make a difference on a huge rocket....?

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They don't like fat people.

3. Originally Posted by Knowledge_Seeker
when NASA sends either people or rovers into space there always seems to be the pressure of the weight limit, why is this?

surely an extra 10, 50, or 100 pounds won't make a difference on a huge rocket....?
For every pound of payload, you need around a hundred or a thousand pounds of fuel and rocket structure. (thats just a wild stab at a guess so don't jump)

Plus, most of the formulas for orbit, flight, trajectory and the like are weight dependent, so every ounce has to be accounted for to be as accurate as possible.

4. Originally Posted by NEOWatcher
Plus, most of the formulas for orbit, flight, trajectory and the like are weight dependent, so every ounce has to be accounted for to be as accurate as possible.
So.

You can add up all the extra weight into the new formula.

But why does weight matter? ( a reasonable weight of course)

5. Originally Posted by Knowledge_Seeker
So.

You can add up all the extra weight into the new formula.

But why does weight matter? ( a reasonable weight of course)
Let me put it another way...

\$

6. NEOWatcher is exactly right. More payload mass = more fuel to lift it = more fuel to lift the extra fuel. Also the payload maximum tends to be towards the upper limit of the rocket to get the payload where it's going to, so if you exceed that, you have to go up a rocket size, and that's even more extra money. So basically, the more mass you want to sent the more you have to pay and when you have a strict budget, you have a strict mass limit.

7. Not just money, but once more, the mass limit. The more fuel you have to add, the more it weighs down the actual rocket... and after a time, you can't get the rocket into space at all, because it's just too plumb heavy, what with all the extra fuel and metal used to hold the fuel.

8. you can't get the rocket into space at all, because it's just too plumb heavy

that's when....

you have to go up a rocket size, and that's even more extra money.

9. Every rocket has a limit that it can put into orbit. The least bit over that limit, and the satellite burns up in the atmosphere. Rockets are big because it takes a huge amount of energy to get a satellite into orbit.

10. Originally Posted by PhantomWolf
you can't get the rocket into space at all, because it's just too plumb heavy

that's when....

you have to go up a rocket size, and that's even more extra money.
Assuming you can even get up at all, and that you can actually get away with adding extra rocket size. There gets to a point where the design becomes unreasonable, and you can't add anymore to it at all, even with the money.

11. There gets to a point where the design becomes unreasonable, and you can't add anymore to it at all, even with the money.

This is true, but by then we're talking large orbital platforms, not what I suspect the OP was wanting to know about, probes and landers like Cassini and the Mars Rovers.

12. Originally Posted by Knowledge_Seeker
surely an extra 10, 50, or 100 pounds won't make a difference on a huge rocket....?
I think maybe it's the same kind of thinking as, why does a company save on this table? Surely an extra 50 dollars isn't going to break the company. But the problem is, it's a kind of "penny saved" issue. If you say, "well, a pound here won't make a difference, a pound here won't make a difference," then soon you'll have tons of weight more than you wanted.

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It has a lot to do with the fact that to put something into orbit costs, what, something like \$10,000 per lb I think.

14. Here's another slant.
We will choose the smallest rocket possible to lift our payload. A bigger rocket means bigger money and the budget is already squeezed.
Now lets say our rocket is capable of 1000lbs, and our satellite is designed at 985lbs because the design and the rocket choice went hand in hand.
Now someone says "hey we can make a fortune sending remains into space." (Sorry; but thats the best example I can think of at the moment)
So how much can they send?
1) they need an ejection system (maybe)
2) they need something to hold them in
3) they need the extra fuel
Let say they've gone to 1010lbs because they didn't watch too closely. The satellite can't reach orbit now.
It's pretty much a case of "the straw that broke the camel's back."

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It's actually much less than \$50,000 per pound. It costs about 2.5 times more to move something to geostationary orbit than it does to move it to low Earth orbit, and LOE costs as little as \$211 (for the Russian Shtil) to around \$5,000 for the Atlas 2AS and Arianne 44L, which is only about \$300 more expensive than the Space Shuttle. (Note: After the Russian Shtil, the next least expensive rocket is the Ukraine's Xenit 2, at \$1,404)

All costs given in year 2,000 dollars.

Source: http://www.futron.com/pdf/FutronLaunchCostWP.pdf

16. ## \$\$\$

Curiously, NASA missions are rarely subject to being too heavy. In fact, most launches carry ballast to increase the lift weight, because the payload is too light. The real problem is cost.

The routine goes something like this. First, a team will decide what science objectives they want to meet. Then they select a suite of instruments needed to meet the desired objectives. Then they compare that list to the funding limits for the program they are proposing too. Launch is going to cost somewhere between \$100,000,000 and \$200,000,000 (unless you go on a Shuttle, which always costs \$500,000,000 just to launch). Usually the programs fund anywhere from \$100,000,000 to \$500,000,000, which includes the launch cost. So it is almost always the case that the first list of desired instruments breaks the budget and can't be done. Then they have to decide what smaller list of objectives & instruments will make the mission worth flying.

Instruments aren't cheap, so if you decide to add a 1-kg instrument to your mission, it's going to cost a minimum of \$100,000, plus whatever you need to add to the mission to support the "new" instrument (extra power, extra fuel for re-orienting the spacecraft & etc.). The missions are almost always driven by cost, not weight, unless you get close to the weight limit for the launch vehicle. Then it's still cost, because adding the weight and moving up to a heavier lift launcher is a big cost, probably an impossible one.

And do keep in mind that at the moment the NASA budget is as tight as it ever has been. Sofia is zero in the FY2007 budget, WISE & PlanetQuest have been "delayed", and the JWST is so over-budget that a host of other astronomy projects are being delayed or zeroed. Adding as much as \$100,000 to any project is unthinkable.

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