Editing Scramjet (section)

==Theory==
All scramjet engines have an intake which compresses the incoming air, fuel injectors, a combustion chamber, and a divergent [[propulsive nozzle|thrust nozzle]]. Sometimes engines also include a region which acts as a [[flame holder]], although the high [[stagnation temperature]]s mean that an area of focused waves may be used, rather than a discrete engine part as seen in turbine engines.  Other engines use [[pyrophoric]] fuel additives, such as [[silane]], to avoid flameout.  An isolator between the inlet and combustion chamber is often included to improve the homogeneity of the flow in the combustor and to extend the operating range of the engine.

[[Shock wave|Shockwave]] imaging by the University of Maryland using [[Schlieren imaging]] determined that the fuel mixture controls compression by creating backpressure and shockwaves that slow and compress the air before ignition, much like the shock cone of a Ramjet. The imaging showed that the higher the fuel flow and combustion, the more shockwaves formed ahead of the combustor, which slowed and compressed the air before ignition.<ref>Analysis of Ignition Process in a Scramjet at Low and High Fueling Rates, Gareth Dunlap, Elias Fekadu, Ben Grove, Nick Gabsa, Kenneth Yu, Camilo Munoz, Jason Burr.</ref>

[[File:X-43A (Hyper - X) Mach 7 computational fluid dynamic (CFD).jpg|thumb|upright=1.15|[[Computational fluid dynamics]] (CFD) image of the [[NASA]] [[X-43A]] with scramjet attached to the underside at [[Mach number|Mach]]&nbsp;7|alt=Computer-generated image of stress and shock-waves experienced by aerial vehicle travelling at high speed ]]

A scramjet is reminiscent of a [[ramjet]]. In a typical ramjet, the supersonic inflow of the engine is decelerated at the inlet to subsonic speeds and then reaccelerated through a nozzle to supersonic speeds to produce thrust. This deceleration, which is produced by a normal [[shock wave|shock]], creates a total [[pressure]] loss which limits the upper operating point of a ramjet engine.

For a scramjet, the kinetic energy of the freestream air entering the scramjet engine is comparable to the energy released by the reaction of the oxygen content of the air with a fuel (e.g. hydrogen). Thus the heat released from combustion at Mach{{nbsp}}2.5<!-- changed this from a clearly erroneous "Mach 25"; assumed it was just an omitted decimal point --> is around 10% of the total enthalpy of the working fluid. Depending on the fuel, the [[kinetic energy]] of the air and the potential combustion heat release will be equal at around Mach{{nbsp}}8. Thus the design of a scramjet engine is as much about minimizing drag as maximizing thrust.

This high speed makes the control of the flow within the combustion chamber more difficult. Since the flow is supersonic, no downstream influence propagates within the freestream of the combustion chamber. Throttling of the entrance to the thrust nozzle is not a usable control technique. In effect, a block of gas entering the combustion chamber must mix with fuel and have sufficient time for initiation and reaction, all the while traveling supersonically through the combustion chamber, before the burned gas is expanded through the thrust nozzle. This places stringent requirements on the pressure and temperature of the flow, and requires that the fuel injection and mixing be extremely efficient. Usable [[dynamic pressure]]s lie in the range {{convert|20|to|200|kPa|psi}}, where

:<math>q = \frac{1}{2}\rho v^2 </math>

where
:''q'' is the dynamic pressure of the gas
:''ρ'' ([[rho (letter)|rho]]) is the [[density]] of the gas
:''v'' is the [[velocity]] of the gas

To keep the combustion rate of the fuel constant, the pressure and temperature in the engine must also be constant. This is problematic because the airflow control systems that would facilitate this are not physically possible in a scramjet launch vehicle due to the speed and altitude range involved, meaning that it must travel at an altitude specific to its speed. Because air density reduces at higher altitudes, a scramjet must climb at a specific rate as it accelerates to maintain a constant air pressure at the intake. This optimal climb/descent profile is called a "constant dynamic pressure path". It is thought that scramjets might be operable up to an altitude of 75&nbsp;km.<ref name="OrbitalVector">{{cite web |url=http://www.orbitalvector.com/Orbital%20Travel/Scramjets/Scramjets.htm |title=Scramjets |url-status=live |archive-url=https://web.archive.org/web/20160212164212/http://www.orbitalvector.com/Orbital%20Travel/Scramjets/Scramjets.htm |archive-date=12 February 2016 |access-date=12 February 2016 }}</ref>

