1993-5 r.lo, j.leppich emissions due to space activities, facts+models

8/14/2019 1993-5 R.Lo, J.Leppich Emissions Due to Space Activities, Facts+Models

1/13

EMISSIONS DUE TO SPACE ACTIVITIES : FACTS AND MODELS

R. LoJ. Leppich

(TU Berlin, Aerospace Institute, Marchstrae 12, 1000 Berlin 10)

EGS XVIII. General Assembly, Wiesbaden May 3-7, 1993Session OA22/ST19


2/13

2

1. Introduction

Emissions into the atmosphere caused by space flight are more than 10 000 times smaller thanthose caused by aviation. Aviation in turn contributes only about 5% of anthropogenic fossilfuel combustion. However, in astronautics, as elsewhere, the problem of environmental effectsis taken more and more serious.

Astronautical impacts differ from all other sources by their specific distribution of theemissions along the flight path (rockets and other spacecraft are the only anthropogenic sourceabove normal aviation altitudes) and by the kind of propellants that are used in huge amountsand injected into the atmosphere.

The paper gives an overview of typical propellants used in spacecraft and the corresponding products of combustion. The current amount of astronautical emissions is graphicallypresented in comparison with aviation and natural sources. The chemical nature of materialsand their quantity has been calculated from sea level to orbit.

Specific problems caused by re-entry bodies are considered. Aside of air combustion induced

by hypersonic flight, exotic materials are injected into the high atmosphere throughevaporation of alloys and elements that normally don't exist in these atmospheric layers.

The possible future development of astronautical impacts is demonstrated for differentmission scenarios considering the development of the international space fleets and theirlaunch rates. Today the emissions concentrate on climbing paths which start at about a dozenlaunch sites spread over four continents. There may be a change in the future to "off-setlaunches" (SNGER mission) and "air-launches" (HOTOL-Antonov, Pegasus) with

prolonged horizontal flight times to the optimal injection point (near the equator in mostmissions) with the consequence of further increases of space related emissions.

2. Launcher Emissions

Apart from the emissions caused by the main engines of the spacecraft, there exist some othersources of exhaust gases during launch and ascent:

exhaust gases from secondary flows (gas generator)

evaporation of propellants and other liquids

exhaust gases of attitude control systems, retro rockets and other secondary engines

Only the emissions of the main engines during launch and ascent were considered in thefollowing chapter.

2.1 Typical emission distribution along the flight path

Due to the relatively slow velocity of the rocket during take-off within the first seconds ofascent, huge amounts of exhaust gases are distributed locally into a thin atmospheric layernear ground level.

After that, as the velocity of the launcher increases, the emissions are distributed over a widerrange of atmospheric layers but the amount injected per unit length decreases. In addition,relative velocity between jet and surrounding atmosphere decreases until the flight velocity isequal to the exit velocity of the rocket motors. Above that point the jet stream follows thelauncher with an increasing velocity relative to the surrounding atmosphere.

When the launcher reaches altitudes were the density of the atmosphere is assumed to be zero,

the trailing jet stream has a velocity less than orbital one and hence, after collisions with local


3/13

3

molecules, surrounds the earth as a free molecular flow until it comes to rest or descents backto earth. In any case this molecular flow is spread over a wide area.

time (s)

0

50

100

150

200

250

300

0 500 1000 1500 2000 2500 3000

SSME cut-off

SRBs separation

time (s)

-4000

-2000

0

2000

4000

6000

8000

0 100 200 300 400 500 600

flight velocity

rel . exhaust

veloci ty S RB's

rel . exhaust

veloci ty S SME

Fig. 2.1.1: Ascent data of STS (US Space Shuttle)


4/13

4

The following fig 2.1.2 shows all major emission components of the US Space Shuttle atvarious altitudes. As one can see the amount of emissions per unit layer length decreasesduring ascent. After the two solid rocket motors are cut-off at an altitude of 48 km, theyconsist only out of the emissions by the cryogenic main engines (H2 and H2O) and exhaust gasmass drops to a much lower level. H2 is not shown in the chart because its amount is too lowcompared with H2O.

