Journal of Marine Science and Engineering

Article Theoretical Performance Evaluation of a Marine Solid Water-Breathing Propulsor

Nachum E. Eisen * and Alon Gany Faculty of Aerospace Engineering, Technion—Israel institute of Technology, Haifa 3200003, Israel; [email protected] * Correspondence: [email protected]

 Received: 24 November 2019; Accepted: 18 December 2019; Published: 20 December 2019 

Abstract: This work analyzes and presents theoretical performance of a marine water-breathing ramjet propulsor. A conceptual scheme of the motor is shown, the equation of is presented, and the dependence on cruise and depth are discussed. Different propellant compositions, representing a wide variety of formulations suitable for propelling a water-breathing ramjet, are investigated. The theoretical results reveal that the specific of a water-breathing ramjet can increase by as much as 30% compared to a standard , when using a conventional hydroxyl terminated polybutadiene (HTPB)-ammonium perchlorate (AP) propellant, which does not react chemically with the water. When employing a water-reactive propellant containing metal particles such as magnesium or aluminum, the specific impulse may be more than doubled. The thrust coefficient of the propulsor was computed at different cruise and depths and was found to be greater than the predictable drag even at significant depth.

Keywords: underwater propulsion; water-breathing ramjet; marine ramjet; water ducted rocket

1. Introduction The search for underwater high-speed cruising reveals certain high-thrust marine propulsion concepts which are parallel in principle to their aeronautical counterparts [1]. Rocket can propel high-speed underwater vehicles just like aeronautical vehicles. Independent of speed and ambience medium, rocket engines provide very high thrust and do not need air for their operation, hence, they can suite underwater vehicles. However, their specific fuel (propellant) consumption is very high and, thus, they enable only a limited range. Therefore, propulsion principles utilizing the surrounding water as a working fluid as well as a chemical reactant (usually oxidizer), have been considered [2–8]. One of the interesting concepts for propelling high-speed underwater vehicles is the solid propellant water-breathing ramjet [4–8]. It is a powerful propulsion means, independent of atmospheric air, hence, it can operate fully and deeply underwater. The basic principle of the marine water-breathing ramjet is to utilize the high dynamic (ram) pressure, resulting from the high-speed motion, in order to ingest water into the combustion chamber of the motor, thus increasing the thrust and energetic performance of the propulsor. This work focuses on theoretical prediction of the performance of a water-breathing underwater solid propellant ramjet motor. The design of an underwater solid propellant water-breathing ramjet is similar to the design of an aeronautical ducted rocket. Water enters the motor through an inlet located in the front of the vehicle and is sprayed through a porous wall into the combustion chamber. Due to the high temperature in the combustion chamber, the incoming water boils and then mixes with the combustion products, forming a high-speed exhaust jet that provides the thrust necessary to propel the vehicle. A conceptual illustration of a solid propellant water-breathing ramjet is presented in Figure1.

J. Mar. Sci. Eng. 2020, 8, 8; doi:10.3390/jmse8010008 www.mdpi.com/journal/jmse J. Mar. Sci. Eng. 2020, 8, 8 2 of 11 theJ. Mar. vehicle. Sci. Eng. A2020 conceptual, 8, 8 illustration of a solid propellant water-breathing ramjet is presented2 of in 11 Figure 1.

FigureFigure 1. 1. ConceptualConceptual illustration illustration of of a a marine marine solid solid propellant propellant water-breathing water-breathing ramjet. ramjet.

SimilarlySimilarly to solidsolid rocket rocket motors motors (SRMs), (SRMs), the the solid solid propellant propellant grain grain can provide can provide high thrust high without thrust withoutthe addition the addition of water, of aswater, well. as Nevertheless, well. Nevertheless, the mere the additionmere addition of water, of water, entering entering through through an inlet an inletdue due to the to vehiclethe vehicle motion, motion, and and its furtherits further conversion conversion into into steam steam because because of of the the high high heat heat release release of ofthe the burning burning solid solid propellant, propellant, increases increases the the thrust thrust andand thethe specificspecific impulse,impulse, even with no no chemical chemical interaction.interaction. Greater Greater performance performance improvement improvement ca cann be be obtained obtained when when employing employing water-reactive water-reactive propellantpropellant compositions, compositions, which which utilize utilize the the incoming incoming wa waterter as as an an oxidizer oxidizer to to increase increase heat heat release release in in additionaddition to to using using it it as as a a source source for for steam steam which which increases increases the the mass mass flow flow rate rate of of the the exhaust exhaust jet. jet. Certain Certain metalsmetals are knownknown toto reactreact very very exothermically exothermically with with water water and and may may be considered be considered for inclusion for inclusion as water as waterreactive reactive ingredients ingredients in . in propellants. Typically, Typicall the reactiony, the reaction of metals of with metals water with generates water generates hydrogenbesides metal besides oxides. metal Table oxides.1 presents Table 1 theoretical presents th physicaleoretical and physical thermochemical and thermochemical properties ofproperties selected ofmetals. selected While metals. the While gravimetric the gravimetric heat of reaction heat of is reaction related tois therelated mass to of the fuel mass (propellant) of fuel (propellant) carried on carriedboard, theon board, volumetric the volumetric heat of reaction heat reflectsof reaction the energyreflects perthe unitenergy volume per unit and volume is mainly and significant is mainly in significantvolume-limited in volume-limited systems. systems.