Fuel injection and management is also potentially complex. One possibility would be that the fuel be pressurized to 100 bar by a turbo pump, heated by the fuselage, sent through the turbine and accelerated to higher speeds than the air by a nozzle. The air and fuel stream are crossed in a comb-like structure, which generates a large interface. Turbulence due to the higher speed of the fuel leads to additional mixing. Complex fuels like kerosene need a long engine to complete combustion.

The minimum Mach number at which a scramjet can operate is limited by the fact that the compressed flow must be hot enough to burn the fuel, and have pressure high enough that the reaction be finished before the air moves out the back of the engine. Additionally, to be called a scramjet, the compressed flow must still be supersonic after combustion. Here two limits must be observed: First, since when a supersonic flow is compressed it slows down, the level of compression must be low enough (or the initial speed high enough) not to slow the gas below Mach{{nbsp}}1. If the gas within a scramjet goes below Mach{{nbsp}}1 the engine will "choke", transitioning to subsonic flow in the combustion chamber. This effect is well known amongst experimenters on scramjets since the waves caused by choking are easily observable. Additionally, the sudden increase in pressure and temperature in the engine can lead to an acceleration of the combustion, leading to the combustion chamber exploding.

Second, the heating of the gas by combustion causes the speed of sound in the gas to increase (and the Mach number to decrease) even though the gas is still travelling at the same speed. Forcing the speed of air flow in the combustion chamber under Mach{{nbsp}}1 in this way is called "thermal choking". It is clear that a pure scramjet can operate at Mach numbers of 6–8,<ref name="N96-11688">{{cite journal |last1=Paull |first1=A. |last2=Stalker |first2=R. J. |last3=Mee |first3=D. J. |title=Supersonic Combustion Ramjet Propulsion Experiments In a Shock Tunnel |journal=Shock Tunnel Studies of Scramjet Phenomena 1994 |publisher=[[University of Queensland]] |date=1 January 1995 |hdl=2060/19960001680 }}</ref> but in the lower limit, it depends on the definition of a scramjet. There are engine designs where a ramjet transforms into a scramjet over the Mach{{nbsp}}3–6 range, known as dual-mode scramjets.<ref name="AIAA-99-4848">{{cite conference |last1=Voland |first1=R. T. |last2=Auslender |first2=A. H. |last3=Smart |first3=M. K. |last4=Roudakov |first4=A. S. |last5=Semenov |first5=V. L. |last6=Kopchenov |first6=V. |title=CIAM/NASA Mach 6.5 Scramjet Flight and Ground Test |conference=9th International Space Planes and Hypersonic Systems and Technologies Conference |publisher=[[AIAA]] |place=[[Norfolk, Virginia]] |year=1999 |doi=10.2514/MHYTASP99 |hdl=2060/20040087160 }}</ref> In this range however, the engine is still receiving significant thrust from subsonic combustion of the ramjet type.

