Showing papers in "Journal of Jet Propulsion in 1956"

Laminar Heat Transfer Over Blunt-Nosed Bodies at Hypersonic Flight Speeds

Abstract: This paper deals wi th two l imit ing cases of laminar heat transfer over blunt-nosed bodies at hypersonic flight speeds, or high s tagnat ion temperatures: (a) thermodynamic equil ibrium, in which the chemical reaction rates are regarded as "very fas t" compared to the rates of diffusion across streamlines; (b) diffusion as rate-governing, in which the volume recombination rates within the boundary layer are "very s low" compared to diffusion across streamlines. In either case the gas density near the surface of a blunt-nosed body is m u c h higher than the density jus t outside the boundary layer, and the velocity and stagnation enthalpy profiles are m u c h less sensitive to pressure gradient than in the more familiar case of moderate temperature differences. In fact, in case (a), the nondimensionalized enthalpy gradient at the surface is represented very accurately by the "classical" zero pressure gradient value, and the surface heat-transfer rate distribution is obtained directly in terms of the surface pressure distribution. In order to i l lustrate the method , this solution is applied to the special cases of an unyawed hemisphere and an unyawed, b lunt cone capped by a spherical segment . In the opposite l imit ing case where diffusion is ratecontrolling the diffusion equation for each species is reduced to the same form as the low-speed energy equation, except that the Prandtl number is replaced by the Schmidt number . The simplifications introduced in case (a) are also applicable here, and the expression for surface heat transfer rate is similar; the maximum value of the ratio between the rate of heat transfer by diffusion alone and by heat conduction alone in the case of thermodynamic equil ibrium is given by: (Prandtl n o . / S c h m i d t no.)'. When the diffusion coefficient is es t imated by taking a reasonable value of a tom-molecule collision cross section this ratio is 1.30. Additional theoretical and (especially) experimental studies are clearly required before these s imple results are accepted.

A Mechanism for High-Frequency Oscillation in Ramjet Combustors and Afterburners

Abstract: An experimental investigation was made of the behavior of a small two-dimensional combustion chamber, burning a uniform mixture of air and fuel vapor under conditions of high-frequency oscillation or screech. Measurements were made of the limits of stable screech, the amplitude and frequency of pressure oscillations over a wide range of mixture ratio, inlet air temperature, and combustor flow rate. Spark schlieren photographs and high-speed motion pictures taken of the combustion process showed, in agreement with other investigations, that the high-frequency oscillation is accompanied by vortices shed periodically from the flameholder lip with the same frequency as the oscillation. The following mechanism of exciting the oscillations is suggested. A mode of transverse oscillation is excited as the result of periodic transport of combustible material, associated with the vortices, into the hot wake of the flameholder. The vortices, in turn, are generated at the flameholder lips by the fluctuating transverse velocity. When the ignition time delay lies in the proper range, the phase relationship between oscillations in transverse velocity and combustion intensity is such that the oscillation is amplified.

Interference effects during burning in air for stationary n-heptane, ethyl alcohol, and methyl alcohol droplets

Abstract: Experiments have been conducted for the determination of the evaporation constant and flame shapes of two and of five closely spaced droplets burning in air. Droplets of approximately the same and of different diameters were used at various distances between the droplet centers. The apparent flame shape, which was observed only for n-heptane droplets, changes very little during burning. The square of the droplet diameter decreases linearly with time for fixed spacing between droplet centers, at least within the experimental limits of accuracy. In general, the average evaporation constant for two droplets, K', must be assumed either to vary continuously during burning or else to be a function of average initial drop diameter, D^0. The change of K' with time corresponds to the second derivative in plots of the square of the diameter vs. time. These second derivatives are not defined in our work because of unavoidable scatter of the experimental data. Attempts at understanding the observed results by considering published theories for single droplets, as well as groupings obtained from dimensional analysis, have been unsuccessful. It appears that the diffusion model for the heterogeneous burning of single fuel droplets will require serious revision and extension before the burning of droplets arrays and sprays can be understood quantitatively. Furthermore, the effective value of K' for a spray probably depends not only on the fuel-oxidizer system but also on the injection pattern. For this reason additional studies had best be carried out under conditions corresponding to those existing in service models.

