guest blogger john hare
The potential problems I see with developing the original TAN concept could be quite wrong. If I am, I owe several people apologies upon proof. Whether I am or not though, the concept itself is obviously valuable. I am not dismissing the people that disagree with my take, just throwing more concepts out using the basic principles of TAN and ATRs.
When looking at the force accounting on the TAN concept, you notice that much of the performance is from the reduction in over expansion losses. Anything that introduces more gas in the over expanded nozzle should see similar improvements. Whatever method is used to introduce this gas will have and Isp and thrust/weight improvement when all of the net forces are accounted for. So if the original TAN ‘booster section would have had an actual Isp of 150 with a T/W of 200(just guessing, substitute your own if you like), net Isp was 260 with total T/W of well over 300. An ATR in place of the TAN units should improve from an Isp of 1,000 and T/W of 25 to an effective net Isp of well over 1,500 and an effective T/W approaching 40.
An expander deflector nozzle is very short and uses a center body to control the flow. At high back pressures it is supposed to direct all the exhaust against the nozzle walls with the center body creating an area in the center at ambient pressure. At low back pressures, the center fills with the expanding exhaust to increase the effective expansion ratio. I propose placing an ATR in place of the center body of the nozzle. This should be a simpler arrangement than the original TAN locations even with a pure rocket system as it allows more volume and residence time for the TAN propellants .
The gas generator cycle is easier on the equipment than the staged combustion cycles. It is not necessary to pump fluids to thousands of psi higher than the combustion chamber pressure to have enough pressure drop to drive the turbines. The down side of the gas generator cycle is that it doesn’t get very good thrust from the turbine exhaust gas which hurts Isp compared to the staged combustion. Since the gas generator exhaust doesn’t give very effective thrust compared to the main exhaust, developers must try for the most efficient pump system possible to minimize the turbine drive gas requirements.
Suppose you instead go for a possibly less efficient system that is cheaper, and use the turbine exhaust from the gas generator to drive an ATR engine. Then the gas generator exhaust products become part of a much higher Isp engine instead of a liability. With the fuel rich turbine exhaust driving the ATR turbines and burning with air in the afterburner, it becomes desirable to use a much higher chamber pressure on the rocket engines even though it increases the turbine drive gas requirements. If 10% of the total propellant usage goes to the pump drives, that is OK because that 10% is operating a system with an Isp three to five times that of the primary rocket chambers.
Using multiple rocket chambers would seem to be easier than the normal expander deflector nozzle arrangement. It allows incremental development and throttling for either attitude control or acceleration limits. As individual chambers are shut down later in the flight, net expansion ratio improves again.
Tip turbines to drive the ATR compressor as used in the Japanese ATREX are lighter and more compact that the standard arrangements, and makes the plumbing much easier in this application. The downside is that they are not as highly developed as conventional turbines, and may be less efficient.
At launch, all rocket engines and the ATR are at maximum throttle with the ATR fulfilling the TAN function. When the ATR is shut down at altitude, inlet doors must be closed to hold the pressure of the rocket exhaust in. The gas generator exhaust then keeps the ATR area cool with no requirement for additional regenerative plumbing. If possible, LOX can be injected into the ATR afterburner area to get more performance from the gas generator exhaust. In landing mode, the ATR is the only engine operating with the nozzle operation in dual bell mode to assure clean flow separation from the main nozzle wall.
It seems possible that the rocket engines would be operated at 3,000 psi to get an Isp of over 300 at sea level. The ATR would use the gas generator exhaust to get an Isp of well over 1,000, with net being perhaps 1,500. My guess is system net Isp of 400 or so from the ground. At ATR shut down, the gas generator exhaust again becomes a liability, but if it can be after-burned with LOX, it seems possible to get a vacuum Isp of 250-300 from the gas generator products, with main chamber Isp of 350 or more. My guess is net Isp of over 325 vacuum. When in landing mode, the ATR Isp will likely drop under 1,000 with poor inlet possibilities.
With gas generator engines at high pressure, T/W of 125 is possible with high development effort with T/W of 100 attainable by Newspace under these conditions. The ATR, optimised for T/W more than Isp, and sharing the expansion nozzle with the rockets, a T/W of 35 seems reasonable. A system T/W of around 60 seems reasonable.