Methane/oxygen rocket engine with specific impulse enhancement by hot helium infusion

An apparatus and method to enhance the performance of rockets engines which utilize liquid methane/oxygen propellants by injecting optimized amounts of pressurized hot helium gas into the combustion chamber with the propellants. In one embodiment, the pressurized helium gas is stored at low temperatures near those of the cryogenic propellants and is used for regenerative cooling of the combustion chamber and nozzle during rocket operation in order to raise the temperature of the helium gas before being injected into the combustion chamber.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to rocket engines, and more particularly to specific impulse enhancement together with propellant pumping without turbopumps in rocket engines.

2. Description of the Related Art

Liquid propellant rocket engines (LPREs) are an essential vehicle component for placing payloads into earth orbit. Currently, liquid methane/liquid oxygen LPREs are of interest because, besides being viewed as ecologically friendly propellants, they have more adequate tankage properties. However, this propellant combination currently has a lower performance than existing liquid hydrogen/liquid oxygen LRPEs.

Specific impulse is a function of the ratio of the engine's thrust to the propellant mass flow rate. A known technique for maximizing the specific impulse (Isp) of an LPRE is to operate fuel rich because hydrocarbon fuels contain a lot of hydrogen. In the case of methane/oxygen combustion, this method shows a very modest increase in the specific impulse. While a separate hydrogen injection source may be used to lower the combustor's gas mixture's molecular mass, liquid hydrogen has several undesirable tanking properties—extremely low temperatures and densities—and, the hydrogen molecule (H2) dissociates and chemically reacts in the hot combustion chamber environment. Hydrogen gas is also not suitable for propellant pumping.

SUMMARY OF THE INVENTION

In accordance with one embodiment, a rocket engine having specific impulse enhancement by hot helium infusion includes: a pressurized liquid methane propellant connected to at least a first injection port of a combustion chamber of the rocket engine; the first injection port configured to allow injection of the liquid methane propellant into the combustion chamber; a pressurized liquid oxygen propellant connected to at least a second injection port of the combustion chamber; the second injection port configured to allow injection of the liquid oxygen propellant into the combustion chamber; a pressurized helium gas connected to a heat exchanger system surrounding the combustion chamber and a nozzle of the rocket engine; the heat exchanger system connected to a helium infusion control valve, the heat exchanger system configured to heat the helium gas; the helium infusion control valve connected to at least a third injection port of the combustion chamber, the helium infusion control valve configured to control the flow of the heated helium gas to the third injection port; and, the third injection port configured to allow injection of the heated helium gas into the combustion chamber; wherein, during operation of the rocket engine, the liquid methane propellant and the liquid oxygen propellants are injected into the combustion chamber, and the helium gas is heated in the heat exchanger system prior to injection into the combustion chamber such that upon injection of the heated helium gas into the combustion chamber where the liquid methane propellant and the liquid oxygen propellant are burning, the specific impulse of the rocket engine's exhaust gases is enhanced. In a further embodiment, the pressurized helium gas is further connected to a liquid methane storage tank containing the liquid methane propellant and to a liquid oxygen storage tank containing the liquid oxygen propellant, and the helium gas is used as the sole pressurizing agent for the liquid methane propellant and liquid oxygen propellant.

