Two rocket questions

[Flight_Idle]

As a young Airman (Airframe) I remember asking the engine guys "Where does the thrust act in a jet engine." Surprisingly, they couldn't give me a straight answer, so I had to buy the Rolls Royce 'Jet engine book which clearly explained all the forward & rearward gas loads throughout the engine.

Now onto rocket nozzles... I've read all about how the nozzles are optimised for different air pressures & vacuum etc, but nowhere can I find details of the forward/ rearward gas loads within the combustion chamber & nozzle. Would I be right in assuming that most of the 'Forward' gas load takes place in the diverging 'Bell shaped' part? After all, something is pressing on the metal somewhere!

Question #2...
The Challenger disaster... I understand that that the solid fuel on the boosters burn along its length from inside out. This insulates the walls, but would lead to a greater burning area as the fuel is used up. Some solid fuel rockets start with a 'Star pattern' along the length to achieve constant thrust.
Was it just bad luck that a join in the sections of solid fuel lined up with the attachment strut? The O rings failed, but was some solid fuel burning before it should have been?
I hope some of that makes sense!

[TURIN]

I spent about half an hour at the Kennedy Space Centre examining the nozzles of the Atlantis shuttle. Still not sure how that amount of force can push through the bell nozzle, it's combustion chamber and attachment points. Incredible piece of engineering.

[Mr Optimistic]

From distant memory I once had to show that the thrust from a rocket engine can be calculated by integrating force over all the internal chamber walls. Don't recall the force over the external nozzle being of particular significance. The nozzle vent provided the hole in the control volume which gave a net fwd force. Was a hell load of years ago mind.
re burning. The burn rate has a pressure dependency which is critical tto address to avoid a short sharp bang.

[chevvron]

One thing that I wondered about was a monopropellant rocket of the '60s using iso propyl nitrate (AVPIN); some versions of this rocket, made by a US company called 'Turbonique' and intended for use on small vehicles, had the usual conical shaped nozzle whereas others simply had a vent direct from the combustion chamber, ignition being initiated by a spark plug. Did this make any difference to the amount of thrust produced compared to fuel burn?

[Nickul]

Yes you're right where the thrust is acting within a Jet engine is not immediately obvious, but as you say the RR book goes through the theory and it turns out everything upstream of the turbine especially the compressor generates the thrust. Not the nozzle, i.e. if you undid the bolts holding the nozzle on it would travel rearwards. when we loose compressor blades they tend to come out the front of the engine i.e. the forces are acting forwards ! The inlets ducts for supersonic aircraft can generate values in the region of 70% of the thrust of the vehicle at cruise.

Moving onto rocket nozzles, think of the unbalanced area projected internally from the exit throat area of the nozzle. All other regions are balanced with an opposing surface, but the projected area internally from the exit nozzle is not. The force is then P x A. Secondly rocket motors tend to be operated at high NPR's well above critical so as the exhaust gases become sonic at the exit throat there is still plenty of pressure available. The divergent part of the this Con-Di nozzle further expands the exhaust plume until at the exit plane you have fully expanded the gases to the ambient static pressure (ideal situation at one altitude). Therefore there is a significant pressure (and hence force) acting on this divergent section.

hope that helps

Nick

[Flight_Idle]

Yes you're right where the thrust is acting within a Jet engine is not immediately obvious, but as you say the RR book goes through the theory and it turns out everything upstream of the turbine especially the compressor generates the thrust. Not the nozzle, i.e. if you undid the bolts holding the nozzle on it would travel rearwards. when we loose compressor blades they tend to come out the front of the engine i.e. the forces are acting forwards ! The inlets ducts for supersonic aircraft can generate values in the region of 70% of the thrust of the vehicle at cruise.

Moving onto rocket nozzles, think of the unbalanced area projected internally from the exit throat area of the nozzle. All other regions are balanced with an opposing surface, but the projected area internally from the exit nozzle is not. The force is then P x A. Secondly rocket motors tend to be operated at high NPR's well above critical so as the exhaust gases become sonic at the exit throat there is still plenty of pressure available. The divergent part of the this Con-Di nozzle further expands the exhaust plume until at the exit plane you have fully expanded the gases to the ambient static pressure (ideal situation at one altitude). Therefore there is a significant pressure (and hence force) acting on this divergent section.

hope that helps

Nick

Thanks Nick, that was very interesting.

[Mr Optimistic]

Not forgetting the much higher internal pressure acting on the convergent section.

[Nickul]

Not forgetting the much higher internal pressure acting on the convergent section.

Which is a drag force, balanced internally with a corresponding "polo mint" area elsewhere in the system. Again if you undo the bolts holding the nozzle on, it will rapidly accelerate rearwards, it is this internal pressure acting on this "polo mint" area that creates this.

[Mr Optimistic]

Which is a drag force, balanced internally with a corresponding "polo mint" area elsewhere in the system. Again if you undo the bolts holding the nozzle on, it will rapidly accelerate rearwards, it is this internal pressure acting on this "polo mint" area that creates this.
Indeed. There was a confused discussion on tech log a while ago about the contribution of the intake section of a gas turbine to thrust. It almost seemed that some believed the cowling would fly on its own. It's all just pressure times area in the end, integrated over the system.

[PAXboy]

Supersonic intakes: Many years ago, talking to an engineer who worked on Concorde intakes said that she (mostly) 'sucked' her way along!

[Flight_Idle]

Supersonic intakes: Many years ago, talking to an engineer who worked on Concorde intakes said that she (mostly) 'sucked' her way along!

Did it become a slightly 'Ramjetty' like the blackbird? Google may be good at many things, but it doesn't give exact details of the forward & rearward gas loads in rocket nozzles or Concord intakes etc. It should be simple to publish exact details inch by inch.

Inch by inch forward & rearward gas loads within a ramjet would be a start, something is 'Pressing on the metal' & many would like to know exactly where!

We know about shock waves, but we want to know where the metal is being pressed by gas loads.