guest blogger john hare
Compensating nozzles would be useful if they can be done affordably. Affordable is not only the cost to fabricate a nozzle, but also the risk of pursuing things that don’t work, and the time and money to develop it to operational status. Years behind the competition due to excessive chasing chimeras of perfect performance can be more expensive than just taking a performance hit. An affordable compensating nozzle should be something that can be modified from existing equipment and brought to use in minimum time with minimal cost.
One simple and cheap method that shouldn’t require much in resources to build or test is what I call a notchbell nozzle. It is a large expansion ratio bell nozzle with a notch in it up to the plane of the lowest expansion ratio desired by the flight profile. It works in the same manner as a linear spike nozzle except there are no other combustion chambers. The exhaust of the single throat expands against the normal bell on all sides except the atmospheric wall on the notch side. The air controls the plume in the same manner as the spike nozzle that has metal reaching all the way down to the maximum expansion ratio with no aero component to the spike.
The engine will have asymmetrical thrust and need more gimbal authority than normal.
The two sketches here represent two different concepts. The one on the left is the notchbell. The one on the right is a speculative bolt on adapter to get compensation from an unmodified bell nozzle.
When trying to figure out what went wrong with some notchbell units I made, I remembered some things from an intake aerodynamics book. Subsonic intakes on jet aircraft have smooth rounded leading edges of fairly precise design to maximize air pressure to the engine with the minimum reasonable drag. They do not have sharp edges like supersonic intakes. As far as I can tell, it might be possible to add a tiny rounded intake to an over expanded bell nozzle to force it to separate in a planned way to compensate for pc/pa variations. The boot would be on the cool side of the nozzle until it reached the nozzle design expansion altitude. It could be made of light expendable material that burned off after doing its’ job.
Since the boot would only be used on about 25% of the perimeter, it would create asymmetrical thrust by forcing flow separation on just part of the normal nozzle. If the boot is found to work, then two of them could be placed on opposite sides of the nozzle to make the thrust symmetrical. It is possible that some TVC can be done with four boots sliding in and out of position. Once the engine is in vacuum, normal means of TVC must be used as there will be no separation to control.
I made a tiny notchbell unit that hooked into shop air a few years back. The first one worked exactly like it was supposed to with the air flow focused on the wall right down the center line opposite the notch. Then I promptly destroyed any credibility the thing might have had by whipping out a dozen of them and shipping them off untested to friends in the business. I only realized the problem when I did another batch of two dozen and only three of them worked properly the first time. I had to spend a good bit of time tweaking them to get up to nineteen units that worked reasonably well. I am hoping some of the people that got the stupid things will take another look.
The boot is highly speculative. The only saving grace it has is that it would be incredibly fast and cheap to confirm or bust. No major hardware would have to be damaged to test the concept. It is almost to the point of shaping a short length of round stock to fit 25% of the perimeter of your large expansion nozzle, and holding it on with a hose clamp during a characteristics burn. Maybe a little caulk to keep a smooth joint pipe to nozzle. It is possible that a couple hours construction and 4 seconds of burn could tell the tale.
This sketch may help clarify what I suggested in comments as a visual aid in understanding the boot idea. A pipe connects the ambient air to the low pressure center in an over expanded bell. Air should flow from the higher pressure ambient to the low pressure center cutting down losses. Putting a pipe in the exhaust stream like this would melt it. The boot is an attempt to get the same job done without interfering with the exhaust stream.