In accordance with another embodiment, a method for specific impulse enhancement by hot helium infusion of a rocket engine having a high pressure helium gas stored in a helium storage tank, a liquid methane propellant stored in a liquid methane storage tank, and a liquid oxygen propellant stored in a liquid oxygen storage tank, includes: opening a helium control valve to allow the high pressure helium gas to pressurize and pump the liquid methane propellant from the liquid methane storage tank and the liquid oxygen propellant from the liquid oxygen storage tank to a combustion chamber of the rocket engine, and to allow the high pressure helium gas to flow to a separate closed specific impulse control valve; activating one or more rocket control systems that control thrust of the rocket engine; controlling the injection of the liquid methane propellant through at least a first injection port into the combustion chamber; controlling the injection of the liquid oxygen propellant through at least a second injection port into the combustion chamber; igniting the injected liquid methane propellant and the liquid oxygen propellant in the combustion chamber; upon a determination that combustion has started in the combustion chamber and is self-sustaining, opening the specific impulse control valve and regulating the flow of the high pressure helium gas into one or more cooling conduits of a heat exchanger system surrounding the combustion chamber and a nozzle of the rocket engine to heat the helium gas; controlling the flow of the hot helium gas from the heat exchanger system to at least a third injection port to the combustion chamber by a helium infusion control valve; injecting the hot helium gas through the third injection port into the combustion chamber with the injected liquid methane propellant and the injected liquid oxygen propellant; and regulating the flow of the hot helium gas into the combustion chamber until a desired diluent mass ratio is achieved to enhance the specific impulse of the rocket engine

Embodiments in accordance with the invention are best understood by reference to the following detailed description when read in conjunction with the accompanying drawings.

Embodiments in accordance with the invention are further described herein with reference to the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Helium (He) has the lowest molecular mass after hydrogen. Compared to hydrogen, helium is inert and its compressed gas has well-known tanking/mass properties and is also a good regenerative coolant for the nozzle and combustion chamber of a rocket engine. Helium does not dissolve in propellant liquids such as oxygen or hydrocarbons and has been used to pump both fuel and oxidizer from their tanks in small rocket engines. Helium is viewed as non-toxic and does not dissociate at any temperature thereby avoiding the combustion-enthalpy decreases that hydrogen and other molecular species introduce at high chamber temperatures. While helium has been routinely used as the sole propellant in Vernier rocket engines for auxiliary propulsion and while compressed helium tanks are used for propellant pumping in relatively small rockets such as the Space Shuttle's Orbital Maneuvering System (OMS), helium has not been used inside medium to large size second or third stage rocket engines for propellant pumping in any engine toward increases for payload-capability enhancements.

Embodiments in accordance with the invention enhance the performance of rocket engines which utilize liquid methane (CH4)/liquid oxygen (O2) propellants, such as second-stage launch rockets, by adding hot helium gas as a diluent to the propellants to achieve potentially high specific-impulse (Isp) enhancement capabilities. Before being injected into the combustion chamber with the propellants, the helium gas is passed through a regenerative cooling system of the rocket nozzle in order to raise the storage temperature of the helium gas. This translates into low gas weights when added as a diluent and potentially high specific-impulse (Isp) enhancement. The Ispdepends on T1/M, where T1is the gas temperature at the entrance to the rocket nozzle and M is the molecular mass of the flow. In small thruster embodiments, utilization of helium gas injection circumvents the need for any turbopumps. At current prices, increases in vehicle payload capability of the rocket stage enhanced in accordance with the invention and hardware simplifications, together with a greater reliability, is projected to more than offset any costs and hardware of the added helium gas due to the larger rocket unit's ability to store a greater amount of helium gas.

Referring initially toFIG. 1, in one embodiment, a liquid CH4/liquid O2rocket engine100with specific impulse enhancement by hot helium infusion includes a high pressure helium gas storage tank102which stores a helium gas104at high pressure and low temperature. High pressure helium gas104is routed from helium gas storage tank102via a helium control valve106through feed lines to each of a liquid methane storage tank112containing a liquid methane propellant114and to a liquid oxygen storage tank116containing a liquid oxygen propellant118for use in pressurizing tanks112/116. High pressure helium gas104is also routed via helium control valve106through a feed line to a specific impulse control valve120which controls the introduction of helium gas104to a helium injection and pressure relief valve122for use in specific impulse enhancement of rocket engine100following pressurization of tanks112/116. In one embodiment, a control valve108controls the flow of helium gas104to liquid methane storage tank112, and a control valve110controls the flow of helium gas104to liquid oxygen storage tank116. In one embodiment, liquid methane propellant114and liquid oxygen propellant118are cryogenically stored. To manage the heat transfer during storage and pumping, it is advantageous to store helium gas104at or near 100K, between the boiling points of oxygen (90K) and methane (112K). This should also aid with the rocket vehicle's tank arrangements (concentric or other nesting).

