I11111 llllllll Ill 11111 11111 11111l1ll ll11111l lI1 I1IlI1IlIi1 l11111111 lIl1 1111 US005 178002A United States Patent [I91 [11] Patent Number: 5,178,002 Hanson [45] Date of Patent: Jan. 12, 1993 [54] SPECTROSCOPY-BASED THRUST SENSOR ~561 References Cited FOR HIGH-SPEED GASEOUS FLOWS U.S. PATENT DOCUMENTS [75] Inventor: Ronald K. Hanson, Cupertino, Calif. 3,852,741 12/1974 McDonald ..................... 73/117.4 X Primary Examiner-Jerry W. Myracle [73] Assignee: The Board of Trustees of the Leland Attorney, Agent, or Firm-Flehr, Hohbach, Test, Stanford Jr. University, Stanford, Albritton & Herbert Calif. [571 ABSTRACT ' [21] Appl. No.: 780,670 A system and method for non-intrusively obtaining the thrust value of combustion by-products of ajet engine is [22] Filed: Oct. 18, 1991 disclosed herein. The system includes laser elements for inducing absorption for use in determining the axial [51] Int. Cl.5 ............................................ GOlM 15/00 velocity and density of the jet flow stream and elements [52] US. Cl. .................................... 73/117.4; 356/315 for calculating the thrust value therefrom. [58] Field of Search ............................ 73/117.4, 30.03; 356/311, 315 18 Claims, 3 Drawing Sheets 19 TO REF. AND r WAVE- STORAGE SCOPES 23 METER f8 TO STORAGE TO STORAGE 16 UmSm Patent Jan. 12, 1993 Sheet 1 of 3 5,178,002 W c3 52 U.S. Patent 5,178,002 Jan. 12, 1993 Sheet 2 of 3 ANGLED BEAM PERPENDICULAR 13 12 DOPPLER-SHIFT FIG. 2 U.S. Patent 5,178,002 Jan. 12, 1993 Sheet 3 of 3 5178,002 1 2 It is still another object of the present invention to SPECI'ROSCOPY-BASED THRUST SENSOR FOR provide a single system which measures both exit veloc- HIGH-SPEED GASEOUS FLOWS ity and gas density as the elements of a thtust measure- ment. CONTRACT 5 It is a further object of the present invention to avoid the use of tracers or particles in thrust detection. This invention was made with United States Govern- It is yet another object of the present invention to ment support under NASA Contract No. 2NCC-547. enable reliable in-flight thrust measurements. As a result, the United States Government has certain It is yet a further object of the present invention to rights in this invention. 10 obtain spatially resolved thrust measurements. FIELD OF THE INVENTION These and other objects are achieved by a system and method of the present invention for non-intrusively This invention relates generally to jet propulsion determining thrust of a jet combustor apparatus apparatus, and more specifically, to an apparatus and wherein combustible gas by-products are released into method for determining the thrust associated with a I5 the ambient environment. The system enables measure- high-speed stream of gases. ments of both velocity and mass flow rate (which is a BACKGROUND OF THE INVENTION function of density of the combustion by-products) virtually simultaneously so that the thrust may, in turn, In-flight jet engine thrust is of interest to atmospheric be calculated by multiplying the velocity and the mass scientists as well as jet engineers. Accordingly, it is 20 flow rate. By directing light at the jet flow stream at a important to develop methods for accurately determin- wavelength which produces an absorption spectrum in ing the thrust of a jet engine. a combustion by-product of interest, two types of mea- It is well known that both engine power and effi- surements are possible. First, the velocity of the jet flow ciency are proportional to the product of the thrust may be determined according to calculations using the times flight velocity. In order to control power and 25 Doppler effect. Second, the density of the combustion obtain maximum efficiency from, for example, a super- by-product of interest (and hence the mass flow rate) sonic jet, thrust can be controlled by variable conver- may be determined according to calculations using the gent-divergent nozzles. Beer-Lambert law and the ideal gas law. In an on-ground jet testing facility, thrust is com- Other objects and advantages of the invention will monly measured on a thrust stand. While this type of 30 become apparent upon reading the following detailed measurement is accurate, the measurement is not truly description with reference to the drawings: characteristic of an engine's performance in flight. Fur- thermore, thrust stand measurements do not give spa- BRIEF DESCRIPTION OF THE DRAWINGS tially resolved information on the exhaust gases. The accompanying drawings, which are incorpo- Another method for measuring thrust includes add- 35 rated in and form a part of this specification, illustrate ing tracers or particulates to the jet stream to monitor embodiments of the invention and, together with the the velocity of the jet stream; this together with infor- description, serve to explain the principles of the inven- mation on gas density can yield the thrust. This method tion. is used in some ground-testing facilities, but is unaccept- FIG. 1 is a schematic diagram of the components of a able in many cases. For example, tracers and particu- 40 thrust detector of the present invention. lates are often toxic. When used in testing facilities, FIG. 2 is a schematic diagram of a cross beam config- particulates cause health risks to facility personnel. Fur- uration of the present invention using laser induced thermore, particles do not follow the flow of the jet fluorescence. stream accurately and therefore, measurements of ve- FIG. 3 is a schematic diagram of a planar laser in- locity, and hence thrust are inaccurate. 