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Rail Launching Of Aircraft  
User currently onlineStarlionblue From Greenland, joined Feb 2004, 16993 posts, RR: 67
Posted (7 years 3 months 2 weeks 4 days 2 hours ago) and read 2671 times:

Slow day today. Time to start a new topic.

I was reading about "aircraft on rails" such as Schienenzeppelin and Aérotrain (both very cool pieces of kit), so I started thinking about rail take offs of aircraft. This is of course nothing new. Military rockets and even aircraft (V-1) have been launched on rails for decades. However, I was thinking in terms of propelling the vehicle with an exterior fuel source. That is, the rail system would provide initial propulsion, reducing the weight of the vehicle by imparting enough momentum for initial flight. This kind of system has also been in use for decades on aircraft carriers, but mainly for the purpose of allowing short take off rolls, not directly for saving fuel.

This external propulsion can be achieved by steam, electromagnetism, etc. The point being that the vehicle can be made lighter, and thus total fuel savings be achieved.

In this age of rising fuel prices, could rail take offs be an option for the future? I understand there are huge disadvantages, but it's a neat concept.

Let the ritual flaming commence!

[Edited 2007-04-09 18:53:46]


"There are no stupid questions, but there are a lot of inquisitive idiots."
27 replies: All unread, showing first 25:
 
User currently offlineKELPkid From United States of America, joined Nov 2005, 6343 posts, RR: 3
Reply 1, posted (7 years 3 months 2 weeks 4 days 2 hours ago) and read 2642 times:

Well, this was the Wright Flyer's method of launch...  old 


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineKalvado From United States of America, joined Feb 2006, 489 posts, RR: 0
Reply 2, posted (7 years 3 months 2 weeks 4 days ago) and read 2604 times:

most apparent objections:
You cannot really land on rails, so it has to be some procedure to put the plane on rails. Extra cost - people and equipment on the ramp; possibly loss of flexibility in motion ( can you enter runway at intermediate point if needed?), and flexibility of switching the runways (is it possible to land on rails, if properly setup?)

Next, you'll have to setup a hard point for attachment to launch trolley - something similar to landing hook for carrier landing - I bet it's not going to be light structure. Weight increase - more fuel (probably most important point here)

Rails themselves - I don't think it's reasonable to use same track width for ERJ and B747/A380, so there will be multiple tracks, adding more varieties of launch equipment, and launch trolleys themselves would tend to accumulate at one end of runway (and they're not free either)..

Some of these can definitely be worked out, but hardpoint issue is probably the worst one.


User currently onlineStarlionblue From Greenland, joined Feb 2004, 16993 posts, RR: 67
Reply 3, posted (7 years 3 months 2 weeks 3 days 23 hours ago) and read 2556 times:

Quoting KELPkid (Reply 1):
Well, this was the Wright Flyer's method of launch...

So it was. Counterweight as I recall.

Quoting Kalvado (Reply 2):
You cannot really land on rails, so it has to be some procedure to put the plane on rails.

Well, I wasn't really envisioning landing on rails. I saw it more as a launch from a rail like from an aircraft carrier for example, thereby saving some amount of fuel needed for acceleration. After that, it's just a normal ERJ or whatever that can land wherever.

Also, for airport without rail launch the aircraft could just take off normally.

Quoting Kalvado (Reply 2):

Next, you'll have to setup a hard point for attachment to launch trolley - something similar to landing hook for carrier landing - I bet it's not going to be light structure. Weight increase - more fuel (probably most important point here)

I don't know if it needs to be that beefy. Aircraft do get towed every day and I bet if you could attach to the main gear you could put quite a bit of load there. That's how the French Navy used to launch aircraft.



I am also curious how much fuel is used during the take-off run. Say that the aircraft can be accelerated up to just under the max tire speed. As Captain Click would say, that energy is still being produced somewhere, but by using energy off vehicle weight is saved if nothing else.

[Edited 2007-04-09 22:33:42]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (7 years 3 months 2 weeks 3 days 22 hours ago) and read 2540 times:

A) Very little fuel is used for the acceleration, compared to what is used for climbout or flight. The fuel savings possible are miniscule.

B) You do not want to come off the rail, push the throttles forward only to find that uh-oh, the power isn't coming as expected for whatever reason. Engine failures in flight are worse than engine failures on the runway. The solution is to get the engines up to power prior to actuating the launching system. However, this means there's no real saving of anything but runway length for a given MTOW. Basically a carrier takeoff. The structural reinforcements required to get the launcher force into the aircraft structure would probably eat up any benefits - and more. Carrier capable aircraft are very beefed up, from a structural point of view, to cope with the stresses of cat shots and no-flare arrested landings.

