Another thing–the SABRE engine is an exception to “rocket engines are not air breathing.”
I might argue that a rocket engine must use some form of convergent-divergent nozzle, and that this pretty sharply delineates rockets from other propulsive sources like jets, but there’s probably another exception I’m missing.
Convergent-divergent nozzles can be present in jet engines, too. I’d say the “air breathing” part is the real distinction between rocket and jet engines – the SABRE isn’t really an exception, it’s a hybrid that can operate as a jet in the atmosphere and then inject LOX to continue operating beyond the atmosphere.
But you’re right, a lot of this terminology is pretty arbitrary.
But on the OP, lest the point be forgotten, the use of the term “rocket” is basically from that fact that these are simple flying pipes that have typically been powered by a mixture of melted sugar and fertilizer and topped with homemade explosives. They’re more like oversize fireworks than military missiles.
Alternatively, it’s a pure rocket bolted to an atmospheric oxygen collector. You could saw off the intake/heat exhanger, weld the pipes closed, and the remainder would be pretty similar to normal staged-combustion engine (albeit with an unusual helium loop).
Yup. An “oversize firework” is definitely a rocket, but if you add a guidance system it becomes a missile… unless it’s not used directly as a weapon, in which case it stays a rocket, or maybe turns into a launch vehicle.
And sometimes it’s a matter of context. The Start-1 is definitely a rocket and a launch vehicle, but most of the hardware is from the RT-2PM Topol ICBM. Put a nuke on it and it’s a missile; put a fourth stage and payload on it and it’s a rocket.
That page is addressing hobbyist model rocketry in which (as a previously indicated) non-standard terminology is used. However, if you talk to anyone working with solid propellant systems from any of the major manufacturers of tactical and large diameter stages (ATK, Aerojet Rocketdyne, Nammo-Talley, or the multitude of former manufacturers such as United Technologies Chemical Systems Division, Lockheed Propulsion Company, Grand Central Rocket Company, Hercules Powder Company, et cetera) the term motor is used consistently in referring to solid propellant systems, as it is in technical literature such as the AIAA Journal of Propulsion and Power.
Engines such as that proposed for but never flown or even demonstrated for the Hotol/SABRE spaceplane are referred to as combined cycle engines, which use both air breathing (ducted or “duct jet”) and rocket propulsion modes. This is not the arbitrary distinction it is made out to be; carrying the working fluid and oxidizer onboard as with a rocket engine has an enormous mass penalty associated with it in comparison to ducted propulsion where the working fluid and oxidizer are extracted from the atmosphere. A turbofan engine may have a propellant specific impulse (a measure of the theoretical efficiency in terms of thrust per unit mass of propellant expended) of 3000 to 6000 seconds at sea level whereas the best bipropellant rocket engines can’t reach 400 second at sea level and struggle to exceed 450 seconds in a vacuum at orbital speed. Of course, an air-breathing engine won’t take you into space, or indeed, into the upper atmosphere where air is scarce, nor will it function at very high Mach (M>5) airspeeds without an unreasonably long chamber or high complexity in slowing airflow down to a workable speed. The distinction between air-breathing/ducted propulsion and rocket propulsion is made clear in every propulsion textbook and reference book I have ever seen; for instance, Sutton’s Rocket Propulsion Elements immediately introduces ducted propulsion and rocket propulsion in clearly delineated sections and describes the differences between them, as does Hill and Peterson’s Mechanics and Thermodynamics of Propulsion.
Axisymmetric converging-diverging nozzles (also referred to as “de Laval nozzles” after Swedish inventor Gustaf de Laval) are the most common type of nozzle used on rockets today because of their thermodynamic properties (specifically, accelerating flow to M=1 at the throat which prevents back pressure from affecting combustion but uses the available energy to compress the fluid which can be recovered as additional impulse when it expands beyond the throat) but this is hardly exclusive to the domain of rocketry; they’re also used on all high bypass turbojet and turbofan engines, and they’re commonly used in steam turbines and other compressible flow applications which benefit from heating or combustion followed by expansion (often called jet orifaces or Venturi tubes). While they are widely used on rocket motors and engines owing to the thermodynamic efficiency and thrust vector controllability they provide, they aren’t fundamentally inherent to rocket cycles, and other types of nozzles–in particular, the aerospike and plug nozzles–have been proposed and tested for chemical rockets. Ion and plasma engines don’t generally use de Laval nozzles because the high speed of the propellants would make them unreasonably long in order to gain any additional benefit even though the exhaust products are extremely hot and take considerably energy away with them. Future rocket engine technologies such as nuclear or solar electric propulsion, magnetically confined plasma propulsion, or continuous wave detonation engines, will probably not use de Laval style nozzles at all.
Stranger
Well, there’s also this page, which mentions several solid-fuel applications beyond model rockets. At any rate, my point was simply that you can’t blame the ambiguity on a few hobbyists. It’s NASA talking about model rockets, not hobbyists talking about them.
Yeah, I believe it. I looked around on ULA’s site and a couple of others, and they are consistent in calling them motors, or (more frequently) just solid boosters.
Well, sure, that’s the whole point. And I can certainly believe that certain parties make a clear distinction. My argument is just that from the air supply rearward, a combined cycle engine looks a heck of a lot like a “normal” rocket, and very different from other air-breathing engines like turbojets.
So if you treat engines as black boxes, or nodes on a Visio chart, then sure, a combined cycle engine looks more like other air-breathing engines. But mechanically it has almost nothing in common.
I suppose I should have added “where most of the thrust comes from the expansion of the gas after it passes through the constriction.” As you know, turbofans get the vast majority of their thrust from the main fan. For a turbojet… well, wolfpup pointed out that some turbojets do have them, but it seems very rare to me and I can’t find an actual exception.
Obviously I’m not counting internal components like Venturi tubes since they don’t directly contribute to the thrust.
I had those in mind when I said “convergent-divergent nozzle,” since they do in fact still converge and diverge the flow–as they must, for maximum thermodynamic efficiency. Is there a more general name for any nozzle that accelerates the flow to sonic and then expands the flow so that it performs work on the nozzle walls?
Several of those, such as the VASIMR, still have a nozzle that chokes the flow and expands it. It’s tough to build a de Laval with a magnetic field, so they come as close as they can.
No idea about continuous wave detonation engines, though.
I think the general understanding is that missiles possess internal guidance and/or computing while rockets are generally aimed but not further guided.
Also, there is a cost and complexity connotation. A warplane carrying two sophisticated radar-guided rockets costing $750,000 apiece, with sensors and onboard computing in each, and each rocket capable of many maneuvers in-flight, is likely to be described as carrying “missiles.”
A warplane carrying a hundred cheap unguided rockets costing $400 each, spraying them at the enemy at close range, is likely to be described as carrying “rockets.”
If it’s carrying Alan Shepherd, to space, it’s a rocket. If it’s got a warhead on it it’s a missile.