45 duced fluorescence configuration of the present inven- Exit velocity is often measured using laser-Doppler tion. anemometry (LDA) which requires the presence of particles. LDA, however, does not provide a method DETAILED DESCRIPTION OF THE ' for determining gas density. Hence, LDA must be com- INVENTION bined with an auxiliary measurement in order to infer 50 Reference will now be made in detail to the preferred mass flux. Therefore, jet engine thrust measurement embodiments of the invention, examples of which are apparatus typically have several measurement compo- illustrated in the accompanying drawings. While the nents requiring support systems and data handling sys- invention will be described in conjunction with the tems for each component. preferred embodiments, it will be understood that they There is an emerging need for non-intrusivem easure- 55 are not intended to limit the invention to those embodi- ment of thrust of new hypersonic aircraft engines and ments. On the contrary, the invention is intended to mass flux into an engine. There is also a need for a cover alternatives, modifications and equivalents, system which utilizes a minimum number of compo- which may be included within the spirit and scope of nents. Methods utilizing rugged, compact and economi- the invention as defined by the claims. cal laser sources are also needed. 60 As described above, the primary flow field parame- ters of interest are mass density and velocity. Thrust is OBJECTS AND SUMMARY OF THE calculable from velocity and mass flow rate according INVENTION to vari!tions of the following relationships: In light of the aforementioned problems with the prior art, it is therefore a general object of the present 65 Thrust=vclocityxmars flow rate, invention to provide an apparatus and method for non- intrusively and accurately measuring the thrust of a jet and engine. 5,178,OO2 3 4 Mass flow rate=vclocity Xdensity, stream. Detector 22 detects and records resultant inten- sity, Ires, in order to calculate AIre6 such that AIreFI- therefore, ,-I,. A reference frame to compare to the Doppler shifted absorption signal is therefore established and a ~~s~=velocityXvelocityXdensity. 5 reference frame output signal is sent to computer 23. The velocity reference frame output signal is received Techniques for monitoring velocity and density de- by computer 23, and is used as the stationary reference scribed below include utilizing absorption characteris- frame in the Doppler effect velocity calculation. It tics of the combustion by-product of interest. Absorp- should also be noted that instead of an active reference tion is effected by utilizing a laser beam source in a line-of-sight transmission measurement, or by utilizing 10 frame, the reference frame may be provided by any appropriate static sample of the absorbing compound. laser-induced fluorescence, capable of providing both The absorption line spectrum of the jet stream is single-point and multi-point (24 and 3-d) measure- detected and recorded by detector 24 for fluorescence ments. The choice of whether to utilize line-of-sight absorp- detection or detector 25 at the terminus of the beam for tion or laser-induced fluorescence is determined by the 15 line-of-sight absorption measurements. The axial veloc- characteristics of the combustion by-product of interest. ity is inferred from the shift in the spectral line position Where the absorption line spectrum of the combustion with respect to the stationary reference frame caused by by-product is strong, laser-induced fluorescence can be the Doppler effect. A spectral feature monitored in a utilized instead. In either case, the absorption is under moving gas shifts from its known reference frame’s 17 observation. 20 directional component along the flow direction. Mea- Laser sources include tunable continuous wave (cw) surement of this Doppler shift, Av, and knowledge of lasers (ring dye lasers and semiconductor diode lasers) the geometry allows the determination of the axial gas and tunable pulse lasers (excimer-pumped dye and nar- velocity v, where row-line width excimer). Wavelength modulation and high-speed frequency modulation (FM) spectroscopy 25 v=(Av/v)c/cos8 techniques are utilized to provide increased data re- cording rates. Furthermore, other flow field parameters where c is the speed of light and 6 is the angle between which can be measured simultaneously with those of the beam and the flow direction. present interest include species concentrations, temper- Accurate Doppler-based velocity measurements de- ature and pressure. 