TableTable 1. MaximumMaximum theoretical gravimetricgravimetric andand volumetric volumetric heat heat of of reaction reaction and and hydrogen hydrogen generation generation for forselected selected metals metals with with liquid liquid water, water, listed listed in the in order the order of their of volumetric their volumetric heat of heat reaction of reaction (data adapted (data adaptedfrom [9]). from [9]).

Gravimetric Heat of Volumetric Heat of Specific VolumetricSpecific H Density Specific H Mole 2 Fuel GravimetricReaction VolumetricReaction Specific2 𝑯𝟐 Mole Production [g/cm3] Production [mol/g] Volumetric 𝑯𝟐 Density Heat[kJ/g] of [kJHeat/cm3] of Mole [mol/cm3] Fuel 𝟑 Mole Be𝒈/𝒄𝒎 1.85 Reaction 36.1 Reaction 66.7 Production 0.111 0.21 Production 𝒌𝑱/𝒈 𝒌𝑱/𝒄𝒎𝟑 𝒎𝒐𝒍/𝒈 B 2.35 18.4 43.2 0.139𝒎𝒐𝒍/𝒄𝒎 0.34 𝟑 BeAl 2.701.85 15.2 36.1 40.9 66.7 0.055 0.111 0.15 0.21 BZr 6.492.35 5.7 18.4 37.2 43.2 0.022 0.139 0.14 0.34 AlMg 1.742.70 13.0 15.2 22.6 40.9 0.041 0.055 0.072 0.15 ZrLi 0.546.49 25.5 5.7 13.8 37.2 0.072 0.022 0.039 0.14 MgNa 0.971.74 2.8 13.0 2.7 22.6 0.022 0.041 0.021 0.072 Li 0.54 25.5 13.8 0.072 0.039 NaIt is apparent0.97 from Table1 2.8 that beryllium reacts 2.7 extremely exothermically 0.022 with water 0.021 both in terms of and volume, yet, beryllium and its products are notoriously toxic and therefore are ruledIt is apparent out. Second from to Table beryllium 1 that isberyllium boron, which reacts demonstrates extremely exothermically a promising theoreticalwith water heat both and in termsgas (i.e., of weight hydrogen) and volume, release. yet, In practice,beryllium it and is hard its products to obtain are high notoriously combustion toxic e ffiandciency therefore of boron are ruled(though out. Rosenband Second to etberyllium al. [10] haveis boron, demonstrated which demonstrates combustion a ofpromising boron in theoretical steam), and heat in additionand gas (i.e.,boron hydrogen) is a relatively release. expensive In practice, ingredient. it is hard Hence, to obtain for practical high combustion applications, efficiency aluminum of boron is commonly (though Rosenbandconsidered. et Aluminum al. [10] have reacts demonstrated with water tocombustion form aluminum of boron oxide in steam), (alumina) and or in aluminum addition boron hydroxide, is a relativelyand hydrogen, expensive while releasingingredient. asignificant Hence, for amount practical of heat, applications, as presented aluminum in Equation is (2). commonly However, considered.aluminum does Aluminum not combust reacts readily with as water a thin layerto form of aluminum aluminum oxide oxide naturally (alumina) forms onor thealuminum exposed hydroxide,surfaces of and the aluminumhydrogen, particleswhile releasing and passivates a significant the metal amount substrate of heat, to as inhibit presented oxidation in Equation progression. (2). However,Different approachesaluminum does for overcoming not combust the readily alumina as a coatingthin layer problem of aluminum are discussed oxide naturally extensively forms in theon theliterature exposed [11 surfaces–14]. Zirconium of the alumin hasum high particles volumetric and heatpassivates of reaction, the metal yet lowersubstrate than to that inhibit of aluminum.oxidation progression.In addition, itsDifferent other energetic approaches parameter, for overcomi gravimetricng heatthe ofalumina reaction, coating is substantially problem inferior are discussed to that of