Effect of Vibrations on the Motion of Small Gas Bubbles in a Liquid

Abstract: : An understanding has been obtained of the mechanism governing the motion (migration) of small gas bubbles in vibrated vessels. In case of a rigid tank it was found that the bubbles above a certain level h will rise towards the surface, while those below will sink to the bottom. In the case of elastic vessels the regions have complicated shapes. It is concluded that, if many bubbles are present, clusters of bubbles will collect in certain locations near the wall or bottom. The chief concern here is with the behavior of a single small bubble. This analysis indicates that the region just below the surface should always be one in which bubbles rise towards the surface and vent. The theory therefore does not yet contain the explanation for the streams of small bubbles moving from the surface towards the bottom which were observed at high accelerations. This phenomenon may be caused by the pressure field of the sloshing motion which is present whenever the bubble stream is seen.

Experiments on the burning of single drops of fuel

Abstract: Experiments have been performed in order to measure the mass rate of consumption of single drops of liquid fuel suspended on a quartz filament and burning under various ambient conditions. The influence of increased oxidizer concentration, increased pressure, and elevated temperature in the surrounding atmosphere on mass burning rate has been studied. Comparison is made with theoretical calculations based on the concept of a heterogeneous diffusion flame, with burning rate controlled by heat and mass transfer. The influence of forced convection on burning rate and extinction of burning has also been investigated.

A Solid-Liquid Rocket Propellant System

Abstract: S years ago, an investigation of a hybrid (solidliquid propellant) rocket propellant s}^stem was conducted by the General Electric Company as part of an Army Ordnance Contract. The system studied involves the use of 90 per cent hydrogen peroxide, the oxidizer, in combination with a solid fuel, generally polyethylene. The purpose of this report is to review the reasoning which led initially to the interest in such a system and to the choice of the particular materials used; in addition, some of the results will be briefly discussed. The advantages to be gained from systems of this kind are not immediately obvious; in fact, there are probably not many solid-liquid propellant combinations which do offer any advantages without introducing other difficult problems. The following summary of the desirable characteristics of the system described here will show why it appeared to be interesting after some rocket engine tests: 1 The theoretical specific impulse is good, with a maximum value of 228 lb-sec/lb at 300 psi and oxidizer/fuel weight ratio of about 6. 2 The average density is high; at the ratio corresponding to the maximum specific impulse, the average density is 1.34 g/cc. 3 By decomposing all of the 90 per cent H202 catalytically, spontaneous ignition of the fuel is obtained with a delay of usually less than 0.5 sec. In this way, the combustion of the fuel may be considered as augmenting the performance of hydrogen peroxide as a monopropellant. Ignition is generally reliable and combustion is smooth over a wide range of oxidizer/fuel ratios. 4 With proper design of the engine and fuel charge, "hard" or explosive starts should never occur; the peroxide decom-

Generalized Theory of the Optimum Thrust Programming for the Level Flight of a Rocket-Powered Aircraft

Abstract: Two theories are presented for the analysis of the opti m u m burning program for horizontal flight. The first theory is based on Green's theorem which leads to a s imple straightforward proof of the necessary and sufficient conditions for the existence of a m a x i m u m . A linear relat ionship between thrust and engine mass flow is assumed. Hibbs' results are generalized by lifting any restriction concerning the shape of the drag polar and by considering a finite m a x i m u m burning rate for the engine. Two regions of best range are detected, one subsonic-transonic, the other one supersonic. The influence of the boundary conditions on the o p t i m u m technique of flight is discussed. A simple similarity rule for stratospheric solutions is pointed out . The second theory, based on the variational method of Lagrange multipl iers, considers the general case of an arbitrary oneto-one correspondence between thrust and propellant mass flow. Particular a t tent ion is devoted to the case of a polygonal thrust characteristic. An example of application is worked out . The concept of index value presented in a previous note is here emphasized. Such an index value appears to be a very effective device in controll ing the composit ion of the o p t i m u m path for discont inuous Eulerian solutions.