All in all, you are much better off just adding those extra few pounds of fuel required for the take-off roll.

Now, if you consider putting the aircraft on a giant conveyor belt, which is accelerating as the aircraft accelerates. Then you fill the cabin with chickens (or penguins, for polar routes) and have them all take off at the moment of rotation...  Wink

Cheers,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineKalvado From United States of America, joined Feb 2006, 489 posts, RR: 0
Reply 5, posted (7 years 3 months 2 weeks 3 days 22 hours ago) and read 2526 times:

Quoting Starlionblue (Reply 3):
I don't know if it needs to be that beefy. Aircraft do get towed every day and I bet if you could attach to the main gear you could put quite a bit of load there. That's how the French Navy used to launch aircraft.

I'm afraid it _needs_ to be that beefy - when the plane is towed, acceleration (and force) is quite small - here we're talking about full takeoff power. I would say something upward of 100 klb for 777. MLG takes plenty of load from brakes action, but load would be higher, something like the load during aborted takeoff without spoilers or reversers. Besides, it's the load in opposite direction - which probably adds some extra metal needed.

Regarding take-off fuel, as far as I understand take off thrust is about 5 times that in cruise; assuming thrust is proportional to fuel burn - 1 minute of take-off roll is equal to 5 minutes in cruise. But then - what limits the MTOW? If it is weight (wing lift/engine power) limited, that MTOW would go down by exactly the weight of fuel burned on takeoff.


User currently offlineKalvado From United States of America, joined Feb 2006, 489 posts, RR: 0
Reply 6, posted (7 years 3 months 2 weeks 3 days 22 hours ago) and read 2515 times:

Quoting FredT (Reply 4):
The solution is to get the engines up to power prior to actuating the launching system. However, this means there's no real saving of anything but runway length for a given MTOW.

Actually I wrote, but didn't post a similar comment. IMHO you can save some gas here (not in a global setup - just during takeoff) - either you takeoff roll is shorter time-wise (less gas), or you can spool up engine little bit later, so that you get full engine power while there is still enough room to stop, or before you separate from the trolley. SOP for that would be a nightmare, though.


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 7, posted (7 years 3 months 2 weeks 3 days 22 hours ago) and read 2515 times:

Quoting Starlionblue (Thread starter):
Let the ritual flaming commence!

Rare, Medium or Well Done?  flamed 

Rail launching might not work too well in the UK. We're not very good at things which run on rails. Flight cancellations due to "leaves on the line" could cause a riot! Big grin



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineKELPkid From United States of America, joined Nov 2005, 6343 posts, RR: 3
Reply 8, posted (7 years 3 months 2 weeks 3 days 21 hours ago) and read 2504 times:

Quoting Jetlagged (Reply 7):
Rail launching might not work too well in the UK. We're not very good at things which run on rails. Flight cancellations due to "leaves on the line" could cause a riot! Big grin

And definitely don't make it electrically powered with overhead catenary as the power source, as the Brits will figure out how to make it regularly pull down the catenary cable (like the Eurostar...)  duck 



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 9, posted (7 years 3 months 2 weeks 3 days 20 hours ago) and read 2490 times:

What if the rails were moving like a conveyor belt; moving in the opposite direction of the aircraft's thrust?  wink 

User currently onlineStarlionblue From Greenland, joined Feb 2004, 16993 posts, RR: 67
Reply 10, posted (7 years 3 months 2 weeks 3 days 19 hours ago) and read 2464 times:

Quoting FredT (Reply 4):
Now, if you consider putting the aircraft on a giant conveyor belt, which is accelerating as the aircraft accelerates. Then you fill the cabin with chickens (or penguins, for polar routes) and have them all take off at the moment of rotation..

Somehow I knew it would come to this.  rotfl 

Anyway thanks for brightening an otherwise slow Monday!



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offline3DPlanes From United States of America, joined Apr 2006, 167 posts, RR: 0
Reply 11, posted (7 years 3 months 2 weeks 2 days 17 hours ago) and read 2337 times:

Late to see this and chime in...

I agree that the fuel savings would likely be smallish given that (like a carrier shot) you'd want to be at full juice before they launched you...

