# A very interesting question

1. May 9, 2004

### stuckie27

A serious problem in our use of planes for transportation is the possibility that they might crash. What if we could stop this?

Here is the proposal, from a physics professor at my university.

If at the time that a plane lost its ability to stay in the air a rocket was sent from the plane and launched up at a certain velocity causing the plane to slowly fall to the ground and rescue the plane from certain disaster.

Here is the problem, how much energy would the rocket need to stop the plane from crashing.

Known,
Plane weighs 123,600 kg (Boeing 757)
Height 9146.34m (30,000 ft)

Ignore the fact that the cable would break, and the wind resistance/air friction of the plane. (consider it a ball of mass). Essentially using the information come up a simplified solution.

Also post if you see any problems with the original proposal

2. May 9, 2004

### Bob3141592

I see several problems with this proposal.

First, there would be some real concerns about a plane filled with passengers carrying explosives, which is basically what rocket fuel is. The danger posed by this might well be much greater than the danger of the plane crashing.

Second, ignition of the rocket would probably pose a serious threat to the passangers and crew.

Thirdly, the weight of the rocket would increase fuel costs.

Fourthly, the volume of the rocket would increase drag on the plane and lower it's speed and increase it's fuel costs.

Fifthly, there would be significant costs involved in the installation and maintenance of such a rocket system. Solid rocket fuel can degrade over time, and liquid fuel needs to be loaded and unloaded. All this infrastructure would be recurrant expenses that would have to be accounted for.

Also, it would make airplanes much for effective as terrorist weapons, and much more vulnerable as targets of terrorists.

I don't know, but not even considering if such a system would even work, and without doing any kind of cost/benefits analysis, this sounds like an absolutely terrible idea.

3. May 9, 2004

### stuckie27

we agree that it's a bad idea, but we still need to find a suitable answer to turn into the teacher.

4. May 9, 2004

### JasonRox

My opinion, concentrate on car accidents, and not plane accidents.

I don't see any real threat from an airplane. Chances of one happening are minimal. They just seem bad because it's like guaranteed death.

Again, in my opinion, I rather die in an airplane crash than become a vegetable because some drunk guy hit me.

Car accidents happen all the time!

I did a research project about this in high school, and it is proven that the media does a good job at scaring people from planes.

5. May 9, 2004

### JasonRox

What do you have now?

We can work from there.

6. May 9, 2004

### JasonRox

If you launch the rocket up, wouldn't the plane increase its speed into the earth?

7. May 9, 2004

### stuckie27

Weight of rocket we decided is 10% of plane=12,360 kg

Potential Energy of plane at altitude 9146.32= 1.1079E10

We want to know what enery is needed to bring the plane to a stop on the ground safely or give the plane the abiltiy to glide to saftey.

Any equations that you use will help and please post them.

8. May 9, 2004

### stuckie27

Launch the rocket agianst the air.

9. May 10, 2004

### JasonRox

You said to ignore wind resistance.

I'm not trying to be an idiot or anything, but this isn't clear.

Are you saying shoot it forward?

It doesn't matter what you do to horizontal velocity it will never affect vertical velocity.

The problem here is that the plane is going down at 9.8 m/s.

If you are saying to shoot the rocket down to propel the plane up, that is a different story.

In the end, it would take too much force to stop the plane. Not even worth scratching a number.

10. May 10, 2004

### stuckie27

should I tell my professor that?

11. May 10, 2004

### arildno

NB!
At no point in your thread have I seen the use of the condition "the cable will not break"
Evidently, the rocket is attached to the plane by a cable, whose tension on the plane reduces the acceleration of the plane.

12. May 10, 2004

### enigma

Staff Emeritus
Your rocket would need to dissipate the energy of the plane.

If you had a plane flying at X velocity when it suddenly had its engines stop and wings snap off, you've now got an object in freefall. Its energy is KE+GPE. To keep it from crashing, it would need to have 0 kinetic energy (no speed) when it has 0 potential energy (no altitude). Plug & chug.

For a real life application, you wouldn't use rockets... that's for sure.

Throttling those things is damn near impossible for solid fuels, and liquid fuels are either cryogenic, REALLY toxic, or are both expensive and have mediocre performance. A more realistic idea, which is actually being tested for smaller planes is to have an emergency parafoil deployment mechanism which automatically fire off if the plane goes into a spin, runs out of fuel, has structural damage, etc. They don't slow it down to zero, but they slow it down enough to allow a survivable crashlanding.

Bob is definitely correct... You don't have a "fire in the hole" staging system when you've got a manned first stage. :tongue:

Last edited: May 10, 2004
13. May 10, 2004

### pallidin

Yeah, a cabled-rocket scenario is definitely out. A "stay on-board" rocket stabilization system is possible, and indeed used in certain applications. However, those systems are designed for minor corrections. Catastophic de-stabilization of an aircraft in flight is more than likely to generate forces and mathematical corrective dynamics of a very high order, surpassing the ability of known or practicle rocket thrust stabilization systems.
So, we are left with the parafoil scenario, which basically deploys one or more parachutes, using explosive, gas or rocket assisted deployment methods.
This can be highly effective in light aircraft implementation, but a full class passenger airliner posses serious engineering challenges.
One of these challenges is the size of the deployed parachutes.
I read somewhere that to implement a parachute recovery system on a 747 would require 2 deployed chutes with a suface area of a football field, each! And, even with that moumental engineering feat, would also require that the entire plane NOT be recoverable. That is, the passenger section of the 747 would detach and the chutes deployed and attached only to that section.

14. May 10, 2004

### Staff: Mentor

The question is based on a flawed premise: The possibility of a crash is not a serious problem in our use of planes for transportation. They are far and away the safest way to travel. Show your teacher http://www.ntsb.gov/aviation/Stats.htm [Broken]

Last edited by a moderator: May 1, 2017
15. May 11, 2004

### Gokul43201

Staff Emeritus
No Enigma,

Energy required=GPE=mgh=1.1079E10. No other data or (major) assumptions are required. The KE can remain unchanged, so the plane retains its velocity, but it's mostly horizontal. You only need the vertical component to be small. That's why there's no data about the plane's speed. I know it's not a great idea to land at 300 mph but that's what you have to live with for this problem.

16. May 11, 2004

### enigma

Staff Emeritus
Uh... we'll have to disagree here.

The only reason planes can land as fast as they do is because they are really stable at touchdown. I wouldn't want to be on the engineering team which designs the devices to allow a plane to touch down at 150% of the Space Shuttle's landing speed while being controlled by rockets/thrusters. When doing VTOL, Harriers land with extremely small lateral velocities.

Sure, it may be possible, but not likely.

17. May 11, 2004

### Gokul43201

Staff Emeritus
I agree entirely, but there is no data provided in the problem that talks about speeds. Perhaps you can assume reasonable speeds but this also means that you'll need to fire the rockets slightly backwards at some carefully calculated angle. And the firing angle will be a function of the velocity. Or maybe there's some way to reduce speed using conventional means such as airbrakes or flaps (assuming they still work) or throwing out attached parachutes. A drag type of deceleration is nicer than a thrust type (you won't fly backwards because of too much drag).

Maybe the answer is to design all planes like Harriers with vertical thrusters for emergencies.

Wonder what else we can come up with to "help" the airline industry ?

18. May 24, 2004

### stuckie27

Thank you all for you your wonderful replies.

Give yourselfs a pat on the back.