Liquid methane propellant114and liquid oxygen propellant118are respectively routed from liquid methane storage tank112and liquid oxygen storage tank116via respective feed lines124and126into one or more injectors feeding a combustion chamber130with production of resultant thrust from rocket engine100exiting a communicating nozzle132.

For rocket engines below about 222 kN thrust operating for only a few minutes, helium gas104can be used in the total or partial pumping of both propellants, i.e., liquid methane propellant114and liquid oxygen propellant118, depending on the overall injection mass flow rates, as, typically, more robust helium storage would be available. This arrangement is simpler overall, faster and more reliable than a LPRE working with turbopumps. Further, as helium is also routinely used for purging lines after propellant use, and stored as a gas, there can be no propellant sloshing issues.

In one embodiment, initially, specific impulse control valve120is closed to prevent the flow of helium gas104to helium injection and pressure relief valve122until liquid methane storage tank112and liquid oxygen storage tank116are pressurized to a specified pressure to allow sufficient flow and injection of the propellants114,118into combustion chamber130. When liquid methane storage tank112and liquid oxygen storage tank116are fully pressurized, specific impulse control valve120is opened to allow helium gas104to flow via the feed line to helium injection and pressure relief valve122. Helium injection and pressure relief valve122then controls the flow of helium gas104into feed line128from where helium gas104is heated and utilized for specific impulse enhancement of rocket engine100as further described with reference toFIG. 2. It can be understood by those of skill in the art that fewer, more or different control valves and flow line configurations, together with additional switches, regulators, transducers, and filters, can be utilized to control the flow of helium gas104, liquid methane114, and liquid oxygen118dependent on the specific control systems (not shown) implemented in rocket engine100.

FIG. 2illustrates specific impulse enhancement of rocket engine100in accordance with one embodiment of the invention. As earlier described, after pressurization by helium gas104, liquid methane propellant114and liquid oxygen propellant118flow via respective feedlines124,126to one or more injection ports202(a first injection port),204(a second injection port) where propellants114,118are injected into combustion chamber130. In some embodiments, one or more propellant control valves212can be utilized allow control and shutoff of the injection of liquid methane propellant114and liquid oxygen propellant118into combustion chamber130.

Helium gas104input from helium injection and pressure relief valve122flows via feed line128into a heat exchanger system206surrounding combustion chamber130and nozzle132. As helium gas104passes through the cooling conduits of heat exchanger system206, the chamber walls of combustion chamber130and nozzle132are regeneratively cooled and helium gas104is heated. Dependent on the injection rates of helium gas104, which can vary with application, other cooling mechanisms may also utilized separately or in conjunction with heat exchanger system206. For example, in some embodiments, other cooling mechanisms may utilize one or more of the propellants as regenerative coolants separately or in conjunction with heat exchanger system206dependent on the rocket engine size and overall engine cooling needs. The now hot helium gas104leaves heat exchanger system206and flows into a feed line208where the injection of hot helium gas104into combustion chamber130through an injection port214(a third injection port) is controlled by a helium infusion control valve210based on inputs from a helium infusion control system (not shown) in communication with helium infusion control valve210, and which may be part of the control systems to rocket engine100. When injected into combustion chamber130, hot helium gas104mixes with the burning liquid methane propellant114and liquid oxygen propellant118in combustion chamber130with a resultant increase in the specific impulse of rocket engine100. In one embodiment, injections ports202,204,214are configured such that hot helium gas104is injected at the back plate of combustion chamber130, where liquid methane propellant114and liquid oxygen propellant118are introduced, at or near the centerline of combustion chamber130for good mixing. The flows of helium gas104, liquid methane propellant114and liquid oxygen propellant118and thrust levels are then allowed to increase under control of the control systems to rocket engine100until they reach full-rated values of combustion chamber pressure and thrust. In some embodiments, the control systems to rocket engine100may be used to prevent exceeding limits of the propellant mixture ratio or rates of increase during transient or exceeding material temperatures.