30 pend on the use of absorption with a sufficiently large Turning now to the drawings, wherein like compo- Doppler shift to total line width ratio. The spectral nents are designated by like reference numerals in the feature must also have a suitable absorption coefficient figures, attention is initially directed to FIG. 1. A com- to ensure a large signal to noise ratio (SNR). Ideal ab- bustion chamber 11 is depicted therein having an exit sorptions are in the 20-80% range. end 12 at the nozzle’s 13 distal end. A jet stream 14 exits 35 Density, and hence mass flow rate, is simultaneously the combustion chamber 11 at exit end 12. The combus- calculated from a measurement of the transmitted frac- tion chamber 11 depicted therein can operate in flight tion of the incident light, as a function of laser wave- or in an on-ground testing facility. length yielding a spectrally resolved record which can As stated above. thrust is the Droduct of the axial be analyzed via the Beer-Lambert law to infer the Dath- velocity and the m&s flow rate (wiich is the product of 40 average density of the absorbing species (assuming the density and velocity) of the combustion by-products. In absorption coefficient is known). the present invention, the velocity and density of a FIG. 1 shows detector 21 which detects and records species of combustion by-products of the jet stream 14 the incident intensity, Io and detector 24 or detector 25 are measured by one system simultaneously. for detecting and recording the absorption line spectra Axial velocity measurement means as shown in FIG. 45 and therefore the transmitted intensity of the jet stream 1, includes laser 16 with its laser beam 15 directed at the 14, Irram.C omputer 23 calculates the absorption inten- exit end 12. The laser 16 operates at a wavelength corre- sity, according to AIabs=Io-Itmm for use calculations sponding to the absorption spectrum of the combustion using Beer-Lambert’s law. by-product of interest SO that the laser light is absorbed Detector 18 receives wavelength and/or frequency Producing absorption lines, such Providing a Doppler input values from wavemeter 19 and sends the values to shifted absorption signal. TO resolve velocities, the computer w for use in calculations. n eB eer-bbert Doppler effect is utilized wherein a non-Doppler- law is as follows: shifted reference is scanned concurrently with the Dop- pler shifted absorption signal. A non-Doppler shifted uobJIo= 1 -M--LvPobsL), velocity reference frame is depicted as reference flame 55 17. The laser beam 15 is split so that a portion is directed where AIabsis the absorbed laser intensity, Iois the inci- to reference flame 17. dent laser intensity, kdatm- 1cm- 1) is the absorption The h ~beram 15, in FIG. 1, is incident at an angle, coefficient and Pabs is the partial pressure (atm) of the in this case, 45 degrees relative to the axis Of the jet absorbing spies and L (cm) is the path length of ab- stream 14 flow. The cosine of the angle provides the 60 sorption. axial velocity vector. A measurable Doppler shift in the By knowledge of the partial pressure from the Beer- spectral position of the absorption/fluorescence signal calculations, the total mixture pressure can & is compared to the reference flame 17 by computer 23. determined directly when the mole fraction of the ab- Any angle, however, which provides an axial compo- sorbing’ species is known. The density @) can then be nent may be used. The known reference frame 17 has a 65 calculated according to the ideal gas equation which directional component along the flow direction. yields, FIG. 1 shows a detector 21 which detects and re- cords incident intensity, I,, of the laser beam on the jet p =M P/R T. 5,178,002 5 6 line center frequency is affected by pressure as well, and where P is pressure, M is the molecular weight of the an unwanted non-Doppler component may contribute combustion by-products species, R is the universal gas to the total relative frequency-shifti f the sample and the constant and T is the temperature. reference volumes are at different pressures. An alternative means of determining the mixture 5 An alternative is to utilize the flowing gas itself as the pressure P is from the width of the recorded spectral non-Doppler-shifted reference. This strategy, depicted feature (i.e. the linewidth), since this width is propor- in FIG. 2, employs two laser-induced fluorescence tional to pressure. (LIF) beams: a first beam 26 passes obliquely and the Therefore, the calculations of velocity and density second beam 27 perpendicular to the flow direction. (and hence mass flow rate) are Performed VirtUallY 10 Accordingly, detection of absorption which ultimately simultaneously using the Same system for both calcula- produces fluorescence allows spatially resolved veloc- tions. Computer 23 receives the reference frame output ity measurements. closely the two volumes signal and the absorption line output signal from dew- reduces errors, owed to differing pressure shifts, in the tor 24 or detector 25 provides axial velocity comparison reported velocity. Monitoring the fractional absorption means according to the Doppler effect. Furthermore, 15 (not the emission of absorbed light) of the two transmit- computer 23 vifluallY simultaneously receives the ab- ted beams of FIG. 2, for purposes of extracting