Editing Liquid air cycle engine (section)

==Advantages and disadvantages==

The use of a winged launch vehicle allows using [[lift (force)|lift]] rather than [[thrust]] to overcome gravity, which greatly reduces [[gravity drag|gravity losses.]] On the other hand, the reduced gravity losses come at the price of much higher [[aerodynamic drag]] and [[aerodynamic heating]] due to the need to stay much deeper within the atmosphere than a pure rocket would during the [[boost phase]].

In order to appreciably reduce the mass of the oxygen carried at launch, a LACE vehicle needs to spend more time in the lower atmosphere to collect enough oxygen to supply the engines during the remainder of the launch. This leads to greatly increased vehicle heating and drag losses, which therefore increases fuel consumption to offset the drag losses and the additional mass of the [[atmospheric reentry#thermal protection systems|thermal protection system]]. This increased fuel consumption offsets somewhat the savings in oxidizer mass; these losses are in turn offset by the higher [[specific impulse]], ''I''{{sub|sp}}, of the air-breathing engine. Thus, the engineering trade-offs involved are quite complex, and highly sensitive to the design assumptions made.<ref>
{{ cite book
 | first = Benjamin
 | last  = Orloff
 | title = A Comparative Analysis of Singe-State-To-Orbit Rocket and Air-Breathing Vehicles
 | url   = https://apps.dtic.mil/sti/pdfs/ADA453934.pdf
 | archive-url = https://web.archive.org/web/20110604220820/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA453934&Location=U2&doc=GetTRDoc.pdf
 | url-status = live
 | archive-date = June 4, 2011
 | id    = AFIT/GAE/ENY/06-J13 
}}</ref>

Other issues are introduced by the relative material and logistical properties of [[LOx]] versus [[LH2|LH{{sub|2}}]]. LOx is quite cheap; LH{{sub|2}} is nearly two orders of magnitude more expensive.<ref>
{{ cite web
 | url = http://www.astronautix.com/props/loxlh2.htm
 | archive-url = https://web.archive.org/web/20020313135834/http://www.astronautix.com/props/loxlh2.htm
 | url-status = dead
 | archive-date = March 13, 2002
 | title = LOX/LH2: Properties and Prices
}}</ref> LOx is dense (1.141&nbsp;kg/L), whereas LH{{sub|2}} has a very low density (0.0678&nbsp;kg/L) and is therefore very bulky. (The extreme bulkiness of the LH2 tankage tends to increase vehicle drag by increasing the vehicle's [[drag equation|frontal area]].) Finally, LOx tanks are relatively lightweight and fairly cheap, while the deep cryogenic nature and extreme physical properties of LH{{sub|2}} mandate that LH{{sub|2}} tanks and plumbing must be large and use heavy, expensive, exotic materials and insulation. Hence, much as the costs of using LH{{sub|2}} rather than a [[rocket propellant#Propellant density|hydrocarbon fuel]] may well outweigh the ''I''{{sub|sp}} benefit of using LH{{sub|2}} in a [[single-stage-to-orbit#Dense versus hydrogen fuels|single-stage-to-orbit rocket]], the costs of using more LH{{sub|2}} as a propellant and air-liquefaction coolant in LACE may well outweigh the benefits gained by not needing to carry as much LOx on board.

Most significantly, the LACE system is far heavier than a pure rocket engine having the same thrust (air-breathing engines of [[Reaction Engines SABRE|almost all]] types have relatively poor [[thrust-to-weight ratio#Examples|thrust-to-weight ratios]] compared to rockets), and the performance of launch vehicles of all types is particularly affected by increases in vehicle dry mass (such as engines) that must be carried all the way to orbit, as opposed to oxidizer mass that would be burnt off over the course of the flight. Moreover, the lower thrust-to-weight ratio of an air-breathing engine as compared to a rocket significantly decreases the launch vehicle's maximum possible acceleration, and increases [[gravity drag|gravity losses]] since more time must be spent to accelerate to orbital velocity. Also, the higher [[parasitic drag|inlet and airframe drag losses]] of a lifting, air-breathing vehicle launch trajectory as compared to a pure rocket on a [[Gravity turn|ballistic launch trajectory]] introduces an additional penalty term <math>\frac {1} {1 + \frac {gD} {aL}}</math> into the [[rocket equation]] known as the ''air-breather's burden''.<ref>
{{ Cite web
 | url   = http://www.islandone.org/Propulsion/SCRAM-Spencer1.html
 | title = Liquid Air Cycle Rocket Equation, Henry Spencer Comment
}}</ref> This term implies that unless the [[lift-to-drag ratio]] (''L''/''D'') and the acceleration of the vehicle as compared to gravity (''a''/''g'') are both implausibly large for a [[hypersonic]] air-breathing vehicle, the advantages of the higher ''I{{sub|sp}}'' of the air-breathing engine and the savings in LOx mass are largely lost.

Thus, the advantages, or disadvantages, of the LACE design continue to be a matter of some debate.