Is the Rocket Equation Holding Back Space Shuttle Safety?

[ScienceGurl]

Well I don't have very much knowledge on this topic, but doesn't it just irritate you that NASA never even bothered to fix the hole in the Space Shuttle Endeavour when it went up a few months ago? Please correct me if I'm wrong about this. But you figure that after what happened to Columbia, that's what they would do when that situation happened.

 

[neutrino]

You probably would not like to read this then.
http://www.floridatoday.com/floridatoday/blogs/spaceteam/2007/10/debris-analysis-ice-struck-shuttle-at.html

 

[mgb_phys]

Ice hitting the tiles isn't really a problem - it only happened once and so that was just a statistical freak, all the other times that it didn't crash prove that it's safe. Similair 'manager logic' can be applied to proving that icebergs are not a statisticaly significant threat to ships called Titanic.

The problem with the shuttle is the rather brittle nature of any advanced technology. Because it is difficult to get into space the shuttle is only just capable of doing it if everything works - so pretty much anything failing is a disaster. Compared to aircraft which regularly take off with a list of problems (snags) but still manage to fly safely because they are over engineered.

 

[Michael Mozina]

Well I don't have very much knowledge on this topic, but doesn't it just irritate you that NASA never even bothered to fix the hole in the Space Shuttle Endeavour when it went up a few months ago? Please correct me if I'm wrong about this. But you figure that after what happened to Columbia, that's what they would do when that situation happened.

Generally the shuttle team has to weigh a variety of "risks" to the shuttle and crew before deciding what to do with damaged tiles. The shuttle tiles have been damaged on many occasions, but only rarely has that damage posed a significant and life threatening problem. Most of the small cracks and holes in the tiles are not "mission threatening". Even more relevant is the fact that adding material into the hole poses the threat of that patch material dislodging during reentry and causing more tile damage than they started with. The NASA team (and flight team) have to weigh a variety of factors before they decide to attempt to "patch" the system in flight.

Our ability to "patch" things up in space is still pretty limited by the way. A major amount of tile damage would probably result in the shuttle having to dock with the space station while the folks on the ground decide what they want to do. Smaller cracks and nicks are often not "messed with" in flight because they are small, or they occur in areas that don't receive as much heat during the reentry process. Columbia was a special case scenario, where the damage was severe (more severe than they realized), and it took place is a very vulnerable area of the tiles, an area that receives a lot of heat buildup during reentry, and an area that has to open up to allow for the wheels to come down. Because patch materials can come loose, it's sometimes much safer to leave the crack alone, than to attempt to patch it up in space. It's definitely a "judgment call" type of process. You can be sure that NASA is highly sensitive to this issue now, and they run through a variety of computer simulation scenarios before they decide what to do with damaged tiles.

 

[ScienceGurl]

Oooh okay I understand now. Thanks! :)

 

[mgb_phys]

What would you do with a broken shuttle? After you dock with the ISS and book a flight back with the Russians, you have a shuttle stuck to the ISS.

Even assuming it isn't a problem sitting there and doesn't add enough drag to compromise the ISS. The space station is due to be dumped in a few years.
The shuttle is a fairly solid lump of metal and is designed to survive re-entry.
Would you open the cargo doors and deliberately try and re-entrer it upside down so it would break up, or install demolition charges to break it inot smaller pieces that would re-enter safely, or would you have enough control to aim it at the middle of the pacific (hopefully missing Hawaii) ?

 

[SticksandStones]

What would you do with a broken shuttle? After you dock with the ISS and book a flight back with the Russians, you have a shuttle stuck to the ISS.

I thought the re-entry process was entirely controlled by computer so they couldn't NASA fly the unmanned shuttle back? If it blew up nobody would get hurt and if it didn't they could reexamine it to find out what went wrong where.

 

[mgb_phys]

I didn't know if they can do the full orbit - landing automatically ( which of course begs the question of why crew it in the first place!)

The danger is that if you know it's going to blow up and you are flying it over the whole of the USA to land it then you risk hitting a lawyer.

 

[SticksandStones]

I didn't know if they can do the full orbit - landing automatically ( which of course begs the question of why crew it in the first place!)

I'm unsure if they can do the entire orbit (I would imagine so, as computers should theoretically be more accurate than humans) but I remember reading in Richard Feynman's autobiography detailing his accounts with the Challenger disaster that NASA had computers doing taking care of all landing details. I could, of course, be remembering incorrectly.

As for your second part...well you win. Excellent point. :)

 

[russ_watters]

They can do an entire shuttle flight without a crew. Why have a crew? Why have a shuttle if you don't want to send people up?