J. Mar. Sci. Eng. 2020, 8, 8 3 of 11

J. Mar. Sci. Eng. 2020, 8, 8 3 of 11 extensively in the literature [11–14]. Zirconium has high volumetric heat of reaction, yet lower than that of aluminum. In addition, its other energetic parameter, gravimetric heat of reaction, is moresubstantially regularly inferior used metals to that such of as more aluminum regularly and used magnesium. metals such Relatively as aluminum to aluminum, and magnesium. magnesium hasRelatively a lower to energy aluminum density,, magnesium but the magnesium-water has a lower energy reaction density, is but easier the to magnesium initiate. Therefore,-water reaction some researchersis easier to initiate. suggested Therefore, using magnesium some researchers [4,5,7,8 ]suggested or using anusing aluminum-magnesium magnesium [4,5,7,8] mixtureor using [an6]. Thealuminum high-temperature-magnesium stoichiometric mixture [6]. reactionsThe high of- boron,temperature aluminum, stoichiometric and magnesium reactions with of water bor areon, presentedaluminum here:, and magnesium with water are presented here: kJ 2B + 3H O B O + 3H + 397 푘퐽, (1) 2퐵 + 32퐻 푂→ → 2퐵 푂3 + 3퐻2 + 397mol , (1) 2 2 3 2 푚표푙 kJ 2Al + 3H O Al O + 3H + 818 , (2) 2 → 2 3 2 mol푘퐽 2퐴푙 + 3퐻2푂 → 퐴푙2푂3 + 3퐻2 + 818 , (2) kJ푚표푙 Mg + H2O MgO + H2 + 312 . (3) → mol푘퐽 푀푔 + 퐻 푂 → 푀푔푂 + 퐻 + 312 . (3) One may note that lithium has a very2 high gravimetric2 heat푚표푙 of reaction in addition of its high reactivity with water. However, besides the difficulty in handling lithium, it has been considered for One may note that lithium has a very high gravimetric heat of reaction in addition of its high application only as a liquid fuel in its molten phase and not as solid fuel [15]. reactivity with water. However, besides the difficulty in handling lithium, it has been considered for The chamber pressure in the marine water-breathing ramjet is dictated by the cruise speed, and application only as a liquid fuel in its molten phase and not as solid fuel [15]. its maximum value at a given speed is equal to the stagnation pressure of the incoming water flow: The chamber pressure in the marine water-breathing ramjet is dictated by the cruise speed, and

its maximum value at a given speed is equal to the stagnation1 2 pressure of the incoming water flow: P0 = Pa 1+ ρwu , (4) 푃 = 푃 + 휌2 푢2, (4) 0 푎 2 푤 where, P0 = stagnation pressure; Pa = ambient pressure; ρw = water density; u = cruise velocity. where, 푃0 = stagnation pressure; 푃푎 = ambient pressure; 휌푤 = water density; 푢 = cruise velocity. TheThe stagnationstagnation pressurepressure increasesincreases withwith thethe cruisecruise speedspeed (as(as shownshown inin FigureFigure2 2);); the the increased increased chamberchamber pressurepressure atat higherhigher velocitiesvelocities enableenable enhancedenhanced conversionconversion ofof thermalthermal energy toto kinetic energy inin thethe exhaustexhaust nozzle,nozzle, increasingincreasing thethe energeticenergetic performanceperformance ofof thethe motor.motor.

33 Figure 2. Stagnation pressure vs. cruise speed in water (assuming: ρ휌w푤 = 1000 kg푘푔//m푚 ).).

2.2. Theoretical ModelModel

2.1.2.1. Governing Equations Equations and and Evaluating Evaluating Performance Performance InIn orderorder toto evaluateevaluate thethe performanceperformance ofof aa marinemarine solidsolid propellantpropellant water-breathingwater-breathing ramjet,ramjet, thethe governinggoverning equationsequations areare presented.presented. TheThe generalgeneral thrustthrust equationequation ofof aa jetjet engineengine is:is: . . 퐹F==푚ṁ eu푢e −mw푚u̇ +푢(P+e (푃Pa−) 푃 ) (5(5)) 푒 푒− 푤 − 푒 푎 where, 퐹 = thrust;. 푚̇ 푒 = exhaust mass flow rate; 푢푒 = exhaust jet velocity. ; 푚̇ 푤 = incoming water where, F = thrust; me = exhaust mass flow rate; ue = exhaust jet velocity; mw = incoming water mass mass flow rate; 푃푒 = exhaust jet static pressure. flow rate; Pe = exhaust jet static pressure.

J. Mar. Sci. Eng. 2020, 8, 8 4 of 11

The specific impulse (Isp) of a propulsion system is an important index defined as the thrust divided by the weight flow rate of propellant consumed in order to produce the thrust, (as shown in Equation (6)). The density specific impulse (ρIsp) of a propulsion system is defined as the specific impulse multiplied by the density of the propellant. For volume-limited vehicles (such as many underwater marine vehicles), the density specific impulse may be of more significant since it represents the thrust produced per volume flow rate of propellant. Thus, the specific impulse and density specific impulse can be used to compare and grade the performance of different propulsion methods. Assuming the nozzle is adapted so that the exhaust pressure is equal to ambient pressure (i.e., optimal expansion ratio for maximal thrust), the specific impulse of a marine ramjet can be calculated as following:

