TRCC Engine Modes
So far we have identified 8 major sub-categories for the TRCC propulsion system as shown here.
The Flight Profile
The flight profile selected for our initial investigations is a 700 pounds per square foot constant Q profile based on our experience with the SR-71. This profile avoids the thermal problems encountered by the NASP program and allows us to build the vehicle with existing materials. The advantages of this are reduced risk and a smaller initial investment, which has a significant effect in helping to keep the dollars per pound to orbit down. The disadvantages are slightly reduced performance for the ramjet-scramjet and a slightly larger and heavier vehicle as compared to a high Q flight profile vehicle. So far our cost estimates are showing that the low Q flight profile results in a lower dollars per pound to orbit then the higher Q flight profiles.
The Flight Performance and Sizing Programs
In order to determine the size and cost impacts of all these engine modes in a cost-effective manner, a pair of computer programs were developed.
The first of these, the Flight Performance Program, includes all the engine performance data, drag calculations, and trajectory data. The output is the amount of propellant required. This information is then transferred to the second program, the Weight and Sizing Program, where the vehicle's geometry is adjusted to meet the volume and balance requirements. The output is the vehicle's dimensions, weight, and cost estimates. The dimensions and weight data are then input back into the Flight Performance Program and the process repeats itself until it converges on a solution.
The graphs shown in this chart are examples of the data that is generated by these programs.
Left. This graph shows the launch vehicle's gross weight, propellant burned, thrust and drag, as a function of time.
Upper Right. This graph shows the vehicle's specific impulse as a function of time. The spike in the middle is the hydrogen-fueled ramjet-scramjet.
Lower Right. This graph shows the vehicle's free stream dynamic pressure (Q) during the boost phase of the flight profile.
These programs also calculate the size and location of all the major components in the vehicle, balance the vehicle's center of gravity (both full and empty) on the wing's aerodynamic center, calculate the vehicle's overall geometry, and determine the vehicle's development cost, unit cost, recurring cost and user cost.
TRCC 12-5-96 #13
To give you an idea of the results we are getting, this is a chart of the initial results for engine mode #13. Be aware that the cost estimates include debt service, amortization of the investment, all recurring costs, $100m for vehicle specific engine development, ZPI and profit. These estimates do not include the technology development costs for the combination ramjet-scramjet ducted rocket motor propulsion system.
As you can see from the graphs, this particular case makes for a vehicle with an overall length that is getting a bit on the long side for currently existing hangar facilities. This is due to the high percentage of liquid hydrogen in the propellant mix for this engine mode. As a result, there are a number of other cases that will need to be run before we can settle on the results for this engine mode. For example;
1. The impact of varying the number of turbine engines in the vehicle.
2. Different fineness ratios for the fuselage.
3. The impact of varying the bypass ratio for the ducted rocket motors.
4. Varying the capture area for the ramjet-scramjet.
5. Looking at other rocket re-ignition velocities for the ducted rocket motors.
Other worthwhile studies are;
investigate the weight and cost impacts of increasing the maximum turbine velocity from Mach 2.5 to Mach 3.0,
investigate the weight, size and cost impacts of using a dual fuel ducted rocket for the final boost portion of the flight profile.
Turbine Based Combined Cycle (TBCC)
TBCC as it is usually defined, is very similar to the TRCC concept we just covered in that it has turbine engines and a ramjet-scramjet, but it does not normally have ducted rockets, and the turbine engines do not have afterburners.
The main difference TBCC and TRCC is that TBCC uses the turbine engines to create an ejector effect with the ramjet which has the potential of improving the ramjet's performance in the zero to Mach 2.5 speed range.
Since TBCC is usually looked at as a propulsion system for a Mach 5 to 8 aircraft and is not expected to be capable of going faster then Mach 8, we will investigate adding either rockets or ducted rockets to the system in order to increase its top speed.
TBCC
This chart shows the range of possible engine operating modes that are possible with this type of propulsion system.
As shown, this vehicle takes-off and accelerates to approximately Mach 2.5 using the turbines and the ramjet. One of the options with this propulsion system is what fuel to use for the ramjet during this portion of the flight profile (hydrocarbon or liquid hydrogen) and what velocity is optimum for the transition to liquid hydrogen if hydrocarbon is used.