velocity, sorption intensity output signal and the incident inten- temperature and pressure, becomes plausible when the sity reference Output sipa1 and provides density mea- flow is spatially uniform. Absorption signals collected ’llrement according to the Beer-Lambert law and from either (or both) the Doppler-shifted or reference the ideal gas law. Moreover, computer 23 generates the 20 volumes be reduced from inferring temperature and calculated density measurements and axial velocity pressure (and thus density) while velocity is measurements and multiplies their values to give a sig- between the two recorded profiles. nal representing the mass flow rate, therefore providing Moreover, when flows are nonuniform or unknown, mass flow rate means. Furthermore, computer 23, gen- multiple beam imaging may be used to combine absorp- erates the axial velocity output signal and the mass flow 25 tion with tomographic analysis methods, rate output signal and multiplies their values to give a especially in the case that low power lasers are used and signal representing the sought after thrust value, there- optical fibers are employed to provide multiple beams fore providing thrust means. absorption paths at different While fixed-frequencym ethods have the capability of determining the velocity, pressure and temperature, 30 without requiring large windows in the device being mass flow rate measurements are also obtainable using monitored. planar laser-induced fluorescence (pLIF) mea- rapid-tuning laser absorption. Rapid-tuning provides the capability of fully resolving, at kilohertz rates, the surement in which a large number Of point LIF mea- surements in a planar region are recorded simdta- frequency variation of a given absorption feature. Data, which may be in the form of fractional absorption of the 35 neously On an intensified CCD detector (not laser beam, are recorded continuously as the laser repet- shown) may be used* shows a schematic for itively sweeps in frequency. The variation of the laser velocity imaging by PLIF of a supersonicj et Stream 14. frequency in time is simultaneously measured in order Laser 16, which in this case is a dye laser, generates to the data’s time base to a frequency scale. light which is transmitted through a series of sheet once this is accomplished, the recorded absorption 40 forming optics 28. A mirror 29 is provided for retrore- feature may be analyzed to infer temperature and pres- flection to enable imaging of both forward and back- Sure (which are used to infer mass density, as discussed wards-directed light sheets (SA beams). The image to below). Frequency modulation can be effected in Some be collected iS transmitted through collection optics 31 solid state lasers sources as well as ring dye lasers. Solid and is and recorded by a 24. state lasers have the added advantages of economy, 45 Recording information can be effected by a high- ruggedness and compactness for use in process control speed digital camera for recording instantaneous 3-d and flight instrumentation. images and fast 2-d image “movies.” The basic concept Furthemore, wavelength modulation can also be inVOlVeS modifying a COmmerCial image Converter Cam- used. By rapid scanning of a laser across a spectral era (for example, an hacon 790) to incorporate CCD region encompassing two absorption lines of a species, a SO detection. In brief, a Specially designed tapred fiberoP- time-dependent signal which contains complete absorp- tic bundle is used to transfer the output Plane image of tion line profiles of both transitions is generated. Tem- the converter camera Onto a high-resolution (i.e.9 perature can be inferred from the ratio of the line inten- 4ooX 1200 Pixels), low-noise CCD array detector. This sities, pressures from the line width, density and veloc- system is capable of recording a modest number of ity from the Doppler shift of the lines relative to those 55 images at UP to 10 million frames per second. Together of a static sample. Density measurement means can also with a high-energy, long-pulse dye laser source to pro- include calculations made from the total absorption if vide “CW” flow field illumination, this camera allows the temperature is known. In cases where the mole rapid recording of multiple PLIF images, constituting fraction of the species is either known or can be mod- either 811 instantaneous multiple-plane 3d image data eled, these can be converted to total density. Density 60 set or an multiple-image movie over a brief time interval can also be determined using the broadening of the line (i.e., 2 microseconds for the current pulsed dye laser). shape if this effect is large. Accordingly, using fre- Combustion of jet fuel leads to a number of combus- quency modulation or wavelength modulation, the mass tion by-products depending upon the fuel under com- flow rate, and hence thrust, is determined from the bustion’and the atmospheric parameters. For example, product of the mass density signal output and the veloc- 65 combustion requires intake of air, which in part is com- ity output signal. posed of molecular oxygen. Because the present inven- While stationary samples such as reference flame 17 tion is applicable to any combustion by-product, molec- depicted in FIG. 1 are convenient reference sources, ular oxygen in the jet flow stream may be monitored. 