STS Emission

altitude (km)

0

100

200

300

400

500

600

0-0.5 0.5-10 10-50 50-67 67-

H2O

Al2O3

Cl2

HCl

CO2

CO

Fig 2.1.2: STS emissions at different altitudes [1]

2.2 Typical propellants and their emissions

The main propellants used in launch vehicles today can be roughly divided into the followinggroups:

cryogenic propellants(H2/O2)

semi cryogenic propellants (where RX denotes hydrocarbon fuels )(RP-1/O2, RG-1/O2,RJ-1/O2)

storable liquid propellants (with various nitrogen-hydrogen compounds as fuels)(UDMH/N2O4, UH25/N2O4, MMH/N2O4, Az-50/N2O4, Az-40/N2O4)

The exact composition of exhaust gases for each propellant type changes from engine toengine as function of chamber pressure, mixture ratio and expansion ratio or area ratiorespectively.

Fig. 2.2.1. presents an overview of the emissions of propellants used today. Using the NASA-Lewis Code [4], exhaust compositions were calculated for a wide range of engines. The valuesshown are mean values of these calculations.


5/13

5

solid Propellant

HCl

Al2O3

CO

CO2

N2H2

H2O

UH25/N2O

CO

CO2

H2

H2O

N2

UDMH/N2O

CO

CO2

H2

H2O

N2

O2/kerosen

CO

CO2

H2

H2O

H2/O2

H2O

H2

Fig 2.2.1: Composition of exhaust gases for various propellants

2.3 Emissions

Launches per Ye

(1957 - 2000)

year

0

20

40

60

80

100

120

140

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Fig. 2.3.1: Launch rates since 1957


6/13

6

For the last 6 years (1986 - 1991), the following charts survey the propellants used, theamount of emissions and the distribution at different altitudes. These charts are quiterepresentative for the period from 1965 until today because since 1965 the launch rate variesaround 80-120 launches per year and is expected to be nearly constant for the next years.

used propellants1985 - 19(all nations)

year

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

1985 1986 198 7 198 8 198 9 1 990 1 99 1

CTPB-solid

HTPB-solid

PBAN-solid

H2/O2

RJ-1/O2

RP-1/O2

RG-1/O2

Az50/N2O4

Az40/N2O4

UH25/N2O4

UDMH/N2O4

UDMH/HNO3

Fig. 2.3.2: Propellants used between 1985 and 1991 (in Mg)

The annual amount of propellants used by launch systems is nearly 3000 times smaller(including oxidisers) than the amount of aviation fuel. (Rocket propellants consumed between1986 and 1991 vary from 30.000 to 40.000 Mg/yr, aviation fuel burned between 1970 and1989 lies between 100.000.000 and 170.000.000 Mg/yr [10]). It should be noted that this fuelis transformed into roughly the 4.4 fold amount of exhaust products by way of combustion,while the rocket exhaust mass is almost identical with propellant mass. Thus space induced

pollution is more than 10 000 times below the one related to aviation!


7/13

7

emissions per year 1986 - 1991

year

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

85 86 87 88 89 90 91

HCL

Al2O3

N2

H2O

H2

CO2

CO

Fig. 2.3.3: Space flight emissions (in Mg)

A comparison between these and other main emission sources [1], [8], [10], [11] shows thateven aviation induced pollution is small compared to other sources. However, it is well knownthat local effects must be considered. A striking example is the effect of vapour trails of jetaircraft on unstable strata of air. Above the highest airline traffic, space missions are the onlylocal anthropogenic source of pollution.