However, I think folks are thinking too much along the lines of cat shots or zero length rails. If the acceleration was gradual enough, you wouldn't need _any_ additional strengthening. And I'm not talking zero to Vr in 4 minutes, either. Just nothing like a push-your-eyeballs-through-your-brain cat shot.

To continue the discussion (although, I'm not sure that Starlion wants that,) what about a combination system. Low-force cats embedded in the runway? Hook up would still be interesting on an active runway, but if the Navy can do it...

Still though, aside from a shorter takeoff run (meaning shorter runways,) would there be any real benefit? Even halving the takeoff roll would only reduce the TO thrust duration by what? 20 seconds or so?



"Simplicate and add lightness." - Ed Heinemann
User currently offlineScrubbsYWG From Canada, joined Mar 2007, 1495 posts, RR: 0
Reply 12, posted (7 years 3 months 2 weeks 2 days 14 hours ago) and read 2313 times:

dont forget that the energy accelerating a plane would still have to come from somewhere. Electromagnets, steam, all require energy, and propelling the same plane by a different means still requires the same amount of energy from whatever the source is.

User currently offlineKalvado From United States of America, joined Feb 2006, 489 posts, RR: 0
Reply 13, posted (7 years 3 months 2 weeks 2 days 7 hours ago) and read 2261 times:

Quoting 3DPlanes (Reply 11):
If the acceleration was gradual enough, you wouldn't need _any_ additional strengthening.

Wrong. Either you're talking about small forces - which makes very little sense for the problem, or you're confusing jerk (which is another thing to keep an eye on) with force (which has nothing to do with how gradual acceleration is)


User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 24
Reply 14, posted (7 years 3 months 2 weeks 2 days 7 hours ago) and read 2255 times:

Quoting Kalvado (Reply 13):
you're confusing jerk (which is another thing to keep an eye on) with force (which has nothing to do with how gradual acceleration is)

What you call jerk is the sudden application of a force. It's still a force though. Never heard of the term jerk as a technical term in this context before.

Force has nothing to do with how gradual acceleration? What about

F = m * a ?

In words, acceleration is proportional to force.

Whether strengthening is required depends on how and where the force is applied.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineKalvado From United States of America, joined Feb 2006, 489 posts, RR: 0
Reply 15, posted (7 years 3 months 2 weeks 2 days 5 hours ago) and read 2234 times:

Quoting Jetlagged (Reply 14):


Whether strengthening is required depends on how and where the force is applied.

I've seen few reliability specs with jerk defined as change in acceleration, d2F/dt2.. Not a very common term, but it is being used.
Regarding force - you need something comparable with engine thrust for entire thing to be meaningful. Engine attachment points are probably strong enough, but I'm unsure if increasing force on those points is possible - just look up any re-engine discussion here.
Landing gear is not designed to transfer strong forward acceleration (OK for strong negative acceleration, aka stopping though - but that doesn't guarantee anything for forward acceleration). In general, attaching any external device (with high external force being applied) to any point of structure without reinforcement means that either structure is designed for that, or the structure was excessively strong (aka heavy) in that point, since it can handle much more than required in normal operation..


User currently onlineStarlionblue From Greenland, joined Feb 2004, 16993 posts, RR: 67
Reply 16, posted (7 years 3 months 2 weeks 2 days 5 hours ago) and read 2234 times:

Quoting 3DPlanes (Reply 11):
I agree that the fuel savings would likely be smallish given that (like a carrier shot) you'd want to be at full juice before they launched you...

Not necessarily. The reason carrier aircraft are balls to the wall before the cat releases is that they need to ensure they are balls to the wall when they reach the end of the run all of 2 seconds later. With a long runway, there's much more time for run-up during the shot. But then we get into decision speeds.

Quoting ScrubbsYWG (Reply 12):
dont forget that the energy accelerating a plane would still have to come from somewhere. Electromagnets, steam, all require energy, and propelling the same plane by a different means still requires the same amount of energy from whatever the source is.

Indeed. But by saving fuel on the aircraft, the aircraft is lighter overall, so you save fuel overall.

Quoting Kalvado (Reply 13):
Wrong. Either you're talking about small forces - which makes very little sense for the problem, or you're confusing jerk (which is another thing to keep an eye on) with force (which has nothing to do with how gradual acceleration is)

Well, planes get towed all the time. That proves the gear legs can drag the whole plane after them. So the weight of the plane. It's a question of how fast the aircraft is accelerated. By hooking onto the mains, you can apply at least twice as much force as a current tug. Probably more.