At Mach 2.5 the turbofans are shutdown and the vehicle continues to accelerate using only the ramjet-scramjet.
Somewhere between Mach 8 and Mach 10 the rocket motors are ignited with liquid oxygen and liquid hydrogen. The vehicle then continues to accelerate using both the scramjet and the rocket to somewhere between Mach 14 and Mach 18 when both are shutdown. The vehicle will then coast up to its staging altitude of 150 km where the payload with expendable upper stage is released.
Rocket Based Combined Cycle (RBCC)
This chart shows the basic arrangement of a Rocket Based Combined Cycle propulsion system. It consists of a ramjet-scramjet and a ducted rocket. The main difference between this propulsion system and the previously mentioned propulsion systems is that this one does not have any turbine engines.
RBCC
This chart shows some of the propulsion and propellant options, along with their respective speed ranges, for a pure Rocket Based Combined Cycle propulsion system.
It takes-off using the liquid oxygen / hydrocarbon ducted rocket only. Additional thrust is generated by the ramjet starting at approximately Mach 0.8.
At approximately Mach 2.2 the ducted rocket is shutdown and the vehicle continues to accelerate using only the ramjet-scramjet.
Somewhere between Mach 8 and Mach 10 the ducted rocket motors are re-ignited with liquid oxygen and liquid hydrogen. The vehicle then continues to accelerate using both the scramjet and the ducted rocket to somewhere between Mach 14 and Mach 18 when both are shutdown. The vehicle will then coast up to its staging altitude of 150 km where the payload with expendable upper stage is released.
Based on our sizing and cost studies to date, the boost phase performance of an RBCC powered HTOHL launch vehicle is very similar to the TRCC powered vehicle.
Unfortunately the RBCC's operational limitations;
1. no subsonic self-ferry capability,
2. extreme engine noise during take-off,
3. and its need for non-standard propellants during ferry operations, make it very unattractive for remote operations from existing commercial airports.
Since ease of operation at existing airports is one of the main reasons for considering the HTOHL configuration in the first place, and since we are getting similar performance from the TRCC powered vehicles without any of these drawbacks, it appears unlikely that the pure RBCC propulsion system will be competitive in a HTOHL vehicle. However, based on the trends we are seeing, we do expect the VTO RBCC powered vehicle to have an advantage over the turbine systems when operating from over water launch sites.
The Earth Orbiting Elevator
As part of our investigation into the evolutionary growth potentials of this vehicle concept, we have examined it for both SSTO flight and having it serve as a single stage sub-orbital launch vehicle that flies to the bottom end of an Earth Orbiting Elevator. So far, our analysis shows the elevator concept to be the most economical of the two growth options by a large margin.
The Earth Orbiting Elevator is based on the elevator-into-space idea that was described by Arthur C. Clark in his book, The Fountains of Paradise. That particular concept consisted of hanging a cable from geostationary orbit down to the surface of the Earth and moving people and freight into space on an elevator which would ascend and descend along the cable.
The elevator concept being considered here is an intermediate version of that idea that can be built with existing materials. The concept works by starting from a much lower altitude orbit and hanging the cable down to just above the Earth's atmosphere. The length of the lower half of the cable is selected such that the launch vehicle can deliver a much larger useful payload to the lower end of the elevator without the need for an expendable upper stage. This results in an estimated user cost to Earth obit of only $550 per pound. This cost estimate includes debt service, profit and recurring costs, for both the Next Generation Reusable Launch Vehicle and the Earth Orbiting Elevator, at a flight rate of 72 flights per year.
As the market for launch services continues to grow the elevator system has the potential of growing with it. Increasing the length of the cable increases the size of the useful payload that can be delivered with each flight of the launch vehicle. Increasing the size of the elevator system's ion propulsion system increases the number of flights per year that the elevator can accept. Both of these changes increase the total payload capacity of the system and as a result reduce the user cost for getting into Earth orbit. Eventually these incremental improvements will lead to user costs of less than $100 per pound to Earth orbit.
The length of the upper half of the cable is chosen so that the endpoint is traveling at slightly less than escape velocity for its altitude. This is done so that satellites and spacecraft bound for higher orbit, the Moon and beyond, can do so with only minimal use of onboard propellants. This will have a significant impact on reducing the cost of getting to these higher orbits.