On Laminar Boundary Layers With Heat Transfer

Abstract: In p a r t I , a t r a n s f o r m a t i o n is given wh ich reduces t h e l a m i n a r b o u n d a r y layer e q u a t i o n s approx imate ly t o t h e incompress ib le fo rm in t h e case of a n imper fec t gas . T h e t r a n s f o r m a t i o n is r e l a t ed t o t h e S t ewar t son IUingwor th t r a n s f o r m a t i o n , a n d involves f i t t ing a n a p p r o x i m a t e re la t ion to t h e dens i ty profile. I n p a r t I I , t h e p r o b l e m of u t i l iz ing s imi la r so lu t ions in a m o m e n t u m i n t e g r a l m e t h o d is t r e a t e d wi th a n a p p r o a c h in which t h e b o u n d a r y layer t h i c k n e s s is charac te r i zed by a s ingle func t i on of d i s t a n c e a long t h e b o u n d a r y layer ; t h e m o m e n t u m , d i s p l a c e m e n t , a n d en tha lpy -de fec t t h i cknes s a re expressed in d imens ion less fo rms di rect ly ca lculable f rom t h e s imi la r so lu t ions . I t is n o t e d t h a t a bas ic incons i s tency appears in t h e energy r e l a t ion in t h a t two different expressions for t h e h e a t t r ans fe r a t t h e wall a re o b t a i n e d if t h e classic m o m e n t u m i n t e g r a l m e t h o d is used . Var ious a l t e r n a t e m e t h o d s a re discussed.

The Application of the Shock Tube to the Study of the Problems of Hypersonic Flight

Abstract: This paper discusses some of the new scientific and technical problems introduced by the high temperature conditions encountered in hypersonic flight. Modifications of the shock tube which have been studied at Cornell Aeronautical Laboratory for the investigation of these problems are presented. The shock tube is one of the few laboratory instruments capable of generating the high temperature conditions in air encountered in hypersonic flight studied. The conventional shock tube is limited since the maximum flow Mach number that can be achieved behind a normal shock wave in air is approximately 3. When the conventional shock tube is terminated by an expansion nozzle, high Mach number flows can be achieved as well as high stagnation temperatures. Actual flow conditions can therefore be closely simulated for the investigation of heat transfer rates and other aerodynamic problems. The application of the shock tube to other areas of high temperature research is briefly discussed. In particular, a technique for the study of high temperature chemical reaction rates is described.

Flame Spreading From Baffles

Abstract: Flame spreading from 0.5 to 2.25-inch V-gutter baffles in two-dimensional turbulent ducted flow was studied experimentally. Measurements of combust ion efficiency were made at distances of 10 to 43 inches downstream of the flame stabilizing baffle(s) in a 9 X 5-inch duct . The effects on efficiency of several geometric and operating variables were observed. Little effect of baffle size was noted, but efficiency increased when reducing the spacing between baffles (or between baffle and wall) by adding more baffles. Pressure dependence was variable. Efficiency increased wi th pressure (over the range of Vs to /3 a t m abs tested) when using geometries giving h igh flame spreading rates, but there was l i t t le change when using geometries giving low rates. Smaller effects were found on changing equivalence ratio (faster burning occurred nearer stoichiometric mixtures) and velocity (efficiency decreased when velocity increased). Miscellaneous data are noted for changes in tailpipe l ength , fuel vaporization, and nature of turbulence. An empirical correlation is presented which groups together in a s imple way m o s t of the data taken. Many variables interact complexly in flame spreading and no accurate s imple description seems likely. Thus , neither the concept of a wrinkled laminar flame, nor of an extended reaction zone, nor of homogeneous combust ion fits the data generally—although any one concept may be acceptable over a restricted range. Burning is probably discontinuous in microt ime making inapplicable theories assuming cont inuous burning.

Medical Problems Involved in Orbital Space Flight

Abstract: After ana lyz ing t h e var ious k i n d s of space ope ra t i ons t h a t m i g h t be expected in t h e n e a r or r e m o t e f u t u r e (space equ iva len t flight, c i r c u m p l a n e t a r y space flight, a n d i n t e r p l ane t a ry space t ravel) , t h e second phase*—circumplaneta ry space flight or o rb i t a l space flight'—is chosen as a p l a t fo rm for t h e d iscuss ion of s o m e of t h e m o s t i m p o r t a n t med ica l p r o b l e m s involved in space ope ra t i ons . F i r s t , t h e s t a t e of weight lessness is d iscussed w i t h regard to i t s effect u p o n t h e genera l wel l -being of t h e o c c u p a n t s of a sa te l l i te vehicle a n d in r ega rd to i t s s e n s o m o t o r effects. In c o n n e c t i o n h e r e w i t h , t h e op t ica l s i t u a t i o n is considered wi th regard to t h e p roper t i e s of t h e e n v i r o n m e n t a n d t h e visual a p p e a r a n c e of t h e l i gh t sources . F u r t h e r m o r e , physiological d a y n i g h t cycling is d iscussed in a n env i ronm e n t whe re t h e r e is n o n a t u r a l day a n d n i g h t . And finally some p r o b l e m s involved in h u m a n eng inee r ing of t h e space cab in conce rn ing p res su r i za t ion , supply of oxygen a n d remova l of ca rbon dioxide, p h o t o s y n t h e t i c gas exchange , a n d t h e event of s u d d e n decompress ion of t h e cab in a re d iscussed. Some of t he se p r o b l e m s a re p re sen t ly u n d e r s tudy in a n e x p e r i m e n t a l space cab in s i m u l a t o r .

