guest blogger john hare
I received a disturbing email today. John Bossard at http://plasmawind.typepad.com critiqued my cagejet idea at my request. I have been down for various reasons for the past month or so and mentioned that the criticism had disturbed me until a few unrelated issues resolved. He apologised for torpedoing my idea. That’s disturbing, bad ideas and poorly explained ones need torpedoing to make room for something else.
This picture of my EZ Rocket model after I got it out of storage is symbolic the last month or so. It is the first or second scale model that XCOR had made as far as I know, and now it is filthy and has pieces missing and broken. When my attitude is bad, I either have few ideas, or stupid ones like the IRS/NASA joke. Now that a few issues are on the way to resolution, I’ll try blogging some more.
The idea that John B. critiqued for me was posted in October 2008. It is the cagejet variation on the turborocket. When a legitimate critique is received, the proper response is to address the issues, not the emotions. I have paid folding money for worse information on occasion. He gave me permission to address the issues he raised in a public blog post.
First problem he points out is that the burn would be at a fairly low pressure with inherent poor efficiency because squirrel cage fans are high volume/low pressure ratio machines. The radial type compressors obtain their pressure rise in the diffuser downstream of the rotating machinery. I was seeing it as a uniform distribution sideways axial type flow with better boundary layer behavior. While I didn’t draw and label diffusers in the post, I don’t see them being left out in the first serious design pass. While he is correct that I didn’t address this point, I think any reasonable designer would correct it early.
Second major problem is that radial flow compressors, of which this is a subset, have large frontal areas compared to axial machines. My solution is to turn the ‘wheel’ sideways and mount it in the vertical tail. This has the multiple advantages of making the machine frontal area smaller than the inlet, making use of an area that is normally empty, providing a structural justification for beefing up the tail that is not ‘charged’ to the airframe, providing a reason for increasing the size of the vertical tail that is not ‘charged’ to the airframe, and placing the inlet drag above the center of lift to reduce supersonic trim drag.
In the cartoon above I have the vertical tail section below the vehicle with air coming in left to right. There is a fixed supersonic inlet at the leading edge of the tail. A normal shock just before entering the center of the cage. The volute, which must be well designed as John B said, is inside the tail also. In the upper sketch you can see the afterburner chamber right at the root of the tail. I believe this layout would bring out the best in the concept.
The things I left out of the original post make John’s opinion correct. The things I add in here might make the difference.
He also mentioned that my Isp values for ATRs was much too low. The proper hydrocarbon bi-prop Isp should have been in the 1,200-1,500 second range.
I believe the proper application for this concept is as a flyback or air launched fly forward first stage. The vehicle climbs out of an airport to safe airspace on ATRs alone. In the clear, the ATRs are throttled up all the way for acceleration to mach 2-3 at 15-20km altitude. The rockets are then lit in addition to the ATRs for the acceleration to ~mach 6 at 40-60km altitude when staging takes place. The upper stage has about 6,000 m/s to go for reaching orbit.
Mass ratio of the first stage should be less than two. Torpedo’s away, if I can’t handle criticism, I have no business posting controversial ideas here.