LEE%SU-STAR@sri-unix.UUCP (05/13/84)
Indeed, a scramjet might be very interestingas part of a hybrid propulsion system. They could be used during the supersonic/hypersonic atmospheric flight phase of a Shuttle-type vehicle ascent to orbit. Unfortunately, according to Ben Rich, the new boss of Lockheed's Skunk Works, a scramjet has yet to achieve any net positive thrust.(note that this was a public statemet) Any propulsion people out there willing to give an opinion? Emilio P. Calius Dept. of Aero & Astro Stanford ------
BRUC%MIT-MC@sri-unix.UUCP (07/04/84)
From: Robert E. Bruccoleri <BRUC @ MIT-MC> The idea behind a scramjet is that the flow of air through the combustion chamber is supersonic. Even in turbojets which operate at supersonic speeds, the air flow in the combustion chamber is subsonic. The air flow is slowed by the inlet and the compressor; after combustion, the heating then accelerates the air back up to supersonic speeds. The reason this distinction is important is that an ordinary flame cannot be maintained in supersonic flow (in effect, the molecules are moving too fast for any reaction to propogate). All I've heard beyond this is that no scramjet has generated more thrust than its own drag, presumably because the inlets and flame holders obstruct the flow. After hearing about this problem, I had a thought which I'd like those reading this digest who really know something about it to criticize. Since the propogation of combustion requires a chain reaction which continually maintains free radicals of oxygen and of the fuel, perhaps one could use an continuous ultraviolet laser operating at a frequency corresponding to the dissociation energy of one of the electrons on either oxygen or the fuel. This laser would maintain a population of radicals that would maintain combustion. Bob Bruccoleri (BRUC@MIT-MC)
rivero@kovacs.UUCP (07/15/84)
Regarding Scramjets
When the airflow inside any airbreathing engine, either
turbine or ramjet, exceeds the speed of sound, the interior of
the engine become filled with shockwaves produced by anything
protuding into the airstream. This includes the fuel
injection mechanism.
When fuel is pumped into a supersonic airstream, it creates
another shock wave, trapping the air on one side, and the fuel
on the other. Hence, no mixing occurs, and no combustion. The
internal airflow snuffs it out.
All attempts to create a supersonic combustion ramjet
involve first slowing the airflow enough to permit mixing
(which includes dealing with the airflows mass and inertia).
This means that the engine must produce enough thrust to move
the airflow back up to speed, and still move the aircraft.
Has anyone considered the old "pulse jet" approach?
Mike Riveroal@vger.UUCP ( Informatix) (02/19/86)
A while back there was a discussion of scramjets on the net. Unfortunately, I wasn't paying much attention at the time. My impression of them is that they use atmospheric oxygen to make major reductions in weight for most of the launch phase. Is this true? 2. If the 'orient express' can get Washington -> Tokyo flights for $5-6,000 then you can get to LEO for about the same amount. How many people do you know that would spend $10,000 for a week or weekend in orbit? If the tourist thing ever gets cheap enough there will be so much space development it'll make our heads spin ...
steve@jplgodo.UUCP (Steve Schlaifer x3171 156/224) (02/24/86)
Essentially, a scramjet is a ramjet designed to operate at much higher speed (I have heard numbers like Mach 12 to Mach 25). If they can be made to get up to Mach 25 then they will have achieved orbital velocity. The big advantage is that they don't have to carry an oxidizer for the fuel (they use atmospheric oxygen). There was an interesting article in a recent issue of High Technology about them and the current work being done. Maybe you could find it in a local library. -- ...smeagol\ Steve Schlaifer ......wlbr->!jplgodo!steve Advance Projects Group, Jet Propulsion Labs ....group3/ 4800 Oak Grove Drive, M/S 156/204 Pasadena, California, 91109 +1 818 354 3171
kwan@smeagol.UUCP (Richard Kwan) (02/25/86)
> A while back there was a discussion of scramjets on the net. Unfortunately, > I wasn't paying much attention at the time. My impression of them is > that they use atmospheric oxygen to make major reductions in weight > for most of the launch phase. Is this true? I wasn't on the net when that discussion took place, and since I haven't seen much comment yet, I'm gonna stick my neck out a bit. (...attempting to revitalize old memory cells... sputter.. reboot.....) Many years ago (197X?), I attempted to study various types of propulsion technology. As I remember, there are a couple significant parameters in picking your engine type. 1. Specific impulse (Isp): thrust per pound of propellant. At least, that's the way I learned it, a carry over from non-metric engineering. (Thrust per unit mass is probably more meaningful.) Propellant naturally includes both fuel and oxidizer. You are correct that in the case of air breathers, they get their oxidizer from the atmosphere. Thus, their Isp's are higher. Rockets tend to have Isp's in the low 100's; turbojets in the 3000's (?), and ramjets somewhere in between. 