Space Station Question - Shuttle Firing Rockets Inside Station - Effect?

[cubby]

The ISS has engines of it's own (attached to the Russian segment).

Depending on what type of thrusters you're talking about (the orbital manuevering system and not the Reaction Control System I presume), the shuttle used them every mission to reboost the ISS to a higher oribt.

The ISS doe snot normally have any engines firing. It uses the RCS and gyros to control its orientation, but otherwise it's orbit decays. With the shuttle retired, currently NASA utilizes Soyuz, Progress and ATVs to reboost the ISS as needed. It's all a function of fuel.

And the ISS does have a "shuttle bay".

Here is the finished ISS with the US NASA Space Shuttle (far left), Soyuz (bottom right), and ESA ATV (far right) all docked.



NM this is different than I thought.

Yeah, sorry for the confusion - I'm working a hypothetical. I'd appreciate your insight, however.

[Azadina]

Right, exactly - assume big (and totally unnecessary) hangar inside a (now much larger) Space Station. I see what you're saying about the thrust hitting the opposite wall and nullifying the movement.

What if the hangar was so large that the wall behind the shuttle was 100 yards away? Would it still have no effect because the thrust eventually does hit the wall behind the shuttle and therefore "zero's out" the affect?

Then it's a costly way of heating up the room, nothing more

[bloodrunner]

I think he is thinking about if you fly into/out of a shuttle bay on a large station. think DS9 or bigger. The mass of the station would determine what reaction any force against it would cause. Also the thrust of a secured shuttle would need to be directed to open space or multiple vectors of force would be applied to the shuttle bay walls/floor/roof.

--edit fingers can't spell today

[Endus]

It's complicated, but the net effect would likely be a small chaotic push generally in the direction the shuttle's pushing. If it were outside in open space, it would be a clear vector, but in an enclosed area, the propellant blast is going to hit the far wall behind the shuttle, and that will expend energy into that wall, working against the thrust of the shuttle. But since the shuttle thrust is chaotic, some of the force will get deflected, and hit the roof, the floor, the other walls, etc. So you lose energy, and there's chaotic thrust pressures in every direction, but it'll still probably cause a "twist" mostly in the direction the shuttle was firing, but far less strong than if the shuttle was in open space, because of how the exhaust is pushing the other walls of the shuttle bay.

This will last at least until the shuttle burns through the bay walls, which is likely if we're talking a full burn and pretty standard modern tech levels and not some advanced armored sci-fi version.

In open space, the exhaust doesn't hit the station, and thus doesn't produce any force upon it other than what the shuttle does via its tether/lock. Once you've enclosed it, the exhaust WILL hit the station with nearly the same force it was expelled, and the only reason it doesn't perfectly counteract is because it's "messy" and bounces around, so some will impact the other parts, not just the one wall.

[bladeXcrasher]

Why would you ask a bunch of randoms on a wow forum instead of asking an astronaut on the space station?

[Kaleredar]

Don't get me wrong - I appreciate the video and information. But my question is on a hypothetical - not the actual ISS, but hypothetically if the ISS had an internal shuttle bay, and the shuttle, while "tied down", fired it's thrusters. Would that affect the station's orbit or move the station?

Not in any considerable way, at least, not unless it damaged the station by blowing a hole through the wall.

Think of it this way: if you had a fan in a closed cardboard box and then put that box on wheels and turned the fan on, would the box move forward? No, it wouldn't. Any thrust the fan, or engines, would produce forward would be counteracted by that same force pushing against the wall backwards.

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Why would you ask a bunch of randoms on a wow forum instead of asking an astronaut on the space station?

It doesn't take an astronaut to answer a fairly simple physics question.

[Ripster42]

That's not correct. Propellant doesn't "push off of" anything other that it very self. Otherwise, how would you be able to generate thrust in empty space, where the next thing to push off of might be a few hundred thousand kilometers away?

Propellant pushes off of the vehicle it's leaving. If the propellant then hits the same vehicle its leaving, along the same vector (which is why I say the transfer of energy isn't perfect, a relatively small amount of the propellant will disperse along different vectors), you lose all of the momentum imparted to the vehicle from expelling the propellant.