Just to be clear, I intended this thread to be an intellectual exercise. I don't really think this kind of system is feasible any day soon.

Thx for the great replies and keep them coming.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineKalvado From United States of America, joined Feb 2006, 489 posts, RR: 0
Reply 17, posted (7 years 3 months 2 weeks 2 days 5 hours ago) and read 2233 times:

Quoting Starlionblue (Reply 16):
Well, planes get towed all the time. That proves the gear legs can drag the whole plane after them. So the weight of the plane. It's a question of how fast the aircraft is accelerated. By hooking onto the mains, you can apply at least twice as much force as a current tug. Probably more

OK, so you would probably be able to accelerate the plane to 10 mph instead of 5.
You need a shitload of power to accelerate the plane - and to transfer that to the frame. Just as a simple examle - you can probably roll your car by pushing the bumper (breaks released, transmission neutral). Just imaging same thing being done by another car all the way to 60 mph (100 km/hr)..
Or, just looking at it power-wise, I've never seen a tug powered with anything remotely as strong as GE90..

Quoting Starlionblue (Reply 16):


Just to be clear, I intended this thread to be an intellectual exercise. I don't really think this kind of system is feasible any day soon.

Sometimes such crazy discussions generate really cool ideas. I didn't get anything patentable on this page yet - but I'll keep an eye on it  Wink


User currently onlineStarlionblue From Greenland, joined Feb 2004, 16993 posts, RR: 67
Reply 18, posted (7 years 3 months 2 weeks 2 days 5 hours ago) and read 2231 times:

Quoting Kalvado (Reply 17):
OK, so you would probably be able to accelerate the plane to 10 mph instead of 5.
You need a shitload of power to accelerate the plane - and to transfer that to the frame. Just as a simple examle - you can probably roll your car by pushing the bumper (breaks released, transmission neutral). Just imaging same thing being done by another car all the way to 60 mph (100 km/hr)..
Or, just looking at it power-wise, I've never seen a tug powered with anything remotely as strong as GE90..

There is no need to subject the gear legs to large forces. Slow, gradual acceleration will work, unless it's too slow to get up to speed.  Wink

Power requirements for towing up to a nice speed are something else, and of course vastly exceed those of current tugs. Then again, "direct drive" is probably more efficient than a jet engine while rolling along on the ground. Otherwise high speed trains would be powered by jet engines like Aérotrain.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineKELPkid From United States of America, joined Nov 2005, 6343 posts, RR: 3
Reply 19, posted (7 years 3 months 2 weeks 2 days 4 hours ago) and read 2220 times:

Quoting Starlionblue (Reply 18):
Otherwise high speed trains would be powered by jet engines like Aérotrain.

Don't know which Aerotrain you are referring to, but if it's the American one:

Big version: Width: 720 Height: 542 File size: 83kb


This one was powered by a 1200 HP diesel-electric powering the front truck only...

There was one jet-powered train in the US that I'm aware of:

Big version: Width: 500 Height: 421 File size: 119kb


Which currently holds the North American rail speed record at 184 MPH...and, by the way, that's an engine pod from a Convair B-36 Peacekeeper up on the roof  Wink AFAIK, this is the only rail vehicle to ever be directly powered by a jet engine...(direct propulsion instead of turbine-electric or turbine-hydraulic).



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently onlineStarlionblue From Greenland, joined Feb 2004, 16993 posts, RR: 67
Reply 20, posted (7 years 3 months 2 weeks 2 days 4 hours ago) and read 2215 times:

Quoting KELPkid (Reply 19):
Quoting Starlionblue (Reply 18):
Otherwise high speed trains would be powered by jet engines like Aérotrain.

Don't know which Aerotrain you are referring to, but if it's the American one:

It's the French one. Was not aware of an American one. Cool! That American one really has that brute force US railroad look. Just a big steel plate in the front. Wow. I wonder how much noise it made.

Quoting KELPkid (Reply 19):
AFAIK, this is the only rail vehicle to ever be directly powered by a jet engine...(direct propulsion instead of turbine-electric or turbine-hydraulic).