. . F meue mwu Isp = . = . − , (6) mp g0 mp g0

. where, Isp = specific impulse; mp = propellant mass flow rate; and g0 = universal gravity constant. The exit (jet) flow rate is equal to the sum of the propellant combustion products and water flow rates, thus: . . . me = mw + mp. (7) Defining: . w mw = . , (8) p mp one obtains: h  w i  w  1 + ue u p − p Isp = . (9) g0 Assuming that the combustion chamber pressure is equal to its maximal theoretical value (i.e., the stagnation pressure) and that the ambient pressure is 1 bar (i.e., cruising at sea level), the specific impulse of a marine solid propellant water-breathing ramjet was calculated. The calculations were done according to Equation (9) with the aid of the Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications (CEA) [16], which yields the equilibrium flow conditions and jet performance (adapted nozzle). The specific impulse and combustion chamber temperature were calculated for different propellant compositions and with different water/propellant mass ratios. In this work, a number of propellant compositions were examined. The first composition was a common solid propellant formulation of 85% ammonium perchlorate (AP) and 15% hydroxyl terminated polybutadiene (HTPB), for which the water ingested into the motor undergoes only a phase change into steam with no chemical interaction with the burning propellant. In addition to the non-hydro-reactive AP-HTPB composition, this work deals with certain water-reactive fuels and compositions, representing the potential range of propellants and hydro-reactive propellant ingredients suitable for propelling a marine water-breathing ramjet. One hydro-reactive fuel is pure aluminum, presenting the maximum theoretical performance of aluminized propellants, since it reacts solely with the incoming water with no oxidizer stored onboard. Additional elements examined as potential fuels or propellant ingredients, were boron because of its high heat of reaction (although its practical application may be questionable), and magnesium due to its better reactivity relatively to aluminum and boron. At last, a more practical solid propellant composition, i.e., 70% Mg and 30% polytetrafluoroethylene (PTFE, Teflon), was examined. The small amount of PTFE oxidizer enables initial combustion to take place without the addition of water, but being a fuel-rich propellant, the excessive fuel can react with incoming water. J. Mar. Sci. Eng. 2020, 8, 8 5 of 11

2.2. Thrust Coefficient and Influence of Cruise Depth and Velocity on Performance In addition to examining the specific impulse obtained by different propellant compositions at different water to propellant mass ratios, the thrust coefficient and the influence of cruise depth and velocity on performance have been studied. It should be mentioned, that in contrast to the specific impulse which highly depends on the propellant composition and water/propellant ratio, the thrust, and therefore the thrust coefficient, weakly depends on these parameters. Thus, it was chosen in this work to present only the thrust coefficient of the more practical fuel-rich hydro-reactive propellant composition containing 70% Mg and 30% PTFE, with a constant water/propellant ratio of 3.5 (where maximal specific impulse is achieved). Yet, similar results and trends of the thrust coefficient have been observed with other compositions and at different water to propellant ratios. Assuming the marine water-breathing ramjet is used to propel a cylindrical torpedo-shaped vehicle, the cross-section area of the body may be used as the reference area in computing the thrust and drag coefficients (Equations (10) and (11)). In addition, it was assumed that the nozzle throat diameter is 50% of the body’s external diameter.

F CT = , (10) 1 2 2 Aρwu where, CT = thrust coefficient; A = reference area. It should be mentioned that for a cylindrical torpedo-shaped vehicle with a length to diameter ratio of 15, the drag coefficient is presumed to be smaller than 0.25 at all cruise velocities [17] and may be significantly smaller in a supercavitating vehicle cruising at a high velocity [18]. In order for the vehicle to overcome the drag force, the thrust coefficient has to be equal or larger than the drag coefficient, which is related to the same reference area:

D CD = , (11) 1 2 2 Aρwu where, CD = drag coefficient; D = drag force. Assuming isentropic supersonic flow in the divergent section of the nozzle, the data obtained from the thermochemical code, mentioned above, enable to calculate the exit velocity and pressure as presented in Sutton and Biblatz [19], and thus the thrust (Equation (5)). The expansion ratio is defined as the ratio between the nozzle’s exit and throat cross-section areas. The results presented in this study refer to a ramjet with an expansion ratio of two cruising at a velocity in the range of 50 to 150 m/s at a depth of 0 to 100 m below sea level. Atmospheric pressure (1 bar) is assumed at sea level and a pressure increase by 1 bar every 10 m below sea level.

3. Results and Discussion

3.1. Performance of Different Propellant Compositions

3.1.1. Non-Hydro-Reactive Composition Figure3a presents the calculated specific impulse and density specific impulse for a solid propellant water-breathing ramjet motor with a propellant grain consisting of 85% AP and 15% HTPB versus the water to propellant (w/p) mass ratio for two cruise speeds, 25 m/s (implying chamber pressure of 4 bar) and 100 m/s (implying chamber pressure of 50 bar). The values at a zero w/p ratio represent the specific impulse of a pure rocket (no water addition). In general, the w/p ratio is limited by the decrease of the overall chamber temperature to the water equilibrium boiling (condensation) temperature (Figure3b). It is obvious that the addition of water increases the specific impulse (and thus the density specific impulse) substantially. However, at the low cruise speed (low chamber pressure), the specific impulse is inferior to that of a rocket motor operating at standard conditions (chamber pressure of about 69 bar). J. Mar. Sci. Eng. 2020, 8, 8 6 of 11 J. Mar. Sci. Eng. 2020, 8, 8 6 of 11

Nevertheless,the higher chamber the performance pressure (50 of bar the, corr water-breathingesponding to ramjeta cruise at speed the higher of 100 chamber m/s) is noticeably pressure(50 higher bar, correspondingthan that of the to lower a cruise pressure speed due of 100 to mthe/s) contribution is noticeably of higher the greater than that expansi of theon lower in the pressure nozzle. dueWhen to thecruising contribution at 100 m/s, of the the greater specific expansion impulse inexceeds the nozzle. that of When a standard cruising rocket at 100 motor m/s, theby specificas much impulse as 30% exceedswhen adding that of water a standard at a w/p rocket ratio motor of 2.5. by as much as 30% when adding water at a w/p ratio of 2.5.