FIG. 3illustrates a method300for specific impulse enhancement of rocket engine100by hot helium infusion in accordance with one embodiment of the invention. The starting and stopping of a rocket engine requires various critical timings and valve sequencing to accommodate transient characteristics and are typically controlled by the control system(s) to the rocket engine. Pressure-fed propellant systems have smaller start-delay times and are lighter and more reliable than turbopumps. Prior to start of method300, a pressurization system of rocket engine100has to be activated and any ullage volume in helium gas storage tank102, liquid methane storage tank112and liquid oxygen storage tank116pressurized. With cryogenic propellants, such liquid methane propellant114and liquid oxygen propellant118, the piping system, i.e., the feed lines, needs to be cooled to cryogenic temperatures to prevent vapor pockets. Purging the flow system is needed before the fuel flow control valves are opened and combustion initiated. For pressurized feed systems, the initial flow of each propellant is often considerably higher than the rated flow at full thrust because the pressure differential (ptank−pchamber) is much higher, with the combustion chamber pressure initially being at its lowest value; these higher flows may lead to an initial surge of combustion chamber pressure. To avoid any mishaps caused by propellant accumulation in the combustion chamber the rocket engine must be designed to slowly open the main propellant control valves or to build a throttling mechanism into them.

Referring now toFIG. 3and method300, initially, in operation302, helium storage tank102is filled with high pressure helium gas104, liquid methane storage tank112is filled with liquid methane propellant114, and liquid oxygen storage tank116is filled with liquid oxygen propellant118.

In operation304, helium control valve106is opened and high-pressure helium gas104is fed into liquid methane storage tank112and liquid oxygen storage tank116for tank pressurization, and the flow of liquid methane propellant114and liquid oxygen propellant118for injection to combustion chamber130is initiated. Additionally, high pressure helium gas104flows to specific impulse control valve120which is initially closed.

In operation306, the various control systems to rocket engine100which control thrust, such as control systems that control propellant mixture ratio and other parameters, are activated.

In operation308, injection of liquid methane propellant114and liquid oxygen propellant118to combustion chamber130through injection ports202,204are regulated based on thrust control system inputs to rocket engine100and are ignited in combustion chamber130with a suitable igniter.

In operation310, after a sufficient temperature increase is sensed in combustion chamber130by control system sensors to rocket engine100, specific impulse control valve120is opened and helium gas104flows to helium injection and pressure relief valve122which meters flow of helium gas104via feed line128into the cooling conduits of heat exchanger system206. The flow of helium gas104into the cooling conduits surrounding combustion chamber130and nozzle132should be opened only shortly after control system sensors to rocket engine100indicate that combustion has started and is self-sustaining. The flow of helium gas104needs to be sequenced to provide the required coolant flow to the cooling conduits in both combustion chamber130and nozzle132after propellant ignition and throughout main stages of operation.

In operation312, hot helium gas104from heat exchanger system206flows into feed line208for injection into combustion chamber130through injection port214as regulated by helium infusion control valve210. In combustion chamber130, injected hot helium gas104is mixed with liquid methane propellant114and liquid oxygen propellant118. Hot helium gas104can be injected into combustion chamber130using either a single jet or a showerhead arrangement dependent on the dimension of combustion chamber130and the mass fraction flow rates of helium gas104. With controlled introduction of hot helium gas104by helium infusion control valve210, a relatively small flow rate of hot helium gas104is not expected to induce combustion instabilities or negatively affect the ongoing chemical process.

