I hope this other view makes a bit more sense. You are looking through the side open to the atmosphere with the side opposite bounded by metal through the low expansion ramp area. The aerospike section is the bottom line between endplates. The heavy black lines represent the endplates for both engine types, both keeping the expansion plume focused on the metal expansion ramp you can see. In the linear aerospike, plumes 2-9 are bounded from the sides by adjacent plumes. 1 and 10 are bounded by plumes on one side and metal endplates on the other. The saddlespike uses metal on both sides all the way down.
Money is the first answer almost every time. People with limited funds cannot affort to take serious chances on an unproven technology. Just getting something working is far more important than improving something they don’t have yet. This is probably the first answer for the serious newspace companies. Armadillo, Masten, XCOR and company would not have had any benefit from a compensating nozzle to date. As of October 2008, for any of them it would have been a waste of resources needed for current flight vehicles. The people with the vision don’t have the money, and the people with the money don’t apparently have the vision.
Uncertainty is very high on the list of reasons to leave this tech alone. An aerospike or other compensating nozzle may boost the payload for SpaceXs’ Falcon 1, it would definately have cost schedule time and reliability uncertainty that a young company just cannot afford. The RS 2200 that was intended for the X 33 never did a full test stand burn as far as I am aware. That Dirkson plus program didn’t answer even the basic aerospike questions, much less supply relevant flight history.
Technical issues are third on the list, or maybe tenth for not putting a compensating nozzle in the specs. There is a lot of throat area to be cooled on most versions of aerospike nozzle or expander deflector. There are either many combustion chambers or an annular chamber resembling a bicycle tire. Either one has to be fun to work with compared to a straight forward single large chamber with an understood nozzle type. Talk to anyone that builds hardware about trying to work with 20 or so small chambers that all have to play well with others all the time.
Several conditions must be met for any serious player to consider compensating nozzles.
1. It must be simple to develop.
2. It must supply a financial benefit to pay for the development plus turn a profit.
3. It must be at least as reliable and certain as the rest of the company hardware.
4. It must not increase technical problems for the company.
Current proposed nozzles fail all four.
I propose a variation on the linear aerospike with 2 chambers total instead of 20 or so.
Shrink the 10 chambers per side to 1 and extend the nozzle skirting on both ends to almost enclose both sides. With the endplates turned back into nozzle walls, the gasses expand against metal on 270 degrees of the arc with the atmosphere interacting with the plume on the remaining 90 degrees faceing outward. The recirculation between the two sides operate the same way as the orriginal linear aerospike. Turn it upside down and it resembles a saddle.
1. Any company that is currently burning two chambers simultaneously, could bolt one of these to a test stand within a few weeks. If it works well, moving forward should not be too difficult.
2. While new space is not flying many high altitude vehicles at this time, several are flying throttled VTVL machines. Compensating nozzles are not altitude compensating, they are pc/pa compensating. There is a financial motivation for having a vehicle that operates efficiently at 300 psi and 75 psi near the ground. You don’t want to try precision landings with intermittent flow separation. Nor do you want serious under expansion for the majority of the flight.
3. If it works at all, it adds nothing to the vehicle that can go wrong. If your engines work right now, they will continue to do so. The exception is if you decide to use vane surfaces in the exhaust for TVC. This engine will lend itself to a rudder/elevator type TVC better than most others.
4. This, if it works, adds no technical issues except base flow differences. It may be an improvement or detraction, but it will be an unknown to be answered.
The people I know in this business don’t have time to go haring after wild ideas. There might be at least one group out there that could try something like this and let the industry know if it works. A few percent performance increase would be useful in the coming years for people with profit motive, if it can be made to work.