It sounds counter-intuitive at first, since why would "fresh" propellant hit that which just moved out, since by the law of the conservation of momentum, everything should move equally fast. And that is still true - BUT: The propellant that juuuuust about leaves the nozzel, exactly where it's accelerated by the combustion reaction, it exerts a forward force on the unreacted propellant that is still inside the nozzle.
And I'm fairly sure that the cooling of the propellant that is out for a bit longer also makes it lose kinetic energy, providing yet another contributor to the whole "pushing off itself" thing, albeit a minor one.

It's not counter intuitive. You're just wrong on the magnitude of conditions.

The question is very similar to a rocket engine tied down in a big room on Earth with a vacuum. Would the rocket still exert a force to whatever it is tied to? Yes, it will indeed.

Push-back from burnt fuel hitting a wall WOULD however have an opposite effect, but it would get lost to levels of margins of error of measurements pretty quickly even at "not huge" distances.

So the answer is yes. An internal rocket drive would actually work, and if the room was big enough it wouldn't even be very noticably less efficient than an outside exhaust. The reason we don't do this is simply that it's a lot easier not to worry about cooling walls and having to carry all the exhaust gases around with you, when you can just dump it into space so easily. A lot less prone to damages, too

The reason we don't do this is because it's stupid.


I don't know why you're comparing this to a situation on earth. The conditions for your example are effectively incomparable from the conditions OP gave. The reason there's no noticeable effect when firing a rocket on earth indoors is that the "blow back" (it's not really blow back, it's shooting the rocket at the vehicle it's supposed to be propelling) isn't being dispersed in the same size of vehicle. A space station vs virtually the entire weight of the earth. There's a small difference in scale.

[Endus]

Here's a way to visualize it in your head; imagine you've got a firehose. And you're sitting on a kid's wagon or a skateboard. What happens when you open up the firehose? You go flying in the other direction. That's your thrust vector.

Now, imagine you're inside a wooden box with wheels, and the firehose comes in a hole in the wall. What happens when you fire it at the other wall? Are you going to go flying as fast as you would've in the first case?

No. Because while the water leaving the hose produces a force vector on your vehicle in one direction in both cases, in the second the water hitting the far wall creates a force vector in the other direction.

This isn't a perfect transfer, though, since there's a lot of spray and splashing, and transfers of energy aren't perfect, so you likely would start to roll, just not anywhere close to as quickly.

It's the "equal and opposite reaction" thing. To generate thrust vector X, you expel propellant in the reverse direction. If that propellant then hits the same structure that expelled it, because it's inside, then it's going to impart that same -X force, counteracting the thrust. Since it's not perfect transfer, this gets messy in practice, but you're definitely not getting the same thrust as if it were outside, not anywhere close, most of it will get countered.

[Aitch]

An easy way to visuallize it is by thinking about the station as a whole, closed system.

With the shuttle burning away inside the docking bay, imagine looking at the station from the outside. There is no mass or energy leaving the station in any direction, therefore there is no velocity vector. NONE. The chaotic nature of the thruster is irrelevant, all forces will be balanced and nullified.

[Deleted]

And the ISS does have a "shuttle bay".

That's not a shuttle bay.


THIS is a shuttle bay:

[EliWallach]

As soon as the shuttle ignites and emits gases, the station will change velocity. Conservation of momentum. If those gases hit an interval wall the station will go back to its previous velocity, more or less, again conservation of momentum. If the gases escape the station then the velocity change will be permanent. Of course the velocity is also continuously changing due to gravity but that's a different problem.

[Ripster42]

You're right, I had it a bit wrong. But you did so, too. The important bit is the point of ignition. Fuel at that point will excert a force equally into all directions, as it combusts radially. The shape of the combustion chamber forces most of the energy to disperse into two directions: Back and forth. But since there is already a rocket in the way adhead, it pushes the rocket. And since it's also losing energy due to this and to dispersing into larger and larger confinements, the propellant that is on its way out is also slower than the freshly ignited one providing another contribution forward.

Nothing I said was wrong. The fuel doesn't combust radially. Hydrazine is reacted within a chamber to direct its flow. There isn't much loss in efficiency because hydrazine is expelled so quickly that there won't be much dispersion before the propellant hits the opposite wall of the bay.

