Second string substitute apprentice relief blogger john hare.
In my last post I described a new type of turbojet based on my cage jet of years ago. The engine I described has the capability of good thrust and good fuel economy which is ideal for launch assist platforms. Launch assist platforms want to have the capability of lifting very heavy loads off runway, and taking them to very high altitudes, and pitching up in a gamma maneuver that allows near vertical launch of the orbital vehicle. Sometimes they want to cruise to a particular launch location and cruise back to base.
In order to reduce upfront investment, most of us start by looking at existing aircraft that can be modified for our purposes. That is almost certainly the way to get started, but has the problem of limiting our capabilities to that of whatever carrier aircraft is selected. The problem with designing our own Launch Assist Platform Aircraft, is that it adds a tremendous amount of expense to a project is almost always funds limited. To date, the launch assist platform aircraft that have been designed have been designed by aircraft designers that are going for extremely capable aircraft, but don’t seem to have much input from the launch industry. The White Knight series of lifters exemplifies this.
I suggest what should be done is design the aircraft around the launch vehicle, instead of vice versa. We should also design around available finances, skill sets, and available ground facilities.
First thing is the launch vehicle payload required, which defines the rocket vehicle, and dictates the capabilities of the Launch Assist Platform Aircraft. It is necessary that we make an assumption about the maximum payload that this system will will want to place into orbit. For the purposes of this blog post, I am going to make the assumption that it is desired to place 25 tons in orbit as a maximum payload. While this is much less than the heavy lift vehicle’s several companies are considering along with NASA and the United States Congress, it is quite sufficient for almost any mission we see in the next decade, as long as we assume orbital tugs and propellant depots. By developing the launch assist platform now with its attendant launch vehicles, a revenue stream can be developed first, which can then be enhanced by the orbital tugs, and the propellant depots.
Designing the launch assist platform aircraft, is much like designing the foundation for a multi-story building. When designing a building you do not start with the foundation, you start with a roof. Then you design the top floor which also carries the loads of the roof, then the second from the top floor, all away down to the basement. Only after all that do you design the foundation of the whole building. In a similar manner we have to work backwards from the payload to the launch assist platform aircraft. If we assume a basic launch architecture of launch assist platform, and single stage from there to orbit, the mass ratio can be on the order of 12 with high-performance kerosene engines. The dry mass would be on the order of 4% each for payload and vehicle.
4% net for a payload of 25 tons gives a rocket vehicle of 625 tons. This becomes the desired payload of the launch assist platform aircraft. This is clearly beyond the capability of any existing aircraft including the White Knight 3. This is the technical requirement based on my assumptions.
Available finances dictate the actual capabilities we will end up with. Trying to design a conventional aircraft with the capability of 625 tons in external carrying capacity is not going to work. There’s not enough work for that vehicle to use on other projects which means that the launch assist platform aircraft must carry the entire burden of cost simply on launch revenue. Available finances are the funds that can be spent on the vehicle considering ROI, and not based on some percentage of a billionaire’s net worth, or how much money can be conned from the United States Congress. The 25 ton payloads, at the pricing that can be expected a decade from now when the system would hit its’ prime dictates how much money could be spent now if we assume that the system is flying at least daily. Since it could be competing against $500 a kilogram or less from other companies, the finances suggest a gross revenue of about 12 1/2 million dollars per flight. Subtract fixed and marginal costs from that number, and multiplied by the number of flights expected annually, and we get a number 10 years out that we can work backwards to find the amount of money available today. Since the LAP is only one component of a two unit system, it is only worth a percentage of the total. The rocket stage will get the lions share of the costs and investments leaving perhaps 2 million per flight available to service the debt on the LAP after its’ own fixed and marginal costs. Assuming a flight rate of 250 per year, and revenue available for debt interest is $500M per year. A high risk debt can be expected to have an effective interest rate on the order of 25%. So the vehicle debt at that price range and interest can be no more than $2B.
If we assume that the initial investment covered a development time of six years, and a further four years was spent ramping up business, and the interest on the development money was at 25%, then there would be something on the order of $200 million available to develop the launch assist platform. The only way this can possibly be done for that number is if the vehicle though very large is very very simple.
The second requirement is to design the launch assist platform around the available skill sets of the people available to the project. Since this is a blue sky concept, I am going to assume that reasonably competent but not brilliant designers are available, along with a workforce that is motivated and experienced at the construction method under consideration. This requires that the construction method under consideration be very simple.
The vehicle must also be designed around available facilities. This is fairly simple, runways and available hangers will limit the design. Runways have length, width, and weight limitations. Hangers have length and width limitations unless you build a fancy and very expensive new structure. Since the 625 ton upper stage will probably be matched by a 625 ton launch assist platform, the runway must have the capability of handling 1250 tons. Since this exceeds any aircraft ever built the weight must be distributed over wider areas that any aircraft landing gear has ever experienced before. 1250 metric tons is 2,750,000 pounds. 2,750,000 pounds can be accommodated by using a hovercraft undercarriage of the type that was experimented with 50 years ago. If we assume a very high wing loading, there will be something toward 30,000 ft.² of wing area. It will take a very low aspect ratio wing to fit in the available facilities. The aspect ratio will probably actually be around 1.5.
In the cartoon you can see the hammerhead shroud hanging over the front of the vehicle. The cage jets are inside of the wings. And instead of wheels underneath there are hovercraft skirts to spread the load across the whole runway.
The way I see it this launch assist platform will be a flying wing with a wingspan of about 200 feet and a length of about 200 feet with sweep to wingtips are still 100 feet long. The launch vehicle will ride on top of the wing centerline. The hammerhead shroud will protrude in front of the vehicle. There will be a huge cage jet mounted inside each wing. Each cage jet will mass about 40,000 pounds and have a thrust of 1,000,000 pounds. The airframe should be around 10% of takeoff mass and will be about 125 tons for airframe. With engines and airframe at 165 tons, and other required systems at 35 tons, there will be about 425 tons of fuel available to cruise and accelerate. Enough fuel will have been burned by the time of the gamma maneuver, that the vehicle can accelerate at a fairly high rate during the gamma maneuver on cage jets alone. The rockets on the launch vehicle can be lit before separation allowing rocket systems checkout during the maneuver. When the vehicle separate the launch assist platform will have a higher thrust to weight ratio than the rocket, which will allow it to accelerate away without worrying about rocket plume impingement.