Just a couple of clarification, though - foam hits the space shuttle on virtually every flight and ususally damages tiles. The shuttle used to even lose tiles due to vibrations on launch (not sure if it still does). As much press as it got, the damage to the Endeavour was not newsworthy. It was a minor gouge that iirc did not even penetrate the tile - it wasn't a hole.

Also, docking with the space station is not generally an option unless you are launched with the intent (into an orbit with the right altitude and inclination) of docking with the space station. It takes an enormous amount of energy to reshape an orbit - it can be a significant fraction of the energy it took to get to orbit in the first place.

 

[Michael Mozina]

What would you do with a broken shuttle? After you dock with the ISS and book a flight back with the Russians, you have a shuttle stuck to the ISS.

Personally, I'd be inclined to fix it. If I still can't be sure it's safe I'd put it on autopilot and let it land by it's own instruments. If it survives, great! If not, at least I tried, and no humans need lose their lives in the attempt to save the shuttle. :)

 

[Michael Mozina]

I thought the re-entry process was entirely controlled by computer so they couldn't NASA fly the unmanned shuttle back? If it blew up nobody would get hurt and if it didn't they could reexamine it to find out what went wrong where.

Oh, I see you beat me to it. :) I like the way you think. :)

 

[Chronos]

Risk is inherent to all shuttle missions. Managing risk is a relatively new phenomenon. I have no problem with the assessment on Endeavor. The risk was very low. In the past, risk was a design, not managed factor. That, as NASA learned the hard way, was a dangerous approach.

 

[GTrax]

I am in fun speculation mode here

Depending on how severe the damage, and how much fuel available, there are some (uncomfortable) calculated strategies for getting a shuttle home in a unconventional re-entry posture. Nobody would want to test them. I tried some quick 'n dirty numbers to see if an extended engine driven slowdown, with the inevitable quite fast drop towards the atmosphere could blunt some of the need to use the atmosphere as a friction brake.

Not that my very school standard calculations should go too far here, but the ideal would be slowing from about 17,000 mph to about 1000 mph, matching the ground below, and just waft down on parachutes. There is not a need to use as much fuel as it took to get up there, but even so, its too much.

More dangerous is the drop to a hot re-entry before there is time to slow down to a safe one.
We just have to accept that space travel is a marginal, very risky thing, and there are economics to making it work. There are only 14 (maybe 13) shuttle missions left, I think. I also think that whatever replaces it will still need to have re-entry insulation.

 

[Garth]

Not that my very school standard calculations should go too far here, but the ideal would be slowing from about 17,000 mph to about 1000 mph, matching the ground below, and just waft down on parachutes. There is not a need to use as much fuel as it took to get up there, but even so, its too much.

Would it not take the same amount of fuel to stop the Shuttle going at ~ 17,000 mph as it took to get to that speed in the first place?

Garth

 

[EL]

Would it not take the same amount of fuel to stop the Shuttle going at ~ 17,000 mph as it took to get to that speed in the first place?

Actually not, since the Shuttle has burned some fuel (that is, decreased its mass) on the way up. The effect should be calculable with a rocket equation.

 

[Garth]

But I wasn't including the ride up to orbital height, just orbital speed.

Garth

 

[mgb_phys]

Also, docking with the space station is not generally an option unless you are launched with the intent (into an orbit with the right altitude and inclination)

I thought all flight had to be capable of reaching the ISS in case of an emergency - that's why the shuttle can't be used to launch most satelites anymore ?

 

[GTrax]

But I wasn't including the ride up to orbital height, just orbital speed.

Of course - normally it takes the same burn, (while facing the other way) to get back to any speed you had originally. In the case of the shuttle, the orbital speed was acquired differently. As EL mentioned, the rocket equation applies. NASA do not expend that huge mass of fuel, mostly used up in lifting the remains of itself, without the payoff being a 17,000 mph orbit of shuttle tonnage. Astronauts notice the increasing forces on their bodies as the vehicle gets lighter.

Once up there, it is true that less is needed to accelerate, but there is very little left. You cannot just choose to head off here or there as takes your fancy (Star Wars style ). The slightest extra burn, say to catch up to ISS for docking, or fix Hubble or whatever, immediately increases altitude also, and you miss the target. I think it takes a calculated wasteful over-speed plus braking and dropdown to arrive. So instead, you plan the path to leave out the extras.

That fuel, and the machinery to lift it! Pumps! Think of pumping empty an Olympic swimming pool in about 4 to 5 minutes! I am sure someone here knows how many $K per kilogram it takes to get stuff up there. Whatever it is, I am sure it precludes extra-orbital touring.

 

[EL]

But I wasn't including the ride up to orbital height, just orbital speed.

