Why don't they use ballons to get spacecraft off the ground?

Factual Questions
#1

Here is the thought I have, stay with me if this sounds crackpot-ish, but its a serious question.

I understand that a very, very large pecentage of the weight of a spacecraft ready for launch is its fuel.

Has NASA or any other space body looked (seriously or otherwise) at using large, heavy lifting ballons to get a spacecraft to, say, a few hundred thousand feet and then using engines to gain velocity and go into orbit from there?

Or, would it be just a difficult to get to a sufficient orbital velocity if your launched from 300,000 ft vs sea level?

Stupid idea or possibility?

#2

The Da Vinci team is going to try that to win the X-Prize

#3

I think it’s a matter of finding balloons big enough to carry a large payload (not just the cargo, but all the engines and fuel) up to a sufficient height in an orientation correct for a spacecraft (horizontal won’t work, nor will angled nose-downward).

That, and you have little control over where the launch vehicle ends up, making all other calculations much more difficult: You can’t exactly wander on over to a specific orbit without expending a lot of fuel and making a lot of calculations that otherwise would be unnneeded.

Finally, where would the balloons end up? With rockets, you have a limited area where they’ll land. Launching close to water pretty much solves the problem, hence Cape Canaveral. With balloons, you have large canvas sacks full of lifting gases floating at random with the breeze. Even if you make them outgas immediately upon relase, they’ll still be in circulation (so to speak) for longer than rockets.

All that said, I hope the Da Vinci team makes it all work out. :slight_smile:

#4

Maybe one of the math dopers can figure this out and give us an idea. Let’s say we want to pick up the Shuttle, fully loaded with crew, via giant balloons or whatever. What kind of lifting power would be required to get the Shuttle up to a respectable level to shoot into space?

Or should we just sing “99 Luftballoons”?

#5

The landing weight of the space shuttle orbiter is 104,000Kg. One cubic meter of He will lift 1.12Kg. This means it would take 93,125 Cubic Meters of balloon displacement. That’s about a 56 meter diameter. I have not taken into account the weight of the ballon itself and I have no idea of how much fuel would be needed and thus how much more ballon dispalacement is required.

#6

We do. Sort of. We launch satellites from planes.

The big drawback is the limited benefit. Most of the fuel on board a launch vehicle is used to generate orbital velocity. The proportion varies with final orbital altitude but for low (~130 miles) orbits the fraction of fuel used for velocity as opposed to altitude is on the order of 90%.

So even if you get your launch vehicle up to a fabulous 80 or 90 miles you still have a substantial fuel load on your vehicle to reach orbital speed.

The benefit is better than what it first appears. Aerodynamic drag at the bottom of our ocean of air is subsantial. Thus you see projects like Pegasus launching vehicles from high altitude aircraft (IIRC 45K feet+) where the air drag is about one sixth the value at sea level.

The upshot is although it’s a reasonable strategy, it isn’t a free ride…

#7

Derleth covered it pretty well, but I’ll see what I can come up with off the top of my head. Ok, the Shuttle is 230,000 lbs at landing, so that’s the approx. dry weight of the Orbiter (what we call the part that most people think of as the Shuttle). It would actually be more with the second stage to get it from floating to space, but that gives you an idea of the numbers.

Now think of how big a blimp is. That’s neutrally buoyant - how much bigger would it have to be to lift a quarter million pounds? And how are you planning to fill all of that in a controlled fashion? How will you attach something that big to the spacecraft?

As you ascend the atmosphere gets thinner, which means that the pressure outside of the balloon will drop, making the balloon want to expand. I’m not sure why, but this seems significant.

Now, in order to get the orbital slot you want, you have to set off the second stage at a specific time, pointed in a specific direction, while in a specific location. What happens if the wind is blowing in the wrong direction during the half-hour you have to launch? Of course, you’ll have to have some sort of navigation system, telemetry, etc. which will only add to the amount of lift you need. Also, the balloon may float, but it still has inertia. It doesn’t exactly turn on a dime.

Finally, as Derleth pointed out, having expended launch vehicle fall on the populace is generally frowned upon. Therefore, most countries launch over unoccupied water or land. The falling balloon would be very difficult, if not impossible to control or even predict, especially after having a rocket go off right next to it.

In summation, probably possible, probably not the best solution.

Also, a little detail - the rocket is called a launch vehicle and that’s what’s filled with fuel. The spacecraft is the thing at the top of the rocket and only has a little bit of fuel comparatively.

On preview, thanks to MonkeyMensch for bringing up the whole altitude vs. horizontal velocity thing. No way was I up to that at this time of the night.

#8

Fill the balloon with hydrogen. The shuttle’s rockets will ignite it and burn up the balloon. No balloon to fall on an unsuspecting populace.
What?

#9

Google “rockoon” to get some info on this idea.

Brian

#10

If you look at the recent history of high altitude ballooning, you’ll get an idea how hard it is just to get a balloon to carry a couple people+supplies up high. Like the recent experience in England where the balloon tore during filling.

Would you want a balloon ripping apart, carrying a big load of rocket fuel, “somewhere” over west FL?