Combustion in the Mixing Zone Between Two Parallel Streams

Abstract: Bluff body flame stabilization depends on the existence of a sheltered wake behind the body in which hot gas recirculates. The hot-wake gas ignites fresh combustible mixture; ignition occurs in a mixing zone between hot gas and external stream. Thus the mixing zone is of critical import in bluff body flameholding and demands further study. An experimental arrangement was devised to simulate important features of the bluff body mixing zone while permitting controlled study of the variables involved. The mixing zone between two parallel gas streams, one hot and one of combustible mixture, was studied. Ignition of the combustible stream was achieved with this arrangement, and many interesting phenomena were observed. Ignition was possible only above a certain temperature of the hot stream, ignition temperature depending markedly on fuel type, and related to fuel-activation energy. Burning was first seen in the mixing zone at some distance downstream from first contact of the streams. Further downstream a vigorous flame appeared tha t propagated into the combustible stream. The propagating flame was established only when the residence time of combustible material in the mixing zone was long enough to lead to ignition of a mass adequate to serve as a secondary ignition source. This result was applied to the explanation of bluff body flame stabilization and blowoff.

The Dynamic Unbalance Test Installation

Abstract: TH E necessity for adequate test equipment capable of the dynamic balancing of missile and rocket equipment, such as rocket motors, rotating launchers, flywheels, and turbo machines, has always presented designers with problems of test machine simplicity, size, and application. Aerophysics has recently solved this problem through the design and development of a simple vertical balancing machine capable of the dynamic balancing of equipment up to 5000-lb weight, 20,000-lb thrust, up to 36 in. diam, over-all lengths of 10 ft, and rotating speeds of 10,000 rpm. Dynamic unbalance may be measured on a spinning rocket motor, firing or not firing. The problem of dynamic balancing is one of bringing into coincidence the mass centerline and geometric centerline of a rotating mass. The unbalance is determined by measuring the amplitude and phase of motion of the freely suspended rotating mass. Weights for correction can then be applied in any two corrective planes if it is necessary. At a speed of 1000 rpm, a 200-lb rotor approximately 50 in. long has been balanced to an accuracy of approximately one-millionth of a radian. Displacement pickups on the test stand have measured displacement to an accuracy of within 0.000050 in. This corresponds to a weight correction of only /16 oz or 0.004 lb. The test stand consists of a rigid outer frame, and a stiff lightweight inner frame mounted on two or more flexure rods or cables. These flexure rods serve as vertical supports for the inner frame but do not constrain the lateral or pitching motion of the frame. Adjustments are provided for varying the lengths and attachment points of the flexure rods. In this way the natural frequency of the inner frame can be modified, as desired, to provide optimum response amplitudes. The adjustments will depend on the rotational speed of the equipment mounted in the inner frame. The freely suspended inner frame permits the resolution and recording of extremely minute amplitudes which are due to small amounts of unbalance in the rotating mass. The outer frame is stiff and comparatively heavy in order to minimize external disturbances to the balancing machine and to provide a fixed frame of reference for measurement. Rigid stops are provided in all directions for safety, convenience of operation, and protection of the flexure rods. The stops may be adjusted, as necessary, to allow motion within the measuring range of the pickups. Instrumentation is provided for measuring displacements in two correction planes and a phase angle (angular location of unbalance) at the various rotational speeds. The displacement and phase ! 1 1

Thermal Stresses in Solid Propellant Grains

Abstract: A method is presented for computing the maximum thermal stress in solid propellant grains tha t can result from temperature cycling. The method is based upon a first approximation obtained from the analytical solution for thermal stresses assuming Young's modulus to be independent of temperature. A more accurate solution is then found by numerical integration taking account of the variation of Young's modulus with temperature.