Ok, I admit I did not read all previous posts (and I still havn't), so maybe my reply was just bla bla (or wasn't it?). Sorry in that case.

 

[OSalcido]

They can do an entire shuttle flight without a crew. Why have a crew? Why have a shuttle if you don't want to send people up?

Not quite true, the reentry part can be done by computer but the actual landing has to be done by the captain/pilot

Remember that the Shuttle was designed around 40 years ago and still has the same technology... that's WAY before any computer was capable of crunching the amount of information it takes to land a plane, let a lone the space shuttle, successfully

 

[mgb_phys]

The shuttle has always been able to land on autopilot. It was one of the original design requirements - it was thought that since the shuttle only gets one shot ( it has no engine power for a go around) that it wouldn't be safe for a pilot to land it.
It turns out that the shuttle's aerodynamic behaviour at final approach is better than expected and so it is almost always landed by hand. basically it's so heavy and it's landing speed so high that the approach is a quite simple "point at the runway" plan.
The tricky part is stopping - there is no reverse thrust and fairly small air brakes so you need a lot of runway and a special tyre-eating grooved surface.

This is helped by being able to wait until good weather at the landing site before leaving orbit - the shuttle doesn't have conventional ILS type approach aids, it is autopilot or manual.

Automatic landing systems have been around since before WWII, you don't need a computer it can be doen with analogue electronics.

 

[setAI]

don't be such a wimp- the Apollo guys landed on the moon with what was essentially a large jiffy pop bag full of air- you could poke a pencil through the walls if you gave a little push [I know- my first job was at the Kansas Cosmosphere where we had a complete lunar module- I used to climb up their and eat my lunches when we were closed]

 

[GTrax]

One of the best descriptions of the process I have seen is at this link.

http://www.bbc.co.uk/dna/h2g2/A6381038

The amount of energy that must be dissipated is very large. Stopping a one hundred tonne craft from a speed of 13 km per second in eighty minutes requires nearly two thousand megawatts of power

Why the shuttle scrubs off speed in a series of S-bends, and why it has what it does have ..for wings. A flying brickyard with the handling characteristics of a cannon, doing a dead-stick managed fall. With no engine, and a runway undershoot is not an option. Its a great read.

Good that safety is a concern, and is addressed, but I think no matter how you cut it, a shuttle trip is such a difficult act of skill and training, it can never be routine. Space travel is just very risky, and a lot of things have to go right to pull it off.

 

[D H]

Some corrections first, and then some comments.

Would it not take the same amount of fuel to stop the Shuttle going at ~ 17,000 mph as it took to get to that speed in the first place?

No. All the Shuttle needs to do to come home is a relatively small deorbit burn that makes the perigee of the orbit inside the atmosphere. The atmosphere does the rest.

Not quite true, the reentry part can be done by computer but the actual landing has to be done by the captain/pilot

As mgb_physics noted, the Shuttle can land on autopilot. It's never been done (I think). Pilots, being what they are, would rather drive than watch. The one thing that the Shuttle autopilot cannot do is docking with the ISS.

The space station is due to be dumped in a few years.

It is the Shuttle, not the Station, that will be "dumped" in a few years. All remaining Shuttle flights but one are dedicated to completing the construction of the ISS. Why would we finish building it only to dump it right away?=========

There is only way to make space flight completely safe, and that is to stop doing space flight. There will always be some risk in space flight. NASA trades off those risks every time they fly the Shuttle. In the case of the ice damage to the Endeavor, NASA simulated things to death, looked for signs of similar damage in previous Shuttle flights that did return safely, and held many conferences before deciding not to make the repair. Making the repair would not have guaranteed a safe return and would have introduced unknown risks. This trade was hard, but in the end, fairly clear. Final note: that Shuttle flight did return home safely.

 

[Garth]

I am in fun speculation mode here
Not that my very school standard calculations should go too far here, but the ideal would be slowing from about 17,000 mph to about 1000 mph, matching the ground below, and just waft down on parachutes. There is not a need to use as much fuel as it took to get up there, but even so, its too much.

Would it not take the same amount of fuel to stop the Shuttle going at ~ 17,000 mph as it took to get to that speed in the first place?

Garth

No. All the Shuttle needs to do to come home is a relatively small deorbit burn that makes the perigee of the orbit inside the atmosphere. The atmosphere does the rest.

I think you missed the context of my remark.

Garth

 

[D H]

I think you missed the context of my remark.

Garth

I missed the context. Thanks to the rocket equation, it would not require as much fuel to slow down from 17000 mph to 1000 mph as it took to get there. All it would take is a main engine, and a external tank. Unfortunately, thanks again to the rocket equation, a whole lot more fuel would be needed than is currently used to get that extra main engine and full external tank into orbit.