The high cost of flight testing and the unavailability of ground facilities have hindered scramjet development. A large amount of the experimental work on scramjets has been undertaken in cryogenic facilities, direct-connect tests, or burners, each of which simulates one aspect of the engine operation. Further, vitiated facilities (with the ability to control air impurities<ref name="UoV Ground Testing">{{cite web |url=http://www.mae.virginia.edu/HyV/groundtesting.htm |title=The Hy-V Program – Ground Testing |work=Research |publisher=[[University of Virginia]] |url-status=live |archive-url=https://web.archive.org/web/20160212174005/http://www.mae.virginia.edu/HyV/groundtesting.htm |archive-date=12 February 2016 |access-date=12 February 2016 }}</ref>), storage heated facilities, arc facilities and the various types of shock tunnels each have limitations which have prevented perfect simulation of scramjet operation. The [[HyShot]] flight test showed the relevance of the 1:1 simulation of conditions in the T4 and HEG shock tunnels, despite having cold models and a short test time. The [[NASA]]-CIAM tests provided similar verification for CIAM's C-16 V/K facility and the Hyper-X project is expected to provide similar verification for the Langley AHSTF,<ref>{{cite web |url=http://wte.larc.nasa.gov/facilities/hypersonic/arc-heated.cfm?field=11&id=2&fac=1 |title=Arc-Heated Scramjet Test Facility |publisher=[[NASA Langley Research Center]] |date=17 November 2005 |archive-url=https://web.archive.org/web/20101024014047/http://wte.larc.nasa.gov/facilities/hypersonic/arc-heated.cfm?field=11&id=2&fac=1 |archive-date=24 October 2010 |access-date=18 August 2009 }}</ref> CHSTF,<ref name="Langley HSTF">{{cite web |url=http://wte.larc.nasa.gov/facilities/hypersonic/combustion.cfm?field=12&id=2&fac=1 |title=Combustion-Heated Scramjet Test Facility |publisher=[[NASA Langley Research Center]] |date=17 November 2005 |archive-url=https://web.archive.org/web/20101024014107/http://wte.larc.nasa.gov/facilities/hypersonic/combustion.cfm?field=12&id=2&fac=1 |archive-date=24 October 2010 |access-date=12 February 2016 }}</ref> and {{Convert|8|ft|m|1|abbr=on}} HTT.

[[Computational fluid dynamics]] has only recently{{hsp}}{{When|date=September 2011}} reached a position to make reasonable computations in solving scramjet operation problems. Boundary layer modeling, turbulent mixing, two-phase flow, flow separation, and real-gas aerothermodynamics continue to be problems on the cutting edge of CFD. Additionally, the modeling of kinetic-limited combustion with very fast-reacting species such as hydrogen makes severe demands on computing resources.<ref>{{Cite journal |last1=Guan |first1=Xingyi |last2=Das |first2=Akshaya |last3=Stein |first3=Christopher J. |last4=Heidar-Zadeh |first4=Farnaz |last5=Bertels |first5=Luke |last6=Liu |first6=Meili |last7=Haghighatlari |first7=Mojtaba |last8=Li |first8=Jie |last9=Zhang |first9=Oufan |last10=Hao |first10=Hongxia |last11=Leven |first11=Itai |last12=Head-Gordon |first12=Martin |last13=Head-Gordon |first13=Teresa |date=2022-05-17 |title=A benchmark dataset for Hydrogen Combustion |journal=Scientific Data |language=en |volume=9 |issue=1 |page=215 |doi=10.1038/s41597-022-01330-5 |pmid=35581204 |pmc=9114378 |bibcode=2022NatSD...9..215G |issn=2052-4463}}</ref>
Reaction schemes are [[Stiff equation|numerically stiff]] requiring reduced reaction schemes.{{Clarify|date=September 2011}}

Much of scramjet experimentation remains [[Classified information|classified]]. Several groups, including the [[United States Navy|US Navy]] with the SCRAM engine between 1968 and 1974, and the [[Hyper-X]] program with the [[NASA X-43|X-43A]], have claimed successful demonstrations of scramjet technology. Since these results have not been published openly, they remain unverified and a final design method of scramjet engines still does not exist.

The final application of a scramjet engine is likely to be in conjunction with engines which can operate outside the scramjet's operating range.{{Citation needed|date=September 2011}}
Dual-mode scramjets combine [[Speed of sound|subsonic]] combustion with [[supersonic]] combustion for operation at lower speeds, and [[rocket]]-based combined cycle (RBCC) engines supplement a traditional rocket's propulsion with a scramjet, allowing for additional [[oxidizer]] to be added to the scramjet flow. RBCCs offer a possibility to extend a scramjet's operating range to higher speeds or lower intake dynamic pressures than would otherwise be possible.