guest blogger john hare
One of the problems with the toroid tank based vehicle is that it looks like a UFO. Let’s just call it a saucer and beat the comedians to the punch.
Three main objections came up in comments on the torus tanks. The large surface to volume ratio could result in more insulation mass or boil off than a conventional tank. Toroids will probably have a slosh problem. Tank construction may be a real problem with no existing tooling and experience base. Each of these problems could result in this concept failing on financial or technical grounds. For the purpose of this post, I assume these problems are solved without excessive financial pain. Deep investigation would need to be done on the tank issues before killing too many trees and electrons designing a vehicle.
An 18 foot diameter vehicle could be road transported within the 14′ wide load (mobile home type) and 13’9″ high (tractor trailer) limits by loading it on a tilt in the transport trailer. The ability to easily truck it around for construction, retrieval, and display is an important requirement. Sometimes the people installing the noodlin pin in the woblin shaft really need the vehicle in their facilities to do their best work for the least dollar. 18′ diameter is small enough to be handled and large enough to do real work. Smaller is possible down to 12′ diameter or so, though it would be more limited without the possibility of serious growth in the same mold lines.
An 18″ ‘pipe’ diameter for the outer tank would have about 90 cubic feet of fuel volume. About 4,000 pounds of RP fuel with some ullage volume. A 24″ ‘pipe’ diameter for the interior LOX tank would have a volume of just over 125 cubic feet. 8400 pounds of LOX would give a rich rocket mix ratio of 2.1. The 12,400 pounds of on board propellant would allow 6,200pounds of dry mass if a mass ratio of 3 is required for a sub orbital flight with reserves. GLOW of 18,600 pounds is either enormous or tiny depending on your viewpoint. The central cabin and cargo section is 11 feet in diameter and would allow 4 across seating with two more in the second row center.
If the vehicle has a mass ratio of 3, and is required to ground launch to over 100 km and return with reserve fuel, then an average Isp of 240 will barely get the job done. A better engine system will allow it to take off heavier or go higher. This conservative number is rational for concept design to give serious margins for the gotchas during development.
If pressure fed engines require a tank mass and pressurant gas mass of 10% of propellant, then tank mass would be 1,240 pounds. If the engines have a T/W of 50 and are required to provide an airframe T/W of 1.25, then the 23k engines will mass 460 pounds. If the airframe is 10% of GLOW, then it masses 1,860 pounds. The airframe can possibly be this light without excessive engineering because the tanks provide most of the main structure. There is just over 2,500 pounds left for aircrew, their gear, payload, and margin. Six people could ride this early development model to space and back with experiments.
An airframe mass 10% of GLOW will have to be investigated. It is claimed to have been achieved in some experimental homebuilts. This vehicle needs to handle mach 4 or so if it is to have real utility. By using the tanks as the structural elements for both length and span, the airframe mass is skin panels, control aero surfaces, landing gear, and cabin. Any over weight items will directly affect the crew capacity and payload..
With the skin panels not needing to be structural components, either composite sandwich or aluminum sheets and stringers can be used. Both systems can be very light when there is a strong main structure to attach to. The aero surfaces are the horizontal and vertical tails, plus some small wings to alleviate induced drag and house ailerons. These are well understood low aspect ratio pieces of reasonable weight.
The cabin and landing gear are the serious weight concerns. Done wrong, either of them could bust the mass budget. The cabin will mass as much as on any other pressurized vehicle that will see supersonic speeds. The landing gear can be slightly improved from standard practice. By housing the fixed main landing gear in the vertical tails, considerable mass can be saved compared to a retractable system that must handle GLOW. The vertical tail extends below the vehicle with clam shell doors revealing the wheels when required. In the event of door failure on landing, that section of the tail can disintegrate to leave the wheels exposed. Depending on who you ask, this can save a lot of weight and cost compared to a conventional retractable gear. The nose gear will have to be normally retracted and extended.
A high pressure pump fed engine could reduce tank and engine mass while increasing Isp to an average of 300. If this happened, then the same mission profile could be done with a mass ratio of 2.5. GLOW could increase to 19,800 pounds. Tank mass could drop to 250 pounds. Engine mass at T/W of 100 would drop to 250 pounds. Airframe mass would increase to 1,980 pounds. The result is over 4,900 pounds available for aircrew, gear, payload, and margin. This is a good place to start thinking about using it as a first stage for microsats.
Before any of the above could be seriously considered, a cheap test article would need to be flown to explore the actual flight characteristics of the shape. For the test article, thin glass over foam torus would provide a fairly economical tank simulation for strength. The rest could be fabric over frame, aluminum, composite, or whatever the construction team is most comfortable with. There is plenty of room to install a large piston engine behind a two place cockpit inside the nominal cargo volume. As a light experimental aircraft, a GLOW of well under 2,000 pounds is possible with pilot and engineer. Wing loading of the disk would be less than 8 psf disregarding any lift from other aero surfaces. Power loading of 8 pounds per horsepower would require a 250 hp engine. A modified auto engine would probably be the choice. A propeller drive shaft could extend through the simulated propellant tanks to a puller or pusher prop. A VNE of 200 knots would give plenty of envelope to explore low end handling qualities.
If a serious design study confirmed the possibilities of this operating layout, then a series of vehicles could be built on the same mold line expanding the envelope each time.
0. Simulation, Small rc, and wind tunnel models.
1. 2,000 lb– piston engine and propeller to test low end flight qualities including CG problems, L/D for level flight, and visibility. (As an airplane, it is going to fly like a pig. Test is for spacecraft type qualities.)
2. 4,000 lb –piston engine and propeller with airframe sonic capability. Test bed for rocket handling with propeller back up. Remove piston engine for transonic testing under rocket power.
3. 18,600 lb –pressure fed rocket vehicle. Suborbital capability with 2,500 lbs of aircrew and payload.
4. 19,800 lb– pump fed rocket vehicle. Suborbital capability with 4,900 lbs of aircrew and payload. Possibly used as first stage for microsat.
5. 19,800 lb– pump fed rocket vehicle. Air launch to give 4,000 lb second stage a mach 6 boost with 50+ km staging altitude. Glide forward landing.
6. Orbital upper stage with extra propellant tanks in the cargo volume.
7. Next step up in airframe size.
These four side views of the lenticular vehicle represent a method of using the same mold lines for a vehicle with constantly increasing capability. If it is found that this layout has reached its’ limits in one particular model, the cancel that one and go back to the last one capable of performing at a profit, if any. With each performance level in the series thoroughly exploring the lower end of the next it is possible to go far in an incremental, financially responsible manner. Financially responsible includes walking away if it becomes clear that a dead end is reached.