TAKE A LOAD OFF JACK!
Our vehicle seems to be a bit overweight, but I also carry a load of doubt. I need something like the Easter story to remind me that the joy comes in the morning. I dived into this to make mistakes and corrections publicly so I might hope there would be an avenue or redemption sooner or later.
I launched into this investigation based on a few seemingly safe assumptions. As a designer I have always had a lot of exposure to every intimate detail of a given geometric structure. As such, I may be aware of opportunities of available space. My job includes letting the engineers know about opportunities within the available geometry. If I am aware of other opportunities in materials or vendor solutions I can also make those known. I have worked on a couple of other space planes and these opportunities came to my attention.
Here is a shuttle on a large booster aircraft. The wings of this shuttle aren’t working to produce lift, so they are parasite mass. The shuttle is protruding enough to be slightly parasite drag. By moving our orbiter to the front, the mass of its wings is working to produce lift. This allows the booster wings to shrink, saving mass there. The smaller craft also becomes a nose cone to the larger craft instead of increasing frontal area. By using a blended wing body form we add the efficiency of a lifting fuselage per the concepts of Burnelli. The lifting bodies maximize internal volume and still allow aerodynamic deceleration and runway operations. Both were crafted to emulate the efficient wing of the Concorde, and the wave rider winglets of the XB-70. Every element is aimed to eliminate wasted effort. There is also a possibility for stage separation in the atmosphere in an emergency. We may avoid aerodynamic hazards of staging if we are pushing the two craft apart vigorously. Thus payloads may be rescued even if a booster is lost.
Still we found an issue with packaging fuel tanks in wings and flattened fuselage areas. This led us to consider High Test Peroxide (HTP) instead of cryogenic fuels. Like jet fuel, HTP can fit in wing tanks, but it is heavier than liquid oxygen. Our early studies suggested that we might need as much as 1.5 times as much HTP as LOX for the mission. And rocket formulas pointed towards a ratio of 7 or 8 parts of HTP to 1 part Jet fuel for rockets. We have found new reports that give us better tools to target our fuel ratios.
An AFRL project was published in 2004 called Quicksat. That was a paper airplane study like this one, but with all the heavy engineering and trade studies. It presented us with a good trade comparing LOX and HTP for a vehicle of similar size and mission goals. Their mission differed in using hypersonic speeds with an air breathing booster. It used little rocket thrust on the booster stage, relying on scramjets for acceleration. The orbiter was essentially an X-37 with strap on HTP and JP-7 boosters. Those were expendable. The article acknowledged “inefficiency” of the booster airfoil at low speeds. Gross Take Off Weight (GTOW) could be nearly that of a Boeing 747 in some configurations. Some exciting wing loading on a flying wedge could produce adventures on takeoff. The study still has value though.
I had harvested some crude estimations from data published about the Bristol Spaceplanes Spacecab. I estimated a ratio of 1.5 times the Spacecab cryo fuels for HTP use. That drove mass up on the “Old” Leap (at the bottom) but it was still way less than the Quicksat. When I projected a “new” hypersonic Leap I included some small mass advantages but still saw a huge wet mass. To my surprise the Quicksat mass was not from HTP. They are burning a huge amount of JP-7 in the scramjet mode. If you feed a gas guzzler, the advantage of air-breathing systems LOX savings is negated. Back to the drawing board.
The choice to use more conservative air breathing engines led us back to Spacecab. That appears to be using a gang of turbine engines. These may still yield supersonic performance without negating the value of harvesting atmospheric oxygen. The Quicksat article did publish a table comparing HTP to LOX fuels which I used to establish a more accurate ratio for converting from the Spacecab figures.
A conversation with Michael Carden of X-L Space Systems yielded another valuable clue. Most studies propose a 7 or 8 to 1 ratio for HTP to fuel. Michael suggested a 5 to one ratio where their distillation yields a 100% HTP product. That will also save us a bunch of mass.
Now we are closing in on the target and I think we can boost the payload with no strain. We don’t mind any load that is paying. You also see a very low estimated mass for landing gear because we have a different fix for the takeoff leg. Every ounce counts, but that detail will be published later.
While we began with Spacecab data, we should see other savings. HTP will have lighter tanks than cryogenics, and it is dense so vehicle size is reduced. Since the orbiter changed we have saved some weight in that area too. This helps us to remain reasonable about wing loading and the takeoff roll. Now we have enough fuel tanks with volume for ullage and balance adjustment in supersonic shift. This was also an opportunity to clean up the structural junction to the first stage.
Changes to the orbiter also aligned the fin and junction structures with structures on the booster. We eliminated four stage pin actuators and two separation cylinders as another mass reduction.
Looking forward through part of the booster shows all the mechanisms clustered in the structures. This is an improvement over the previous mid-stage proposal.
This series of illustrations animates the pin release and piston separation sequence. Now the engineers can go to work shaving the un-needed mass off of my heavy-handed models!
PRESSURIZED FOR SEPARATION
We will explore more booster solutions soon, including some of the skins and service considerations. This changes every week as problems and solutions are discovered. We have seen that propulsion choices have a big influence on performance. We want to get back to new possibilities in that area. Is anyone out there interested in innovation in air-breathing propulsion?