(a) (b)

Figure 3. (a) Calculated specific impulse—Isp, density specific impulse—ρIsp, and (b) combustion Figure 3. (a) Calculated —퐼푠푝, density specific impulse—휌퐼푠푝, and (b) combustion chamber temperature—Tc, of an underwater water-breathing ramjet using a non-hydro-reactive solid chamber temperature—푇푐, of an underwater water-breathing ramjet using a non-hydro-reactive solid propellantpropellant (85%(85% ammoniumammonium perchlorateperchlorate (AP)(AP) ++ 15% hydroxyl terminated polybutadiene (HTPB (HTPB)))) vs. waterwater/propellant/propellant massmass ratio,ratio, forfor twotwo didifferentfferent cruise cruise velocities. velocities. 3.1.2. Hydro-Reactive Fuels 3.1.2. Hydro-Reactive Fuels Figure4a presents the theoretical specific impulse and density specific impulse for a 4 water-breathingFigure a presents motor withthe theoretical aluminum specific as a fuel impulse versus and the water density to specific propellant impulse (fuel) for mass a water ratio,- breathing motor with aluminum as a fuel versus the water to propellant (fuel) mass ratio, for two for two cruise speeds, 25 and 100 m/s, corresponding to low (4 bar) and high (50 bar) chamber pressures, cruise speeds, 25 and 100 m/s, corresponding to low (4 bar) and high (50 bar) chamber pressures, respectively. In this case, even at the low speed cruise conditions, the theoretical specific impulse of respectively. In this case, even at the low speed cruise conditions, the theoretical specific impulse of the water-breathing ramjet exceeds that of a standard rocket, the density specific impulse exceeds the water-breathing ramjet exceeds that of a standard rocket, the density specific impulse exceeds that of a standard rocket even more so due to the relatively high density of aluminum (compared to that of a standard rocket even more so due to the relatively high density of aluminum (compared to HTPB-AP based propellants). Figure4b presents the calculated combustion chamber temperature HTPB-AP based propellants). Figure 4b presents the calculated combustion chamber temperature versus water/propellant mass ratio. It can be observed that at low w/p ratios (bellow the stoichiometric versus water/propellant mass ratio. It can be observed that at low w/p ratios (bellow the ratio) the addition of water increases the chamber temperature due to the increase in available oxidizer stoichiometric ratio) the addition of water increases the chamber temperature due to the increase in that enables more aluminum to react and release heat. On the other hand, at large w/p ratios, the available oxidizer that enables more aluminum to react and release heat. On the other hand, at large addition of water reduces the temperature, similarly to the trend shown above in the case of a w/p ratios, the addition of water reduces the temperature, similarly to the trend shown above in the non-hydro- reactive propellant. case of a non-hydro- reactive propellant. To be more realistic, one can take into account the fact that the reaction products of aluminum To be more realistic, one can take into account the fact that the reaction products of aluminum with water contain large amounts of condensed material (Al2O3 or Al(OH) ) that may be retained with water contain large amounts of condensed material (Al O or Al(OH)3) that may be retained within the motor. Hence, calculations were repeated for the case2 where3 the condensed3 material remains within the motor. Hence, calculations were repeated for the case where the condensed material in the reaction chamber and does not exit the nozzle. Even in such case, the motor performance may remains in the reaction chamber and does not exit the nozzle. Even in such case, the motor be doubled and more, compared to standard solid . performance may be doubled and more, compared to standard solid rockets.

J. Mar. Sci. Eng. 2020, 8, 8 7 of 11 J. Mar. Sci. Eng. 2020, 8, 8 7 of 11

(a) (b)

Figure 4. (a) Calculated specific impulse—I , density specific impulse—ρI , and (b) combustion Figure 4. (a) Calculated specific impulse—퐼sp푠푝, density specific impulse—휌퐼sp푠푝, and (b) combustion chamber temperature—Tc, of an underwater water-breathing ramjet using aluminum as a hydro-reactive chamber temperature—푇푐 , of an underwater water-breathing ramjet using aluminum as a hydro- fuelreactive vs. water fuel vs./propellant water/propellant mass ratio, mass for ratio two, di forfferent two different cruise velocities. cruise velocities.