Actually, the French Aérotrain was also jet powered directly. http://en.wikipedia.org/wiki/A%C3%A9rotrain.

http://upload.wikimedia.org/wikipedia/commons/2/29/Aerotrain.jpg

Aérotrain was one of the early TGV options. The next was a turbine powered wheel drive variant called TGV001, which looked pretty similar to the early in service TGVs. TGV001 still holds the record for a turbine powered rail vehicle. After the 70s oil crisis conventional electric drive with a catenary was selected.


Schienenzeppelin was also non wheel powered, using a propeller. http://en.wikipedia.org/wiki/Schienenzeppelin. One of main issues there was how to make the train longer. The prop gets in the way. Schienenzeppelin held the rail speed record for 23 years, and still holds the speed record for a gasoline (petrol) powered rail vehicle, at 230kn/h.

[Edited 2007-04-11 17:11:01]


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineVikkyvik From United States of America, joined Jul 2003, 9772 posts, RR: 27
Reply 21, posted (7 years 3 months 2 weeks 2 days 3 hours ago) and read 2200 times:
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Quoting Kalvado (Reply 17):
OK, so you would probably be able to accelerate the plane to 10 mph instead of 5.
You need a shitload of power to accelerate the plane - and to transfer that to the frame. Just as a simple examle - you can probably roll your car by pushing the bumper (breaks released, transmission neutral). Just imaging same thing being done by another car all the way to 60 mph (100 km/hr)..
Or, just looking at it power-wise, I've never seen a tug powered with anything remotely as strong as GE90..

Just to throw some theoretical numbers in here (cause I'm currently too distracted to do any real work):

Say you have a 100,000 kg airplane accelerating on takeoff to 175 mph (78.4 m/s). Assume it takes 25 seconds to reach this velocity.

So, you have:

v = 78.4 m/s
m = 100,000 kg
t = 25 s

a = v/t = (78.4)/(25) = 3.13 m/s^2

So your average force over the acceleration is:

F = m*a = 100,000 * 3.13 = 313,000 N ( = 70,370 lbs)

And your takeoff distance is:

d = 0.5*a*t^2 = 0.5 * 3.13 * (25)^2) = 978 m (3208 ft)

That force is the average thrust minus the average drag over the takeoff run - and it's a significant amount of excess thrust, which is obviously needed to keep the takeoff distance down to something reasonable.

So if you can only provide, say, 10,000 lbs of excess thrust (assuming this is what the rail launch can provide), then your takeoff stats become:

a = 0.445 m/s^2
t = 176 s
d = 6892 m (22,606 ft)

and the dry lakebed runways at EDW are suddenly looking quite nice  Smile



"Two and a Half Men" was filmed in front of a live ostrich.
User currently onlineStarlionblue From Greenland, joined Feb 2004, 16993 posts, RR: 67
Reply 22, posted (7 years 3 months 2 weeks 2 days 2 hours ago) and read 2192 times:

Quoting Vikkyvik (Reply 21):
So if you can only provide, say, 10,000 lbs of excess thrust (assuming this is what the rail launch can provide), then your takeoff stats become:

Lol. Thanks for the calculation. I was envisaging the catapult to be somewhat more powerful. Something like the combined power of both engines on a twin.

I love when topics generate some fun back of the envelope calculations.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineVikkyvik From United States of America, joined Jul 2003, 9772 posts, RR: 27
Reply 23, posted (7 years 3 months 2 weeks 2 days 2 hours ago) and read 2184 times:
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Quoting Starlionblue (Reply 22):
I was envisaging the catapult to be somewhat more powerful. Something like the combined power of both engines on a twin.

I figured, but I was just playing around with the idea that was suggested that the gear may not be strong enough to support such forces.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently onlineStarlionblue From Greenland, joined Feb 2004, 16993 posts, RR: 67
Reply 24, posted (7 years 3 months 2 weeks 2 days 1 hour ago) and read 2176 times:

Quoting Vikkyvik (Reply 23):
I figured, but I was just playing around with the idea that was suggested that the gear may not be strong enough to support such forces.

Ah.

I have my doubts about the gear struts not being strong enough though. Does anyone really know how strong the gear is that way? Surely tugs occasionally give a good yank to the gear. Also, while braking is pulling "the other way" some of that strength must translate to tugging as well.



"There are no stupid questions, but there are a lot of inquisitive idiots."
25 FredT : But not using forces large enough to get any significant acceleration. Besides, the magnitude is not the only thing you have to consider. There's als
26 Post contains images Starlionblue : See, even O'Leary thinks I'm not nuts!
27 Post contains images FredT : Tomorrows debate: Is that good or bad?
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