guest blogger john hare
The VTVL vs HTHL vs VTHL arguments will probably never end. Many people simply cannot admit that different methods fit different requirements. That I prefer HTHL myself is not a compelling argument for the method. It does mean that I will look for ways to support my preference.
The number one advantage of VTVL vehicles is light airframes. I may have found a method of getting a runway vehicle close to the attainable mass ratio of vertical vehicles. Or an air launched vehicle with a better ratio than wings on a rocket. It all starts with the tanks in spacecraft. Launch vehicles have been described as propellant tanks with auxiliary equipment. The two types of successful tanks are spherical and cylindrical. The multi lobe tank was one of the causes of the X33 failure.
It is my understanding that a torus tank would have very slightly worse mass per volume compared to the normal spheres and cylinders. It offers the possibility of advantages that more than make up the difference in terms of over all vehicle mass, construction cost, reentry shape, and payload volume. The inflatable raft offers a dramatic view of the layout and strength of a toroid based structure. The billions of auto tires on the road demonstrate the same. Picture either one based on an inflated sphere or cylinder. Other than a hand full of special cases, it won’t work.
Though zee Griffenshaft is an extreme example of long and thin problems, most ELVs use considerable mass for intertank structure and payload fairings, not to mention the attention that must be paid to keeping the whole vehicle rigid enough for the job. While these things are understood and handled, they cost. Any VTVL RLVs will also have mass in those places. Torus tanks offer the possibility of eliminating the inter-tank structure and reducing the payload fairing structure. This is because the structure of the two tanks can be concentric and stiff in three dimensions. The donut hole is the payload bay with relatively roomy volume.
The sketch here is HTHL mode. Yank the aero surfaces and use it for VTVL for the same result. Less parasite mass, if my numbers are correct of course. In the very small sizes the tanks can be irrigation pipe formed into a circle to get a more rigid structure from small diameter COTS parts. In the larger sizes it isn’t necessary to have 10 meter diameter cylindrical tank tooling. 5 meters coiled to a torus can do the same job. Stack similar donuts for an even lighter VTVL structure with no serious inter-tank structure or payload shroud structure.
In 2000-2001, a consultant ran a disk plane simulation on Xplane and got a lift/drag ratio of 8. What I am suggesting here would have a different layout than he used and would have more drag. The lifting body program of the 1960s had L/D ranging from 2.5 to 4. This lifting arrangement would most likely split the difference to get 5 or 6. This would be terrible for an aircraft, acceptable for a spacecraft though. For a spacecraft, we should be willing to sacrifice efficient aerodynamics to achieve low dry mass as long as the vehicle is capable of doing the job of getting on the ground safely. The German and American disk planes of the 1940s had poor efficiency due to the very low aspect ratio, and this one will be even worse. The only thing that could possibly make this layout acceptable is the possibility of a very strong airframe that is very light. The lightness is because the tanks provide virtually all the vehicle strength with most of the other structure being skin with stiffeners.
I don’t really like the wings on a rocket school of spacecraft. The objections of the VTVL people are somewhat accurate for most of them. Many have all the structural problems of the totem poles, added to all the structural problems of an airplane. The lifting bodies to me are nearly insane. Terrible flight characteristics and terrible mass problems for no visible gain. The pure VTVL lacks cross range options for the most part, even if the landings are getting well defined.
On reentry the vehicle could come in belly first for a very fluffy profile. At alpha 90, it is hard to picture anything short of a balloot that would have higher drag. The fluffy reentry profile allows simpler lower tech TPS that many people suggest for vehicles that are mostly tank. The big problem I see is keeping it stable at high alpha during reentry. Fortunately there are people that know how to solve that problem.
This idea seems obvious enough that there have probably been studies about it. That I have not seen them doesn’t mean they don’t exist. If there is a good reason this won’t work, it was likely written up in the 1940s or 1950s.