anthropogenic sources [Mt/yr] natural sources [Mt/yr]

space activities(max. values)

aviation fossil fuelburning

all volcanoes oceans stratosphericmethaneoxidation

H2

O 0.0140 222.00 525,000.0 45.0

CO 0.0140 0.26 1,490.0

CO2 0.0050 554.00 20,350.00

N2 0.0100

HCl 0.0013 1.98 5.0 330.0

NOx 3.20 90.0

Tab. 2.3.1: Comparison between anthropogenic and natural sources


8/13

8

emissions at different altitudes(1985 - 1991)

altitude (km)

0

50000

100000

150000

200000

250000

0-50 50-150 ber 150

HCL

Al2O3

N2

H2O

H2

CO2

CO

Fig. 2.3.4: Space flight emissions at various altitudes

The bulk of emissions is injected at altitudes below 50 km as shown in fig 2.3.4. But a com-parison with some molecules of the stratospheric background (tab. 3.3.2) shows that the in-crease due to space activities is insignificant. However, if one considers afterburning and reac-tions with the surrounding atmosphere involving HCl, N2, CO and CO2 there could be a

strong local effect like ozone depletion along the ascent path or similar effects.

space activities

(1985 - 1991)[Mg]

stratospheric

background[Mg]

CO 69,921 15,600,000

CO2 24,846

H2 4,536 340,000

H2O 61,584 15,600,000

N2 40,224

NOx 280,000

Al2O3 7,879

HCl 5,284

Tab. 3.3.2: Comparison with stratospheric background [8]


9/13

9

3. Pollution caused by re-entry

Re-entry vehicles cause two types of pollution:

Hypersonic shock waves induce chemical reactions in the atmosphere

Evaporation of spacecraft inserts the elements of their construction materials into the

atmosphere

3.1 Pollution by shock waves

Space flight uses re-entry capsules and winged vehicles of Space Shuttle or Buran type. Afterdescending from orbital velocities of typically 7 km/s flight velocity at re-entry into thecontinuous atmosphere is about Mach 27 (27 times the local speed of sound). From this upperlimit down to about Mach 6 this flight regime is called "hypersonic" and characterised byshock waves that can induce very high temperatures, causing molecular gases to dissociate. Asa result, nitrogen and oxygen react under formation of nitrous oxides, NOX.

The amount formed depends on flight velocity. Below Mach 5, temperature is too low, while

above Mach 15 it is too high. However in the medium regime, up to 8% of the incoming aircan be transformed into NOX [12].

On average, about 3000 kg NOX are formed during a typical re-entry event.

Since all air traffic at pre-set rates is estimated to produce 3000 tons of NOX, it takes 1000 re-entries to match air flight caused NOX-pollution.

3.2 Pollution by spacecraft incineration

The number of entering spacecraft and space debris depends on the launch rate and thelifetime of the target orbit. The actual launch numbers for each year are shown in fig 2.3.1.

About 50 to 80% of these launches led to the geosynchronous orbit in 36,000 km altitude oreven more distant trajectories that have essentially infinity longevity as far as the purposes ofthe present consideration are concerned. In other words, a fraction of 20 to 50% of thesatellites launched went into near earth orbits, where the lifetime drops from several years in800-900 km altitude to a few days at 120 km. In addition to evaporating satellites, rocketstages must be considered. Since many years, near earth orbits are entered by two-stagerockets, while more distant orbits require three stages. As a rule first stages fall back to thesurface after travelling a distance of several 100 km. They do not evaporate. In contrast,second stages are normally moreless completely incinerated. The large external tanks of theUS Space Shuttle belong to this type of stage. Third stages can remain in the GEO-transferorbit for several years prior to re-entry through the continuous drag of the upper atmosphere

near perigee.According to a statistical investigation published by [9] the interplay of launch rate and thelifetime has caused 2/3 of all objects inserted between 1957 and 1988 into near earth orbits to

be removed from altitudes up to 1800 km.