In operation314, the flow of hot helium gas104is regulated and brought up to a desired mass ratio to enhance the specific impulse of rocket engine100. As earlier described, the flows of helium gas104, liquid methane propellant114and liquid oxygen propellant118and thrust levels are then allowed to increase until they reach full-rated values of combustion chamber pressure and thrust.

Embodiments in accordance with the invention can be utilized in various upper stages of launch rocket engines where the use of cryogenically stored CH4/O2is deemed advantageous over H2/O2because of tanking and other advantages such as reduced storage volume. Unlike hydrogen, there are no dissociations or chemical reactions with helium. As further discussed with reference toFIG. 4, varying degrees of specific impulse enhancement can be achieved utilizing helium injection with CH4/O2propellants.FIG. 4illustrates a graph of specific impulse (Isp) in sec of an O2/CH4mixture versus helium diluent mass ratio (x) with an O2/CH4mixture ratio r=3.6 (stoichiometric=4.0), T1=3500 K, p1=4.36 MPa (combustor pressure), ϵ=385 (nozzle area ratio), vacuum exhaust, liquid storage of CH4/O2propellants, and shifting equilibrium flow (using NASA's CEA code). The mass ratio of helium diluent (x) is calculated as

x≡m.Hem.p=mHemp=nHe⁢MHenp⁢Mp
where x is the mass ratio, the subscript “p” represents reaction products, m represents mass, and n represents the number of mols, and M represents the flow's molecular mass.

After cooling the combustor (i.e., combustion chamber)/nozzle, helium is injected at 1000 K at the combustor entrance. While the mixing of the helium gas does cool the combustion gases while decreasing the molecular mass of the exhaust mixture (because helium gas can only be injected at the maximum operating temperature of the hardware, e.g., at the nozzle throat, usually 1000 K for steels), as seen inFIG. 4, there is an attractive region of specific impulse enhancement during which the flow mixture temperature divided to molecular mass value remains advantageous. The maximum specific impulse (Isp) in sec increase occurs at a helium mass ratio of 0.45 which represents a rather substantial helium mass flow. While a potential 12% increase of payload ratio is theoretically possible, such increases would not be desirable or cost effective beyond x=0.20 (the mass ratio of helium to total propellant).

Additionally, the use of hot helium infusion for specific impulse enhancement is expected to significantly reduce payload costs.FIG. 5illustrates a graph of payload mass-fraction (mpl/m0) as a function of Δu/g0Ispfor relevant propellant mass-fractions, ζi(less than 1.0 reflecting tankage masses, see Sutton, G. P. and Biblarz, O., “Rocket Propulsion Elements”, 9thEd., Wiley (2017)). Using an existing H2/O2comparable single upper stage vehicle with Δu=3,400 m/sec, the red line inFIG. 5demonstrates an estimated (ζi, Isp)-change from (0.88, 465 sec) to (0.95, 377 sec) with the decrease of payload fraction, followed by anticipated increases of the Ispand mpl/m0resulting from He-injection to (ζi, Isp)-values of (0.94, 410 sec) at x=0.1 and (0.93, 422 sec) at x=0.15 shown by the green line.

As shown inFIG. 5, any resulting higher propellant-mass fraction (ζi) can only partially offset their lower maximum Isp. because of a lower initial T1/M. Also, using data fromFIG. 4, the specific impulse of CH4/O2can be enhanced with appropriately optimized hot helium gas mass injection amounts into upper-stage rocket engines to nearly match the payload mass fraction (mpl/m0) of the original H2/O2fueled rocket. Additionally, because of the more robust helium storage required, propellant pumping with compressed helium gas in accordance with embodiments of the invention can be utilized with medium sized second or third stage rockets that now use turbopumps, such as the Russian RD-120 which delivered 85 kN vacuum thrust and the US's RL-10B-2 engine at 110 kN thrust (used as input data forFIG. 4).