If it needed anything disconnected from the rocket to push off of, as I said, not a single thruster we currently use in space would ever work, and the energy of the propellant that DOES hit walls all around a rocket is less than what contributed to the forward motion already. Also, the effect needs to back-propagate to the rocket first to have an effect. A particle hitting a wall somewhere doesn't have magically instant effect on the rocket, that would be a faster than light effect. But to repell the propellant you need to bounce it off the walls, which will heat them, making the propellant lose yet more energy.

It's like you didn't read what I said. The bolded? I'm not contesting this fact. I'm contesting the need for the propellant to "back propagate to the rocket" because all it has to do is hit the other side of the station bay. I'm contesting the fact that the amount of momentum imparted to the station by the thrust delivered by the shuttle tethered to the station is much higher than momentum imparted to the station from the thrust hitting the opposite wall of the station bay. It's not. It's a bit higher, as I've mentioned in every post, but relatively small.

Let's take something else then. If you shoot a gun here on Earth, you feel recoil of the same strength no matter if you shoot at a wall a meter ahead or outside on an open field. Would you expect to get NO recoil if you fired it inside a space station hangar - with your feet glued to the floor?

This is missing the point. Yes, you'd feel recoil if you were inside a space station. That's not the point. This illustrates why what you're saying is stupid. You'd feel the recoil. The station will have been imparted some momentum from you firing your gun. The station then loses all the momentum it gained when the bullet hits to opposite wall. This is the same scenario as the rocket, except it is a closer to perfect transfer of energy (if the bullet lodges in the opposite wall and doesn't just cut all the way through.

Let's do a different scenario. Aim the gun at your head. Pull the trigger. Your hand experiences the recoil, jerks your body in that direction. Your head experiences the force from the bullet, jerks your body in the other direction (if you ignore the mechanics of off center mass applications of force, and swinging limbs). This scenario is much closer to the OP's scenario than just firing a gun off into the distance.

You don't really seem like you have a good grasp of enclosed systems; nor do you understand what enclosed systems we're talking about in either the shuttle/station bay scenario or the circumstances/defined-system required for the gun scenario to be remotely related to the former.

[Endus]

Let's take something else then. If you shoot a gun here on Earth, you feel recoil of the same strength no matter if you shoot at a wall a meter ahead or outside on an open field. Would you expect to get NO recoil if you fired it inside a space station hangar - with your feet glued to the floor?

The problem here is you're assuming the "gun" is imperfectly attached to the system being moved; the "recoil" is your arm flailing about, not you being shifted.

Put yourself on a platform with omnidirectional wheels. If you shoot a gun South, you'll roll North somewhat; that's the "recoil". Now, build a box on that platform around yourself. If you shoot the Southern wall, while the recoil pushes you North, the bullet impacting the Southern wall pushes you South. If there were perfect transfers of energy, the two would cancel out.

What you're essentially arguing is that bullets don't hit with any force at all. Which is sort of the opposite of the intent of bullets. If your conceptualization worked, guns wouldn't, because bullets wouldn't carry any force on impact. You keep ignoring the force they're impacting with, for no reason.

[Endus]

The recoil is the force that moves the arm. That force would still be there with an infinitely tightly mounted gun. But the analogy was shit anyways.

Yes, because you focused entirely on the propellant's force as applied to the gun, and ignored that it applied a similar force to the projectile.

And when that projectile (or propellant/ejecta, in the case of a shuttle engine) strikes the far wall of the shuttle bay, it imparts that force in the opposite direction of the vector it produced on the shuttle. As I said from the beginning, this is likely to be a less-than-perfect transfer for a host of reasons, and will tend to produce a much slower and chaotic push in the direction the shuttle is aiming than it would if the shuttle were docked externally, but ignoring the impact force of the ejecta isn't a fair treatment of the problem.

[ItsRainingMetal]

I'm researching an issue for a book.

And your first stop was MMO-C ?

[D3thray]

Endus has the right of it. The shuttle absolutely does push off something to gain thrust. It pushes off the exhaust gases. If the gases (which absolutely do have mass) then hit the far wall of an enclosed shuttle bay, that would partially (but not fully because of 'messiness') counteract the push of the shuttle.

My question is how is the shuttle tethered? Because if it's an umbilical going to a port on the side for instance, the thrust will impart significant torque to the umbilical and probably shear it right of. And then it'd hit the wall the umbilical was attached to.