5.178.OO2 a 7 The decision to monitor one type by-product instead thrust calculation means for receiving said axial ve- of another in conjunction with the present invention is locity output signal and said mass flow rate output dependent upon ease of spectrographic resolution. For signal and providing a signal representing said example, absorption measurement of molecular oxygen thrust. is possible using either the Schumann-Runge band sys- 5 2. A system as recited in claim 1 wherein said axial tem (in the W and VW) or the A-band system near velocity measurement means and said mass flow rate 760 nm. The emergence of new semiconductor laser measurement means provide measurements simulta- sources, especially at the wavelengths which match the neously. A-band absorption of oxygen near 760 nm, are ideal for 3. A system as recited in claim 1 wherein said mass this application. The absorption of oxygen at 760 nm is 10 flow rate measurement means compriks: weak and therefore frequency modulation is used to density measurement means for measuring the density enable the detection of the weak absorption at relatively of said species of combustion by-products and pro- rapid rates. viding a density output signal; Tunable diode lasers offer the possibility of an eco- axial velocity measurement means for measuring the nomical, rugged and compact alternative to cw ring dye 15 axial velocity of said species of combustion by-pro- lasers for spectrally resolved absorption and fluores- ducts and providing an axial velocity output signal; cence spectroscopy. At present, such lasers are avail- and able in several wavelength intervals at wavelengths mass flow rate means for receiving said density out- generally in excess of 750 nm, but the trend toward put signal and said axial velocity output signal and operation at shorter wavelengths is clear. These lasers 20 providing a signal representing said mass flow rate. have significant potential advantages over ring dye 4. A system as recited in claim 3 wherein said axial lasers, including the possibility of high frequency modu- velocity measurement means comprises: lation of laser wavelength through current modulation. laser means operating at a wavelength corresponding Monitoring molecular oxygen, which is detected at 760 to an absorption spectrum having absorption lines nm (the atmospheric band of oxygen) and water (H20)2 5 of said species of combustible gas by-products, said is detected near 1.35 microns, both by-products of com- laser means for generating light directed at said exit bustion can be detected by these lasers and have great end of said combustor such that at least some of potential for fundamental research and for packaged, said light is absorbed by said species of combustion user-friendly instruments, including flight instrumenta- by-products; tion. 30 absorption lines recording means for recording ab- As described above, the primary flow field parame- sorption line information and generating an absorp- ters of interest are mass density and velocity. Tech- tion line output signal; niques for monitoring velocity and mass flow rate in- velocity frame reference means for generating a ref- clude utilizing absorption effects producing line spectra erence frame output signal; and and absorption prior to fluorescence. In light of the 35 velocity comparison means for receiving said absorp- foregoing, it is clear that the general object of the pres- tion line output signal and said reference frame ent invention to provide an apparatus and method for output signal and providing a signal representing non-intrusively and accurately measuring the thrust of a said axial velocity. jet engine has been met. Moreover, the object of the 5. A system as recited in claim 4 wherein said laser present invention to provide a single apparatus which 40 means comprises an argon ion pumped ring dye laser. measures both exit velocity and gas density, virtually 6. A system as recited in claim 4 wherein said laser simultaneously, to obtain a thrust measurement has been means comprises a semiconductor laser. met. Also, the object of the present invention to avoid 7. A system as recited in claim 4 wherein said velocity the use of tracers or particles in thrust detection has comparison means comprises means for calculating the been met as has the object to obtain reliable in-flight 45 Doppler shift of said absorption lines recorded by said thrust measurements. Furthermore, the object to obtain absorption lines recording means with respect to said spatially resolved thrust measurements has been met. velocity reference frame means. While the invention has been shown and described in 8. A system as recited in claim 3 wherein said axial what is presently conceived to be the most practical and velocity measurement means comprises: preferred embodiment of the invention, it will become 50 laser means in a cross beam configuration operating apparent to those of ordinary skill in the art that many to induce fluorescence in the combustion by-pro- modifications thereof may be within the scope of the ducts; invention, which scope is to be accorded the broadest means for monitoring two slightly separated volumes interpretation of the claims so as to encompass all equiv- of said fluoresced combustion by-products and alent structures and devices. 