2. Engine thrust-to-weight (T/We) ratio: thrust per pound of engine. (How about thrust(newtons)/engine-mass(kilograms), T/Me?) As I remember, turbojets were around 6 (T/We), ramjets higher, rockets ... well, way up there. Thus, although jet engines give you much higher Isp's than rockets, they also require a lot more massive machinery to function. Of course, turbojets are much more massive than ramjets. The problem with ramjets is that you need something else to get them up to a functioning velocity, i.e., rocket (lots of extra propellant) or turbojet (lots of extra machinery). Scramjets need an even higher startup velocity than most ramjets. I would presume greater than Mach 1 (unless there is some mixed mode tricks that can be played; any propulsion scientists care to comment?). Given the engine mass, you get into the rocket/scramjet tradeoff area. ...and then, there are proposals for turbo-ram-rockets... -- Rick Kwan JPL Spacecraft Data Systems sdcrdcf!smeagol!kwan (UUCP) ia-sun2!smeagol!kwan@csvax.caltech.EDU (ARPA) -------------------------------------------------------------------- "...jumpin' into hyperspace ain't like dustin' crops, boy." H. Solo --------------------------------------------------------------------
karn@petrus.UUCP (Phil R. Karn) (02/28/86)
> 1. Specific impulse (Isp): thrust per pound of propellant. At least, > that's the way I learned it, a carry over from non-metric > engineering. (Thrust per unit mass is probably more meaningful.) > Propellant naturally includes both fuel and oxidizer. You are > correct that in the case of air breathers, they get their oxidizer > from the atmosphere. Thus, their Isp's are higher. Rockets tend > to have Isp's in the low 100's; turbojets in the 3000's (?), and > ramjets somewhere in between.... I have several problems with this. Specific impulse is often erroneously specified in "seconds"; the correct units should be "meters/sec", i.e., velocity. The error occurs because Isp is usually defined in English units as pounds-force of thrust x seconds -------------------------------- pounds-mass of propellant and somebody made the mistake of "cancelling out" the pounds-force factor with the pounds-mass factor. A good example of how the English system of measurements befuddles thinking, but I digress... In metric units, things are much clearer: newtons of thrust x seconds --------------------------- kilograms of propellant Since a newton is the force required to accelerate 1 kg by 1 meter/sec^2, it has dimensions Kg-m/sec^2. When the other factors are included, this all reduces to meters/second. This way of expressing specific impulse has a much more elegant and straightforward meaning: it is simply the velocity of the rocket exhaust relative to the rocket. The faster the exhaust, the higher the specific impulse and the less mass (i.e., propellant) that must be ejected to gain a specified impulse (momentum). Since momentum is simply mass times velocity, this is a linear relationship. You only need half as much propellant mass if you kick it out twice as fast. However, the energy that must be imparted to the exhaust increases as the SQUARE of the exhaust velocity (the kinetic energy of the exhaust is 1/2 m v^2). If as a measure of the "energy efficiency" of a rocket you divide the energy imparted to the exhaust by the impulse obtained, you get: energy = 1/2 mass x velocity^2 ==> 1/2 x velocity ------- --------------------- impulse = mass x velocity This means that the amount of power required to sustain a given amount of thrust goes up linearly with exhaust velocity (i.e. specific impulse). This is why people don't generally use rocket motors to propel automobiles. If to cruise down the road at a nice legal 55 mph you need X newtons of "thrust" to balance air and road drag, it is much more energy efficient to do this by exerting a force of X newtons against the road at 55 mph than it is to push with the same force against a stream of hot gases traveling at several thousand meters per second. Similarly with airplanes, it is much more efficient to scoop up as much of the air mass around you and push on that than it is to push solely on the combustion products of your engine. So what this says is that for anything other than spacecraft, where you're not surrounded by something you can grab and push on, you want the LOWEST specific impulse you can attain. Hence propellers and high-bypass turbojets are more fuel-efficient than low bypass jets or rocket engines for air travel. It's not clear to me that "specific impulse" has any meaning, though, for an air-breathing (and air-pushing) aircraft, nor for an automobile. With chemical rockets, the combustion products of the reaction that produces energy are used