Editing Rocket (section)

===Mass ratios===
{{Main|Mass ratio}}
[[File:Tsiolkovsky rocket equation.svg|thumb|The Tsiolkovsky rocket equation gives a relationship between the mass ratio and the final velocity in multiples of the exhaust speed]]

Almost all of a launch vehicle's mass consists of propellant.<ref>{{cite web |url=http://www-istp.gsfc.nasa.gov/stargaze/Srockhis.htm |title=The Evolution of Rockets |publisher=Istp.gsfc.nasa.gov |access-date=2012-12-10 |url-status=dead |archive-url=https://web.archive.org/web/20130108191716/http://www-istp.gsfc.nasa.gov/stargaze/Srockhis.htm |archive-date=2013-01-08 }}</ref> Mass ratio is, for any 'burn', the ratio between the rocket's initial mass and its final mass.<ref>{{cite web |url=http://exploration.grc.nasa.gov/education/rocket/rktwtp.html |title=Rocket Mass Ratios |publisher=Exploration.grc.nasa.gov |access-date=2012-12-10 |url-status=dead |archive-url=https://web.archive.org/web/20130216063603/http://exploration.grc.nasa.gov/education/rocket/rktwtp.html |archive-date=2013-02-16 }}</ref> Everything else being equal, a high mass ratio is desirable for good performance, since it indicates that the rocket is lightweight and hence performs better, for essentially the same reasons that low weight is desirable in sports cars.

Rockets as a group have the highest [[thrust-to-weight ratio]] of any type of engine; and this helps vehicles achieve high [[mass ratio]]s, which improves the performance of flights. The higher the ratio, the less engine mass is needed to be carried. This permits the carrying of even more propellant, enormously improving the delta-v. Alternatively, some rockets such as for rescue scenarios or racing carry relatively little propellant and payload and thus need only a lightweight structure and instead achieve high accelerations. For example, the Soyuz escape system can produce 20&nbsp;''g''.<ref name=soyuzt/>

Achievable mass ratios are highly dependent on many factors such as propellant type, the design of engine the vehicle uses, structural safety margins and construction techniques.

The highest mass ratios are generally achieved with liquid rockets, and these types are usually used for [[orbital launch vehicle]]s, a situation which calls for a high delta-v. Liquid propellants generally have densities similar to water (with the notable exceptions of [[liquid hydrogen]] and [[Liquid methane rocket fuel|liquid methane]]), and these types are able to use lightweight, low pressure tanks and typically run high-performance [[turbopumps]] to force the propellant into the combustion chamber.

Some notable mass fractions are found in the following table (some aircraft are included for comparison purposes):

{| class="wikitable"
|-
! Vehicle
! Takeoff mass
! Final mass
! [[Mass ratio]]
! [[Propellant mass fraction|Mass fraction]]
|-
| [[Ariane 5]] (vehicle + payload)
| 746,000&nbsp;kg <ref name="ariane">{{cite web| url = http://astronautix.com/lvs/ariane5g.htm| archive-url = https://web.archive.org/web/20031225090030/http://www.astronautix.com/lvs/ariane5g.htm| url-status = dead| archive-date = December 25, 2003| title = Astronautix – Ariane 5g}}</ref> (~1,645,000&nbsp;lb)
| 2,700&nbsp;kg + 16,000&nbsp;kg<ref name="ariane"/> (~6,000&nbsp;lb + ~35,300&nbsp;lb)
| 39.9
| 0.975
|-
| [[Titan 23G]] first stage
| 117,020&nbsp;kg (258,000&nbsp;lb)
| 4,760&nbsp;kg (10,500&nbsp;lb)
| 24.6
| 0.959
|-
| [[Saturn V]]
| 3,038,500&nbsp;kg<ref name="saturnv">{{cite web| url = http://astronautix.com/lvs/saturnv.htm| archive-url = https://web.archive.org/web/20020228155213/http://www.astronautix.com/lvs/saturnv.htm| url-status = dead| archive-date = February 28, 2002| title = Astronautix – Saturn V}}</ref> (~6,700,000&nbsp;lb)
| 13,300&nbsp;kg + 118,000&nbsp;kg<ref name="saturnv"/> (~29,320&nbsp;lb + ~260,150&nbsp;lb)
| 23.1
| 0.957
|-
| [[Space Shuttle]] (vehicle + payload)
| 2,040,000&nbsp;kg (~4,500,000&nbsp;lb)
| 104,000&nbsp;kg + 28,800&nbsp;kg (~230,000&nbsp;lb + ~63,500&nbsp;lb)
| 15.4
| 0.935
|-
| [[Saturn 1B]] (stage only)
| 448,648&nbsp;kg<ref name="saturn">{{cite web| url = http://astronautix.com/lvs/saturnib.htm| archive-url = https://web.archive.org/web/20020305051145/http://www.astronautix.com/lvs/saturnib.htm| url-status = dead| archive-date = March 5, 2002| title = Astronautix – Saturn IB}}</ref> (989,100&nbsp;lb)
| 41,594&nbsp;kg<ref name="saturn"/> (91,700&nbsp;lb)
| 10.7
| 0.907
|-
| [[Virgin Atlantic GlobalFlyer]]
| 10,024.39&nbsp;kg (22,100&nbsp;lb)
| 1,678.3&nbsp;kg (3,700&nbsp;lb)
| 6.0
| 0.83
|-
| [[V-2 rocket|V-2]]
| 13,000&nbsp;kg (~28,660&nbsp;lb) (12.8 ton)
| 
| 3.85
| 0.74 <ref>{{cite web| url = http://www.astronautix.com/lvs/v2.htm| archive-url = https://web.archive.org/web/20020302013547/http://www.astronautix.com/lvs/v2.htm| url-status = dead| archive-date = March 2, 2002| title = Astronautix-V-2}}</ref>
|-
| [[X-15]]
| 15,420&nbsp;kg (34,000&nbsp;lb)
| 6,620&nbsp;kg (14,600&nbsp;lb)
| 2.3
| 0.57<ref name="rlv"/>
|-
| [[Concorde]]
| ~181,000&nbsp;kg (400,000&nbsp;lb <ref name="rlv">{{cite web |url=http://mae.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf |title=AIAA2001-4619 RLVs |access-date=2019-02-19 |archive-url=https://web.archive.org/web/20131206012152/http://mae.ucdavis.edu/faculty/sarigul/aiaa2001-4619.pdf |archive-date=2013-12-06 |url-status=dead }}</ref>)
| 
| 2
| 0.5<ref name="rlv"/>
|-
| [[Boeing 747]]
| ~363,000&nbsp;kg (800,000&nbsp;lb<ref name="rlv"/>)
| 
| 2
| 0.5<ref name="rlv"/>
|}