As mentioned above, theoretically, boron as a fuel may provide the uppermost performance of a As mentioned above, theoretically, boron as a fuel may provide the uppermost performance of water-breathing ramjet propulsor (assuming beryllium is ruled out due to toxicity), though there is a water-breathing ramjet propulsor (assuming beryllium is ruled out due to toxicity), though there is uncertainty regarding practical application. Additionally, magnesium is widely considered as a fuel uncertainty regarding practical application. Additionally, magnesium is widely considered as a fuel for a marine ramjet due to its reactivity. Figures5a and6a present the theoretical specific impulse and for a marine ramjet due to its reactivity. Figure 5a, 6a present the theoretical specific impulse and density specific impulse of a ramjet using boron and magnesium as fuels at two cruise speeds (100 m/s density specific impulse of a ramjet using boron and magnesium as fuels at two cruise speeds (100 and 25 m/s). It is apparent from Figure5a that at a cruise speed of 100 m /s boron enables to achieve m/s and 25 m/s). It is apparent from Figure 5a that at a cruise speed of 100 m/s boron enables to superior theoretical performance of up to four times the specific impulse achieved by an SRM, yet, achieve superior theoretical performance of up to four times the specific impulse achieved by an even at 25 m/s a significant specific impulse may be achievable. It should be noted, that due to the SRM, yet, even at 25 m/s a significant specific impulse may be achievable. It should be noted, that immense amount of heat released from the boron-water reaction, maximal specific impulse is achieved due to the immense amount of heat released from the boron-water reaction, maximal specific impulse at a very large w/p ratio. Figure6a reveals that the specific impulse of magnesium is inferior to that is achieved at a very large w/p ratio. Figure 6a reveals that the specific impulse of magnesium is of aluminum by roughly 15% at both cruise speeds. However, due to magnesium’s reactivity, which inferior to that of aluminum by roughly 15% at both cruise speeds. However, due to magnesium’s possibly implies increased combustion efficiency, magnesium is a promising fuel and can be used as an reactivity, which possibly implies increased combustion efficiency, magnesium is a promising fuel energetic propellant ingredient to improve performance of solid propellant marine . Figures5b and can be used as an energetic propellant ingredient to improve performance of solid propellant and6b present the combustion chamber temperature of a ramjet using boron and magnesium as fuels, marine ramjets. Figures 5b and 6b present the combustion chamber temperature of a ramjet using respectively. The trends discussed above in the case of aluminum, of an increase in temperature at a boron and magnesium as fuels, respectively. The trends discussed above in the case of aluminum, of low w/p ratio and a decrease in temperature at a large w/p ratio, can be observed also in the cases of an increase in temperature at a low w/p ratio and a decrease in temperature at a large w/p ratio, can boron and magnesium. It should be commented, that while aluminum (Figure4b) and magnesium be observed also in the cases of boron and magnesium. It should be commented, that while aluminum (Figure6b) present roughly an equal peak temperature of 3100 K, the peak temperature obtained with (Figure 4b) and magnesium (Figure 6b) present roughly an equal peak temperature of 3100 K, the boron (Figure5b) is significantly lower due to the large latent heat and the relatively low temperature peak temperature obtained with boron (Figure 5b) is significantly lower due to the large latent heat of evaporation of the water-boron reaction product, boron oxide. In all cases, where phase change and the relatively low temperature of evaporation of the water-boron reaction product, boron oxide. occurs (observed as “plateaus” in the temperature figures), the phase change temperature is greater for In all cases, where phase change occurs (observed as “plateaus” in the temperature figures), the phase the higher velocity due to the increased chamber pressure, which results in an elevated equilibrium change temperature is greater for the higher velocity due to the increased chamber pressure, which vaporresults pressure. in an elevated equilibrium vapor pressure.

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(a) (b) (a) (b) Figure 5. (a) Calculated specific impulse—퐼 , density specific impulse—휌퐼 , and (b) combustion Figure 5. a I푠푝 I푠푝 b Figure 5. ((a)) CalculatedCalculated specificspecific impulse—impulse—퐼sp푠푝,, densitydensity specificspecific impulse—impulse—ρ휌퐼sp푠푝,, andand ((b)) combustioncombustion chamber temperature—푇푐, of an underwater water-breathing ramjet using boron as a hydro-reactive chamber temperature—T푇c, of an underwater water-breathing ramjet using boron as a hydro-reactive fuelchamber vs. water/propellant temperature— mass푐, of an, for underwater two different water cruise-breathing velocities. ramjet using boron as a hydro-reactive fuelfuel vs.vs. waterwater/propellant/propellant mass,mass,for fortwo twodi differentfferent cruise cruise velocities. velocities.