As an example, in order to arrive at representative quantitative values, consider the second stage of a two-stage rocket that is required to propel a payload (mpl) to Δu=3400 m/sec in a gravitationless, zero drag environment. The thrust and main hardware elements of the rocket engine are to be similar to the RL-10-B2 H2/O2engine, but using liquid methane (CH4) instead of liquid hydrogen and with all propellants pumped to steady flow by the compressed helium without any turbopumps. The example considered here keeps m0, the total of the second stage mass plus payload, fixed which allows costs comparisons because this variable represents the payload for the first stage. All calculations reflect ideal behavior and quality of tankage design values (e.g., ζi) are estimates based on data from Humble, R. W., Henry, G. N., and Larson. W. J., “Space Propulsion Analysis and Design”, Appendix C, McGraw-Hill Space Technology Series, New York (1995). For the RL 10B-2, F=110 kN, p1=4.36 MPa, ϵ=385, and take mpl=3500 kg at x=0.0. The CH4/O2has a mixture ratio r=3.6 with inert gas pumping of propellants, before the hot-helium injection, T1=3500 K and Isp=377 sec (values of Ispfor x>0 are fromFIG. 4and for mpl/m0fromFIG. 5). The total propellant mass (mp) and the propulsion time Δt are given by:

Solving for mpand Δt in constant flow rates, Table 1 below summarizes results for x=0.0, 0.1 and 0.15.

In Table 1, a single asterisk (*) represents helium gaseous storage at 100 K and 69 MPa (perfect gas behavior), a pressure higher than the chamber operating pressure, minimum values are given not accounting for helium uses other than injection. Two asterisks (**) represent estimated income at $5,000/lbm of payload to LEO for entire vehicle (lowest present value)−494 kg for liquid helium (presently at $40/kg to $120/kg) would cost from $20,000 to $60,000 at x=0.1 (less than 0.1% of payload gain).

Noticeable costs of using helium do arise from both added hardware and from the helium propellant itself. Being scarce on Earth, currently liquid helium costs about 30 times more than liquid oxygen, in 2017 fluctuating between $5 and $15/liter, but methane is relatively inexpensive. All these costs are expected to be more than compensated for by enhancements of the rocket's payload capabilities and by potential hardware and starting simplifications (i.e., no turbopumps) utilizing embodiments in accordance with the invention.

While helium-3 would be more attractive, it is extremely rare on Earth's surface and in 2016 large deposits of helium-4 were found in Tanzania. Helium-4 is a liquid at 4 K and one atmosphere, colder than liquid hydrogen, but somewhat denser at 125 kg/m3, and all the many precautions needed for liquid hydrogen usage apply to liquid helium. However, stored in the rocket vehicle as a compressed gas at about 100 K, near the liquid temperature of the liquid oxygen and CH4propellants, and 69 MPa, it has a density of about 332 kg/m3which could make any helium-tank volumes smaller than those for oxygen.

With liquid helium storage, turbopumps would be necessary; however, gaseous high-pressure storage would permit propellant pumping without turbopumps. Helium high-pressure tanks have needed to be thick walled and massive but newer designs using aluminum liners with outer layers of strong carbon fibers or other modern composite arrangements can be lighter and thus more attractive. Also, non-spherical tanks would package better into rocket vehicles. Noticeable enhancements of the propellant mass fraction (ζi) over turbopumped systems should be expected with embodiments in accordance with the invention.

While all helium uses described herein could also work with other hydrocarbon fuels, methane is currently considered to be a practical propellant and more ecologically friendly than RP-1 fuel. However, any unburned methane exhausted to the atmosphere captures more infrared solar radiation than CO2and although short lived is not desirable. At the mixture ratios envisioned herein, there is little or no methane in the exhaust gases, but as with other hydrocarbon fuels, including CO2, some noticeable amounts of CO may be generated.

This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification or not, may be implemented by one of skill in the art in view of this disclosure.