why are jets faster then props?

[Brett_Henderson]

Interesting replies here. I'm surprised that nobody mentioned stall speed . Jet engines, having more blades per row and more rows are more efficient at moving air than propellers

Hmmm, thought I "kinda" said that :

Props (including turbo-probs) are like little wings  trying to "climb" through the air, pulling the plane along. A prop can only cut through the air to a point

I still want my quarter.

[Felix/FFDS]

To add what Brett said -

assume you have a rocketship at rest in space...

If you accelerate from a full rest at a constant 1.0g for 24hrs  ... shut the engine off ... coast (at the final speed reached ...  you will need to turn the rocketship 180degrees so that the thrust will point in the exact direction of the line of travel and turn it on again for 24hrs at a 1.0g acceleration to come to a complete stop.

It's a matter of vector math - x force at y angle applied can be stopped by x force at -y angle... or forces b at angle -u, c at angle -v as long as their vector sum equal force x at angle -y ...

and exactly how do you decelerate in an enviorment with no air if u have one little rocket engine pushing the plane forward?(first person to explain this gets a shiny new quater and a shiny new learjet;)

[Brett_Henderson]

Anyone in my age bracket (40+) surely played Asteriods in the arcades.

That game was a perfect example of how vector math applies to a rocket. Remember when you would run from the little saucer as fast as the ship would fly ? And then when it was safe, you'd have to turn the ship around and apply the thrust just to slow down and gain control.

And that was only on 2 dimensions, imagine trying it in a "3-D" environment  :o

[Rifleman]

this stall you are talking about in front of a propellor is what I was referring to when I addressed the difference in the tip and root speeds of a prop....tips hit Mach 1 faster than root, .....thats the noise we here from a high speed Heli.....as for the cpmpressor stalls, from high density air being forced down the throat of modern jets, thats the reason the variable intake was brought into play....remember the performance of the B-1 ? before the variable intakes and exhausts were removed on this bird to reduce costs, and produce the B-1B , the B-1A was a much higher performance aircraft.....much faster !............backin ya up 100% on this one  :D

[katana_1000]

Hmmm, thought I "kinda" said that :


I still want my quarter.

ok,ok*gives brett his damn quater*

[SyPrO_CaStEr]

wow, i never expected to get this much of a responce.... i've lernt more then i do in school in 2 weeks!!! well maybe i should be paying more attention in school...

[Professor Brensec]

[quote]

I said, "Turn around" , or turn the engine around

[Brett_Henderson]

I love how these threads take on a life of their own  ;D

Any of the brighter designers would have "retro" (meaning reverse or backwards) rockets or engines already facing the other way!!  

I dunno,,  since we've jumped over from "prop vs. jet" to "space-travel", let's ponder it.

A retro-rocket capable of counter-acting the velocity built up by another rocket, would have to be at least as big, powerful and heavy. That would make it "dead weight" for 1/2 the flight.

Small maneuvering rockets (to turn around) or a mechanical sytem to reverse the main rocket (or it's thrust) is more the way to go.

Can't wait to see where this thread goes next  ???

PS. "dead weight" should read, "dead mass", as there is no weight without gravity  ;)

[Felix/FFDS]

Yes, it's more efficient to use smaller maneouvering thrusters than another "equal and opposite" rocket.

As to mechanical means - an internal "flywheel" type arrangement could be considered, using the torque of a spinning mass to rotate the overall ship.   I think, however, that given the masses involved, thrusters are more efficient.

(Where Andy Libby when we need him?)



[quote]I love how these threads take on a life of their own

[Rifleman]

[quote] let's ponder this......
A retro-rocket capable of counter-acting the velocity built up by another rocket, would have to be at least as big, powerful and heavy. That would make it "dead weight" for 1/2 the flight.
PS. "dead weight" should read, "dead mass", as there is no weight without gravity

[Brett_Henderson]

[quote]Brett, first thing to ponder...why would it be necessary to carry a slower-downer the same size as the giddy-upper.......if its half the size, then you just run it twice as long ?

[Loafing Smurf]

One upgrade to the whole idea you present here....in the event of the little boy crouchng under the nozzle trying to strike his match to light the fuel and oxidizer, and winding up finding the sulphur all paste from moisture, they now use a foolproof method of ignition,....the fuel and oxidizer are " hypergolics ".....materials which ingite on contact with each other........been using these types since leaving the moon became a requirement to a successful mission.



oh yeh, one more thing..... airplane engines are called "air breathing engines"

I stand corrected.

[Rifleman]

I did say that I could be confused !.....it does explain one thing, though.....it takes less time to get home from the moon than it does to go there......its all downhill(relatively speaking), with only one sixth of the escape velocity required to break free of the moons influence and allow the earths to take over.....5/6ths of the trip back is accelerating.....without further motive force being applied internally at the vehicle........I guess thats why long distance space travel so far, has made use of other celestial bodies and their imparted gravitational influence to the traveller............Americans call it the singshot effect.....in England a slingshot is a catapult .

[Deputy]

[quote]Yes, it's more efficient to use smaller maneouvering thrusters than another [glb]"equal and opposite"[/glb] rocket.

As to mechanical means - an internal "flywheel" type arrangement could be considered, using the torque of a spinning mass to rotate the overall ship.

[Professor Brensec]

A retro-rocket capable of counter-acting the velocity built up by another rocket, would have to be at least as big, powerful and heavy. That would make it "dead weight" for 1/2 the flight.

We were talking about a Jumbo being moved through space by "the tiniest rocket". This is what I based my comment on. Another "tiny one" would not make that much difference. In fact, depending on the definition of "tiny", it may even have less mass than the thrusters you speak of.

As once a rocket propelled object reaches it's required velocity and remains at that velocity until acted upon by some opposite force (inertia), the extra weight (little as it is) I don't feel would make a difference in this particular scenario.  :P