(a) (b) (a) (b)

Figure 6.6. (a) Calculated specific specific impulse impulse——퐼I푠푝sp, density specificspecific impulse—impulse—휌ρI퐼푠푝sp,, andand ((bb)) combustioncombustion chamberFigure 6. temperature—(a) Calculated Tspecific, of an impulse underwater—퐼푠푝, density water-breathing specific impulse ramjet— using휌퐼푠푝, and magnesium (b) combustion as a chamber temperature—푇푐,c of an underwater water-breathing ramjet using magnesium as a hydro- 푇 hydro-reactivereactivechamber fuel temperature vs. fuel water/propellant vs.— water푐, of/propellant an massunderwater ratio mass, for ratio,water two for- breathingdifferent two diff cruise erentramjet cruisevelocities. using velocities. magnesium as a hydro- reactive fuel vs. water/propellant mass ratio, for two different cruise velocities. 3.1.3. Practical Hydro-Reactive Composition 3.1.3. Practical Hydro-Reactive Composition 3.1.3.In Practical practice, Hydro water-reactive-Reactive Composition ingredients may be embedded within a fuel-rich propellant matrix In practice, water-reactive ingredients may be embedded within a fuel-rich propellant matrix containing a small fraction of oxidizer. Figure7a presents the calculated specific impulse and density containingIn practice, a small water fraction-react ofive oxidizer. ingredients Figure may 7a presents be embedded the calculated within aspecific fuel-rich impulse propellant and density matrix specific impulse for a water-breathing ramjet motor with a propellant grain consisting of 70% Mg and specificcontaining impulse a small for fraction a water -ofbreathing oxidizer. ramjet Figure motor 7a presents with a thepropellant calculated grain specific consist impulseing of 70% and Mg density and 30% PTFE versus the water to propellant mass ratio for two cruise speeds. This propellant composition 30%specific PTFE impulse versus for thea water water-breathing to propell ramjetant motor mass with ratio a propellant for two cruise grain con speeds.sisting This of 70% propellant Mg and is fuel-rich, thus also in this case maximum temperature is achieved when water is introduced in the composition30% PTFE versus is fuel - therich, water thus also to propell in thisant case mass maximum ratio for temperature two cruise is achieved speeds. This when propellant water is combustion chamber as an additional oxidizer reacting with the excess magnesium, as can be seen introducedcomposition in isthe fuel combustion-rich, thus ch alsoamber in as this an caseadditional maximum oxidizer temperature reacting with is achieved the excess when magnesium, water is in Figure7b. It is apparent from Figure7a, that the addition of a hydro-reactive fuel ingredient such asintroduced can be seen in thein Figurecombustion 7b. It chis amberapparent as an from additional Figure 7oxidizera, that the reacting addition with of the a hydro excess- reactivemagnesium, fuel as magnesium, increases significantly the performance relatively to the case of a non-hydro-reactive ingredientas can be seen such in as Figure magnesium 7b. It is, increasesapparent significantlyfrom Figure 7 thea, that performance the addition relatively of a hydro to the-reactive case offuel a composition presented at the beginning. At a cruise speed of 100 m/s it enables one to achieve a specific noningredient-hydro- reactive such as composition magnesium, presentedincreases significantlyat the beginning the . performanceAt a cruise speed relatively of 100 to m/s the it case enables of a impulse double that achieved with a standard solid propellant rocket. onenon to-hydro achieve-reactive a specific composition impulse doublepresented that at achieved the beginning with a. standardAt a cruise solid speed prop ofellant 100 m/s rocket. it enables one to achieve a specific impulse double that achieved with a standard solid propellant rocket.

J. Mar. Sci. Eng. 2020, 8, 8 9 of 11 J. Mar. Sci. Eng. 2020, 8, 8 9 of 11 J. Mar. Sci. Eng. 2020, 8, 8 9 of 11

(a) (b)

(a) (b) Figure 7. (a) Calculated specific impulse—퐼푠푝, density specific impulse—휌퐼푠푝, and (b) combustion

chamber temperature—푇푐, of an underwater water-breathing ramjet using a hydro-reactive fuel-rich FigureFigure 7.7. (a) Calculated specificspecific impulse—impulse—퐼푠푝Isp,, densitydensity specificspecific impulse—impulse—ρ휌I퐼sp,, andand ((bb)) combustioncombustion solid propellant (70% Mg + 30% PTFE) vs. water/propellant mass ratio, for푠푝 two different cruise chamberchamber temperature—temperature—T푇c,, ofof anan underwaterunderwater water-breathingwater-breathing ramjet using aa hydro-reactivehydro-reactive fuel-richfuel-rich velocities. 푐 solidsolid propellant propellant (70% (70% Mg Mg+ 30% + 30% PTFE) PTFE) vs. water vs. water/propellant/propellant mass ratio, mass for ratio two, di forfferent two cruise different velocities. cruise 3.2.3.2. ThrustThrustvelocit Coeies. Coefficient fficient and and Influence Influence of Cruise of Cruise Depth Depth and Velocity and Velocity on Performance on Performance