Flight Mechanics of Ascending Satellite Vehicles

Abstract: The types of satel l ite orbits and types of ascending trajectories to achieve these orbits are discussed. Both analog and digital computor techniques for trajectory c o m p u tations are briefly described. A solution of the o p t i m u m vacuum stage thrust axis pitch program is presented for a satellite using cont inuous thrust all the way from ground to orbit. The procedure is explained for opt imizing the trajectory to min imize propellant consumpt ion for a vehicle using a power-off coast up to the orbit.

Control and Power Supply Problems of Instrumented Satellites

Abstract: Schemes for an attitude control system and for electric power supplies for an unmanned satellite of a few hundred pounds total weight are presented. The attitude of the satellite with respect to the earth's center is controlled within ±10 degrees by utilizing the shadowing effect of the earth on the isotropic cosmic radiation. A number of Geiger counters are arranged so that they sense the location of the shadow cone of the earth. Signals resulting from the counting rates of the counters control fly wheels that cause the satellite to rotate around its center of gravity. Three sources for electric power are described, each of which delivers an average of about 100 watts. The first converts the sun's radiating energy with a silicon junction photoelectric generator. A sun-seeking device keeps the generator oriented toward the sun during daytime. In the second system, the sun's radiation is directed toward a pile of thermocouples made of ZnSb and constantan. Thermocouples have been built of these materials which convert solar energy into electric energy with an efficiency of 5.6%. The third method uses a radioactive, isotope, strontium 90, and its daughter product, yttrium 90, as heating element for a pile of thermocouples. The half life of strontium 90 is 20 years. Each of these three sources has a specific power production of the order of 0.4 to 0.7 watts per pound of weight. The attitude control system and the methods of power supply described are applicable also to larger satellites.

Properties of Fuming Nitric Acid Affecting Its Storage and Use as a Rocket Propellant

Abstract: Some of the general problems of storing fuming nitric acid (the system HNO3-NO2-H2O) in closed metal con­ tainers for periods of the order of months at elevated tem­ peratures around 130 F are discussed. A literature review of some of the properties of importance for satisfactory storage and use as a rocket propellant, as well as a discus­ sion of chemical methods of analysis, is included. Two chemical processes which cause problems in the storage of this widely used oxidant are: (a) the corrosion of metal containers causing change in composition of the acid and its contamination with metallic salts, and (b) the thermal decomposition of acid lean in NO2 and H 20 with accom­ panying high pressures and changes in composition of the oxidant. Liquid and gas phase corrosion of aluminum al­ loys and of stainless steel alloys of the 18-8 class are found to be markedly inhibited by the presence of about 0.5 wt per cent hydrofluoric acid in the fuming nitric acid, thus reducing the first problem of storage. The mechanism of passivation appears to be the formation of an impervious coating of insoluble metal fluoride on the surface of the metal. The second problem of thermal decomposition and correspondingly excessive storage pressures is elimi­ nated in acid containing enough NO2 (12 to 16 wt per cent) to obtain the results desired without causing high partial pressure of this species, and enough H 20 (2 to 3V2 wt per cent) to give satisfactory results without unduly diluting the oxidizing power.

Low Pressure Performance of Cylindrical Can Burners

Abstract: A study was made of the combustion stability and combustion efficiency of cylindrical can burners, 2 to 5 in. diam, at low pressure (/Q to V2 a tm abs). Cans with one, two, and three stages of inlet holes were tested and stability was found to depend primarily on the geometry of the upstream (first) stage of holes and the region inside the can (the recirculation zone) upstream of that stage. Size and location of downstream stages were of minor importance to stability but did affect over-all efficiency. Tests were conducted to determine the effects of varying the number and size of first-stage inlet openings, and the diameter and length of the recirculation zone. The effects of changing pressure, mass flow, inlet jet direction, recirculation zone wall temperature, and tailpipe length were also studied. In an empirical stability correlation, the lean and rich blowout limits are functions of the superficial first-stage velocity divided by the product of can diameter and pressure to the 0.8 power. A can stability mechanism depending on over-all reaction rates in a local zone is suggested.

Climatization of Animal Capsules During Upper Stratosphere Balloon Flights

Abstract: Experiments requiring exposure of animals to primary cosmic radiation at altitudes above 90,000 ft on 24-hr balloon flights have led to the development of environmental control techniques. Simple methods for supplying oxygen and removing carbon dioxide were developed. Internal capsule temperatures have been maintained at very nearly room temperature on 24-hr flights by providing adequate insulation to use animal heat at night, and by cooling the capsule with a water-can cooler during the daytime.