These figures result in the following estimation of the presently annually evaporating mass ofspacecraft:

Spent satellites and orbital garbage: about 40-60 Mg/yr

Upper stages: about 100-150 Mg/yr

Predominantly, this mass comes from aluminium-, titanium- and iron alloys, accompanied to a

lesser extend by plastics and residuals of propellants. It is safe to assume that all of it is


10/13

10

cleaved into the constituent elements which later may recombine with air to form oxides andnitrides.

3.3 Pollution by meteorites

Considering the very modest pollution caused by re-entering spacecraft, the effect ought to benegligible when compared with the natural load caused by meteorites.

After [3] meteorites show the following average elemental composition (in weight %):

Element Stony meteorites Iron meteorites

O 35.71

Fe 23.31 89.70

Si 18.07

Mg 16.67

S 1.80 0.08

Ca 1.73

Ni 1.53 9.10

Al 1.52

Na 0.65

Cr 0.32

K 0.17

C 0.15 0.12

Co 0.12 0.62

P 0.11

Ti 0.11

Cu 0.04

P 0.18

Tab. 3.3.1: Average elemental composition of meteorites

While earlier estimates went as high as 6500 Mg of meteoritic matter per day, presentestimates state 40 Mg/d [5]. Still, this amounts to 14610 tons annually. Since this is twoorders of magnitude above space debris, only such constituents of space alloys that are not

present in meteorites will go beyond natural pollution. Without claiming to be complete, thereis the following collection found in alloys of aluminium, titanium and steel:

Manganum, Molybdenum, Vanadium, Beryllium, Lithium, Niobium (Columbium),Tungsten

These elements are likely to be injected in small amounts, albeit larger ones than naturally present. Any deeper investigations about their distribution, removal and possible catalyticeffects in atmospheric chemistry would go beyond the scope of the present survey.


11/13

11

4. Scenarios of further development

At present, as shown above, space flight induced atmospheric pollution is more than 10 000times lower than air flight induced one.

However, as long as solid propellant rockets are used, spacecraft release hydrochloric acid in

the same order of magnitude as other anthropogenic sources. In general, however, space flightinduced pollution is many times smaller than other human or natural sources.

There is, it seems, room for growth until space travel must be considered a problem.

In a 1991 study [7] an estimation was made about the consequences of lunar industrialisationon world wide space traffic. The scenario used comprised a "Return to the Moon" in thisdecade, the building of a Lunar Far Side Observatory until 2010 and a first step of lunarindustrialisation (by using it as ecologically inert development site) until 2022. Together withsome additional assumptions resulted the traffic depicted in figs.4.1 and 4.2 [6].

The scenario must be considered quite optimistic. The launch rate of a clearly definedassortment of launch vehicles reaches a maximum of about 3300 annual launches in 2030 or

33 times present rates. Leaving aside any higher resolution analysis by assuming that thelarger launchers used will carry proportionally more payload, this corresponds to an increaseon chemical pollutant emission from 10 000 times less than (present day) aviation to 300times less! In other words: there is no reason to worry as far as the amount of emissions goes.If the analysis of more subtle effects reveals other threats, remains to be seen. On should notforget that the launch numbers shown lead to a corresponding number of re-entries of whicheach produces the NOX amount mentioned above.

year

0

1000

2000

3000

4000

5000

6000

7000

1990

1995

2000

2005

2010

2015

2020

2025

2030

Personen in LUO

Personen in LEO

Fig.4.1: Development of passenger traffic for lunar industrialisation.


12/13

12

year

0

5000

10000

15000

20000

2500030000

35000

40000

45000

2

000

2

005

2

010

2

015

2

020

2

025

2

030

addi tional for passengers in

LUO/Mars

addi tional for cargo inLUO/Mars

in LUO, Mars

in LEO

Fig.4.2: Development of payload traffic for lunar industrialisation.