3.2. ThrustFigureFigure 8C presents8oefficient presents the and the thrust Ithrustnfluence coe coefficientffi cientof Cruise and and theDepth specificthe and specific V impulseelocity impulse ofon a P marine erformanceof a marine water-breathing water-breathing ramjet propelledramjet propelled by a solid by propellanta solid propellant grain containing grain containing 70% magnesium 70% magnesium and 30% and PTFE 30% with PTFE a w with/p ratio a w/p of Figure 8 presents the thrust coefficient and the specific impulse of a marine water-breathing 4ratio versus of 4 the versus cruise the speed cruise at speed different at different depths. depths. It is apparent It is apparent from Figure from8 figurea,b thats 8a with and the 8b increasethat with ramjet propelled by a solid propellant grain containing 70% magnesium and 30% PTFE with a w/p inthe depth, increase the in thrust depth, coe thefficient thrust and coefficient the specific and impulse the specific decrease. impulse The decrease. reason isThe that reason the increase is that the in ratio of 4 versus the cruise speed at different depths. It is apparent from figures 8a and 8b that with ambientincrease pressure in ambient reduces pressure the exit reduces nozzle the performance exit nozzle (see performance Equation (5)).(see Yet,Equation even at (5)). a depth Yet, even of 100 at m a the increase in depth, the thrust coefficient and the specific impulse decrease. The reason is that the belowdepth seaof 100 level, m below the thrust sea level, coeffi cientthe thrust is greater coefficient than the is greater drag coe thanfficient the drag predicted coefficient on a torpedo-likepredicted on increase in ambient pressure reduces the exit nozzle performance (see Equation (5)). Yet, even at a vehiclea torpedo discussed-like vehicle above, discussed thus the motor above can, thus suite the a broad motorrange can suite of velocities a broad and range depths. of velocities If necessary, and depth of 100 m below sea level, the thrust coefficient is greater than the drag coefficient predicted on thedepths selection. If necessary, of a different the selection throat diameter of a different will enable throat to diameter achieve a will lower enable or higher to achieve thrust levela lower (that or a torpedo-like vehicle discussed above, thus the motor can suite a broad range of velocities and willhigher dictate thrust a di levelfferent (that propellant will dictate consumption a different rate).propellant consumption rate). depths. If necessary, the selection of a different throat diameter will enable to achieve a lower or higher thrust level (that will dictate a different propellant consumption rate).

(a) (b)

(a) (b) FigureFigure 8. 8.Calculated Calculated ((aa)) thrustthrustcoe coefficientfficient and and ( b(b)) specific specific impulse impulse of of an an underwater underwater water-breathing water-breathing ramjetramjet with with a nozzle a nozzle expansion expansion ratio ratio of two of and two a and throat a diameterthroat diameter 50% of the 50% vehicle’s of the external vehicle’s diameter, external Figure 8. Calculated (a) thrust coefficient and (b) specific impulse of an underwater water-breathing usingdiameter a hydro-reactive, using a hydro fuel-rich-reactive solid propellant fuel-rich solid (70% Mg propellant+ 30% PTFE) (70% with Mg a + water 30%/propellant PTFE) with mass a ramjet with a nozzle expansion ratio of two and a throat diameter 50% of the vehicle’s external ratiowater/propellant of 3.5 vs. cruise mass velocity, ratio of cruising 3.5 vs. atcruise a depths velocity, of 0, cruising 50, and 100 at a m depth belows of sea 0, level. 50, and 100 m below diameter, using a hydro-reactive fuel-rich solid propellant (70% Mg + 30% PTFE) with a sea level. water/propellant mass ratio of 3.5 vs. cruise velocity, cruising at a depths of 0, 50, and 100 m below sea level.

J. Mar. Sci. Eng. 2020, 8, 8 10 of 11

4. Conclusions The concept of a water-breathing ramjet for underwater propulsion has been evaluated theoretically based on thermochemical calculations and available practical propellants and energetic components. It is revealed that theoretically, even without chemical interaction, a solid propellant ramjet motor augmented by water entering from the surroundings through an inlet, can deliver specific impulse higher by 30% than a standard solid rocket. By introducing a fuel-rich propellant containing large amounts of a water-reactive metal such as magnesium, aluminum, or boron, the specific impulse may be doubled or more. At cruise speeds as low as 25 m/s, the performance of a water-breathing ramjet with a non-water-reactive propellant is inferior to that of a standard rocket, however, when the propellant is enriched with water-reactive ingredients, improvement in performance can be achieved even at relatively low velocities. In all cases, the specific impulse increases significantly with speed because of the increase in chamber pressure. At sea level, the thrust coefficient is almost constant at velocities between 50–150 m/s, whereas at greater depths there is a decrease in both the thrust coefficient and the specific impulse. Yet, at high velocities the thrust coefficient decreases mildly with depth, thus, the marine solid propellant water-breathing ramjet is a powerful propulsion method suitable for propelling high-speed underwater vehicles at a wide range of depths.

Author Contributions: Conceptualization, N.E.E. and A.G.; Data curation, N.E.E.; Formal analysis, N.E.E. and A.G.; Funding acquisition, A.G.; Investigation, N.E.E.; Methodology, N.E.E. and A.G.; Project administration, A.G.; Resources, A.G.; Software, N.E.E.; Supervision, A.G.; Validation, A.G.; Visualization, N.E.E.; Writing—original draft, N.E.E.; Writing—review & editing, A.G. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by PMRI—Peter Munk Research Institute—Technion. Acknowledgments: The authors wish to thank David Albagli for his advice with the thermochemical calculations. Conflicts of Interest: The authors declare no conflict of interest.

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