as the ejection mass on which the rocket "pushes". This means that the specific impulse of a chemical rocket is theoretically determined by the propellants' energy density, i.e., joules per kilogram. (I've neglected some other effects here such as the molecular weight of the combustion products and other, non-useful ways that the combustion energy is dissipated, but suffice to say that there is a theoretical exhaust velocity associated with each propellant combination.) Unlike airplanes and cars, spacecraft must carry all their reaction mass with them. Since work must be done to carry this mass to the point where it is finally ejected, for any specified total delta-vee there is an OPTIMUM specific impulse if your goal is to minimize energy requirements. Below this point less power is needed to generate each unit of thrust, but this is outweighed by the extra thrust (and power) needed to loft the extra ejection mass required. On the other hand, above this point you can carry less reaction mass, but the extra energy required to eject it at the higher velocity more than counteracts the savings in lofting propellant mass. So why do rocket designers always seem to be striving for higher specific impulse? One reason is that other considerations besides energy efficiency are important. Rockets are mechanically easier to build if they have lower fuel-to-payload mass ratios; in particular, fewer stages may be needed. The other reason is that in most situations, chemical rocket propellants always seem to have less than the optimum specific impulse, so an increase is almost always desirable. If you go away from chemical rockets, however, the rocket's energy no longer need be stored in its reaction mass. For example, in a nuclear rocket engine energy from a nuclear reactor is applied it to an inert (for the purposes of thrust) material such as hydrogen gas. It is then possible to vary the specific impulse of the engine as an operating parameter. If you want more specific impulse, feed less mass to your reactor (operating at a constant power level), or alternatively, crank up the reactor while feeding it mass at a constant rate. Either causes the mass to be ejected at a higher velocity, increasing specific impulse (and the amount of power required for each unit of thrust). Other engines in which this is possible include the ion engine, the plasma engine and the electrothermal thruster. In many cases, the engine has to be operated at a LOWER specific impulse than it is capable of because it is easier to carry additional reaction mass than additional energy for accelerating it. Unfortunately, all of these non-chemical engines, with the exception of the nuclear engine, are currently incapable of generating enough thrust to overcome their weight; they are useful only in space when you've got plenty of time to accumulate momentum. Phil
kwan@smeagol.UUCP (Richard Kwan) (03/05/86)
In <617@smeagol.UUCP> I originally said... > > 1. Specific impulse (Isp): thrust per pound of propellant. At least, > > that's the way I learned it, a carry over from non-metric > > engineering. (Thrust per unit mass is probably more meaningful.) > > Propellant naturally includes both fuel and oxidizer. You are > > correct that in the case of air breathers, they get their oxidizer > > from the atmosphere. Thus, their Isp's are higher. Rockets tend > > to have Isp's in the low 100's; turbojets in the 3000's (?), and > > ramjets somewhere in between.... And in <34@petrus.UUCP>, Phil Karn responded: > I have several problems with this. With good reason. I blew it. > Specific impulse is often erroneously specified in "seconds"; the correct > units should be "meters/sec", i.e., velocity. I don't know what got into me. You are correct. "Seconds" is the accepted units in the English system. > This way of expressing specific impulse has a much more elegant and > straightforward meaning: it is simply the velocity of the rocket exhaust > relative to the rocket. Some other classical examples: How much thrust do you get if you burn one pound of propellant for one second? Or, how many seconds can you burn one pound of propellant if you maintain one pound of thrust? Hence, we get pound(thrust)-seconds per pound of pound of propellent. The numerator is impulse; thus, the per pound measure is termed "specific impulse." But as you say, the metric version, meters/sec, is probably clearer. > ... It's not clear to me that "specific impulse" has any meaning, > though, for an air-breathing (and air-pushing) aircraft, nor for an > automobile. Perhaps so. Some clarifications are in order. 1. All the figures I gave for specific impulse were in seconds. I have not worked with the metric form. 2. Certainly for air breathers, the velocity analog does not work. The figures I gave for air breathers are for thrust x time / *fuel*. Thus, the unusually high performance rating is due to not carrying oxidizer. (Not my idea; sorry, can't remember the source.) By the way, Hank Walker <dmw@UNH.CS.CMU.EDU> reminded me that there IS such a thing as a variable geometry engine. He points out: ...you can convert a scramjet to a ramjet, and perhaps start as low as Mach 0.5. Scramjets are really only useful above Mach 5... You propulsion specialists can take it from here. -- Rick Kwan JPL Spacecraft Data Systems sdcrdcf!smeagol!kwan (UUCP) ia-sun2!smeagol!kwan@csvax.caltech.EDU (ARPA) -------------------------------------------------------------------- "...jumpin' into hyperspace ain't like dustin' crops, boy." H. Solo --------------------------------------------------------------------