55 generating first and second output signals; I claim: wherein said first output simal is representative of a 1. A system for non-intrusively determining the reference volume &d wierein said second output thrust of a jet combustor apparatus having an exit end signal is representative of a Doppler-shift volume; wherein combustible gas by-products are released into and the ambient environment, comprising: 60 velocity comparison means for receiving each of two axial velocity measurement means for measuring the said output signals and providing a signal repre- axial velocity of at least one species of said combus- senting said axial velocity. tion by-products and generating an axial velocity 9. A system as recited in claim 3 wherein said density output signal; measurement means comprises: mass flow rate measurement means for measuring the 65 laser means for generating light directed at said exit mass flow rate of said species of combustion by- end of said combustor apparatus such that some of products and generating a mass flow rate output said light passes through said combustion by-pro- signal; ducts; 5,178,02 9 10 transmitted intensity measurement means for receiv- 16. A method as recited in claim 15 wherein said mass ing and measuring the intensity of said light which flow rate measuring Step comprises: passes through said combustion by-products and measuring the density Of said Species Of combustion by-products and providing a density outpui signal; generating an transmitted intensity output signal; incident intensity reference means for generating an measuring the axial Of said species Of com- bustion by-products and providing an axial veloc- incident intensity reference output signal; and ity output signal; intensity comparison means for receiving said trans- said density output signal with =id axial mitted intensity output signal and said incident velocity output signal to provide a signal represent- intensity reference output signal and providing a 10 ing said flow rate. density output signal. 17. A method as recited in claim 16 wherein said axial 10. A system as recited in claim 9 wherein said inten- velocity measuring step comprises: sity comparison means comprises the means for calcu- operating a laser at a wavelength corresponding to an lating the ratio of said intensity output signal and said absorption spectrum having absorption lines of said intensity reference output signal. 15 species of combustible gas by-products; 11. A system as recited in claim 3 wherein said veloc- generating light directed at said exit end of said com- bustor such that at least some of said light is ab- ity reference frame means comprises a static gas sample. sorbed by said species of combustion by-products; 12. A system as recited in claim 3 wherein said veloc- recording absorption line information and generating ity reference frame means comprises a calibration tech- 20 an absorption line output signal; nique of said velocity comparison means. generating a reference frame output signal; and 13. A system as recited in claim 3 wherein said signal calculating the Doppler shift in said absorption lines representing said mass flow rate is the product of said with respect to said reference frame from said absorp- density output signal and said axial velocity signal. tion line output signal and said reference frame output 14. A system as recited in claim 1 wherein said signal 25 signal to provide a signal representing said axial veloc- representing said thrust is the product of said axial ve- ity. locity output signal and said mass flow rate output sig- 18. A method as recited in claim 16 wherein said nal. density measuring step comprises: 15. A method for non-intrusively determining the operating a laser and generating light directed at said thrust of a jet combustor apparatus having an exit end 3O exit end of said combustor apparatus such that some of said light passes through said combustion wherein combustible gas by-products are released into by-products; the ambient environment, comprising the steps of: receiving and measuring the transmitted intensity of measuring the axial velocity of at least one species said light which passes through said combustion said by-products and generating an 35 by-products and generating an intensity output axial velocity output signal; signal; measuring the flow rate Of said species of generating an incident intensity reference output sig- bustion by-products and providing a mass flow rate nal; and output signal; calculating the ratio of said transmitted intensity out- multiplying said axial velocity output signal with said 40 put signal and said incident intensity reference mass flow rate output signal to provide a signal output signal to provide a density output signal. representing said thrust. * * * * * 45 50 55 65