At present, there is no indication whatsoever that chemical propulsion might be replaced bysomething better. Futuristic concepts like nuclear propulsion, super propellants, laser

propulsion and the like depend on hydrogen as propulsive fluid. At least in the loweratmosphere this would be converted to water vapour by afterburning. While carbon compound

would disappear as exhaust gases, it is quite likely that NOX formation would increase due tohigher temperatures involved. However, at the present state of early concepts, it is much toearly to make any more definite statements about the possible atmospheric impact of thesetypes of propulsion. The use of nuclear propulsion in the earth atmosphere is unlikely to betolerable due to the hazard of major accidents.

5. Summary

It has been shown that space induced atmospheric pollution by rocket exhaust gases is anegligible quantity at present. However, there is no doubt, that small but relatively significantamounts of pollutants are introduced at altitudes above the main air traffic routes. The load ofre-entering space debris is tiny compared with natural meteoritic sources. Again, some of the

constituents are significant due to their absence under natural conditions. Re-entering spacecraft produce roughly 3000kg of NOX per flight or event. This number could grow

problematic in case of massive future space programs. However, even in this case the exhaustgas load remains very far below other human pollutant sources.


13/13

13

6. Literature

[1] Anonym"Environmental Effects of Space Activities"Report by the Committee on Space Research and the IAF,A/AC.105/420,15.12.1988

[2] Anonym"Atmospheric Effects of Chemical Rocket Propulsion"Report of an AIAA Workshop, Sacramento, 28/29 June, 1991,1.10.1991

[3] Bhler, R. W."Meteorite"Weltbild Verlag, FBN Nr. 389350180Schweiz 1992

[4] Gordon, McBride"Computer Program for Calculating of Complex Chemical Equilibrium Compositions, Rocket Performance,Incident and Reflected Shocks, and Chapman-Jouguet Detonations"NASA-SP-273, Interim RevisionMarch 1976

[5] Gritznerprivate communication by GSE-Gritzner Space EngineeringApril 22, 1993

[6] Lamann, J; Lo, R; Thierschmann, M."Entwicklung der globalen Raumtransportflotte"Deutscher Luft- und Raumfahrt-Kongre 1991Sitzung B5: Raumtransportsysteme II, 12.Sept. 1991;DGLR-Jahrbuch 1991 II,Deutsche Gesellschaft fr Luft- und Raumfahrt,Bonn-Bad Godesberg, Eigenverlag DGLR, S.1213-1220

[7] Lo, R.E.; Lamann, J.; Thierschmann, M."Long Term Modelling of Global Space Traffic"IAF-91-190, 42nd Congress of the International Astronautical Federation, Montreal (Canada), Oct. 1991

[8] McDonald, Alan J."The Impact of Chemical Rocket Propulsion on the Earths Environment"IAA-92-0218, 43rd Congress of the IAF August 28 - September 5 1992 Washington,

[9] Rex, D., et.al."Space Debris - Origin, Evolution and Collision Mechanics"TU-BraunschweigActa Astronautica, Vol.20, pp 209ffOxford 1989

[10] Schumann, Ulrich"On the Effect of Emissions From Aircraft Engines on the State of the Atmosphere"DLR Institut fr Physik der AtmosphreDLR-Report No.1,May 93

[11] Schumann, U.; Reinhard, M. E."Studies on the Effekt of High-Flying Air-Traffic on the Atmosphere"IAF-91-737, 42nd Congress of the International Astronautical Federation. Montreal (Canada) Oct. 1991

[12] Stuhler, H."Experimentelle und Theoretische Untersuchungen zur Beeinfluung atmosphrischer Gase durch Stowellen inder Raumfahrt"TU-Stuttgart, Inst. f. Thermodynamik der Luft- und RaumfahrtKoloquium im Rahmen des DFG Schwerpunkts "Grundlagen der Auswirkungen der Luft- und Raumfahrt,auf dieAtmosphre"Mnchen, 19.4.1993

1993-5 r.lo, j.leppich emissions due to space activities, facts+models

Documents