ted@jplgodo.UUCP (Ted Sweetser x4989 156/224) (03/06/86)
In article <623@smeagol.UUCP>, kwan@smeagol.UUCP (Richard Kwan) writes: > And in <34@petrus.UUCP>, Phil Karn responded: > > Specific impulse is often erroneously specified in "seconds"; the correct > > units should be "meters/sec", i.e., velocity. > I don't know what got into me. You are correct. "Seconds" is the > accepted units in the English system. Wait a minute, I don't think Mr. Karn *is* right. The best definition of specific impulse (Isp) is "thrust / (weight of propellent mixture used per unit time)". With this definition the units for Isp are seconds in both the metric and English systems and no conversion factor is needed for Isp between the two systems. Furthermore, this definition is needed to make the following form of the rocket equation: mass ratio = exp(-(delta-V)/(g*Isp)) work in any consistent system of units; if you use a "meter/second" Isp then you have to use a different form of the rocket equation in the metric system. A short history of the term can be found in "Comment on 'Definition of Specific Impulse'", _J._Spacecraft_, vol.12(1975), no.9, p.576, by Alfred Africano, one of the originators of the concept. Unfortunately, textbook writers have been consistently inconsistent on Isp. Ted Sweetser (...smeagol!jplgodo!ted)
kwan@smeagol.UUCP (Richard Kwan) (03/06/86)
In article <739@jplgodo.UUCP>, ted@jplgodo.UUCP (Ted Sweetser x4989 156/224) writes: > In article <623@smeagol.UUCP>, kwan@smeagol.UUCP (Richard Kwan) writes: > > And in <34@petrus.UUCP>, Phil Karn responded: > > > Specific impulse is often erroneously specified in "seconds"; the correct > > > units should be "meters/sec", i.e., velocity. > > I don't know what got into me. You are correct. "Seconds" is the > > accepted units in the English system. > > Wait a minute, I don't think Mr. Karn *is* right. The best definition of > specific impulse (Isp) is "thrust / (weight of propellent mixture used per > unit time)". With this definition the units for Isp are seconds in both the > metric and English systems ... Furthermore, this definition is needed to make > the following form of the rocket equation: > mass ratio = exp(-(delta-V)/(g*Isp)) > work in any consistent system of units; if you use a "meter/second" Isp > then you have to use a different form of the rocket equation in the metric > system. Yes, but not by much. If you assume that exhaust-V = g*Isp since 'g' is a constant, you get mass ratio = exp(-(delta-V)/(exhaust-V)) Now, of my reading from the ancient past, I only recall once seeing this relationship between Isp and exhaust velocity; so I don't know how widely accepted this is. (Furthermore, that was in my decidedly dumber days :-}) -- Rick Kwan JPL Spacecraft Data Systems sdcrdcf!smeagol!kwan (UUCP) ia-sun2!smeagol!kwan@csvax.caltech.EDU (ARPA)
dsmith@HPLABSC (David Smith) (03/06/86)
> Specific impulse is often erroneously specified in "seconds"; the correct > units should be "meters/sec", i.e., velocity. The error occurs because Isp > is usually defined in English units as > pounds-force of thrust x seconds > -------------------------------- > pounds-mass of propellant > and somebody made the mistake of "cancelling out" the pounds-force factor > with the pounds-mass factor. A good example of how the English system of > measurements befuddles thinking, but I digress... > In metric units, things are much clearer: > newtons of thrust x seconds > --------------------------- > kilograms of propellant > Since a newton is the force required to accelerate 1 kg by 1 meter/sec^2, it > has dimensions Kg-m/sec^2. When the other factors are included, this all > reduces to meters/second. > This way of expressing specific impulse has a much more elegant and > straightforward meaning: it is simply the velocity of the rocket exhaust > relative to the rocket. This is true enough, but note that the metric system is abused almost as frequently as the English system. The above English abuse is in using pounds-mass instead of slugs. The usual corresponding metric abuse is in using kilograms-force. So often we see jet and rocket engines rated in kilograms of thrust; or specific fuel consumption (inverse of specific impulse) as kilograms (mass) per kilogram (force) per hour. Phonograph needle tracking forces are rated in grams. David Smith hplabs!dsmith dsmith%hp-labs@csnet-relay