Choices: Some Considerations in Configuring Launch Systems

Dr. John M. Jurist

Adjunct Professor of Space Studies, Odegard School of Aerospace Sciences, and

Adjunct Professor of Biophysics and Aviation, Rocky Mountain College

Note: This material was used in various seminars at the above institutions, and is not to be reused without attribution

Target Audience:• People not trained in the physical or

engineering sciences• People with a general interest in why

space launch technology is so complex

2

A Great Resource for Those Who Wish to Dig Deeper:

George Sutton’s books: Rocket Propulsion Elements (multiple editions over 60+ years)

3

Goals:• Give a sense of the kinds of choices

that are made in configuring a launch system.

• Show how choices made early in the process affect later options.

4

Relevant Quote:

“Statements in the first 2 years of the company should be disregarded due to idiocy.”

-- Elon Musk, ISDC 2008 remarks at Capital Hilton in Washington, DC on SpaceX scheduling problems

5

Primary Parameters:

Define payload mass to specified trajectory

Example: 2,000 pounds to 200 km LEO at 23 deg inclination launched to east

Translates to mission velocity change (delta-V)

Orbital velocity plus margins for gravity and aerodynamic losses = mission delta-V 6

The Basic Rocket Equation:

Mo/Mf = e(v/c) or v = c * loge(Mo/Mf)

Mo = GLOW = liftoff mass

Mf = burnout mass

c = g * Isp = exhaust velocity

v = ideal burnout velocity7

Useful Reading Material:

For a narrative of the implications of the rocket equation:

http://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html

8

Options Narrowed:

Select propellant (solid vs liquid, hydrocarbon vs liquid hydrogen, etc.)

Propellant combination determines specific impulse from rocket equation

C* = g * Isp = exhaust velocity9

Tradeoffs -- 1:

Solid vs Liquid:• Simple vs complex (especially with

pumps)• Reliable vs less reliable ignition• Storable vs less storable• Lower vs higher potential impulse density• Lower vs higher mass efficiency for

larger (pumped) systems10

Tradeoffs -- 2:

Solid vs Liquid:• Toxic components and/or exhaust

products• Ammonium perchlorate • Liquid oxygen • Hydrocarbon• Hydrazine• Liquid hydrogen

11

Tradeoffs -- 3:

Solid vs Liquid:• Handling• Hydrocarbon vs liquid hydrogen

• Lower vs higher specific impulse• Higher vs lower density (tank mass)• Insulation (tank mass)

12

Options Narrowed -- 1:

Selected propellant C* gives required mass ratio from rocket equation for given mission delta-V:

(Mo/Mf), where Mo = GLOW = liftoff mass

and Mf = burnout mass13

Options Narrowed -- 2:

Staging to Ease Mass Ratio Constraints:• More stages = more complexity and less

reliability• No ullage or separation motors required for

solids but required for liquids unless hot-fire staging used

14

Discussion Limitation:

Assume two stage liquid propellant system from this point forward

15

Mass Budget -- 1:

Once payload mass, mission delta-v, propellant combination, staging, and mass ratio defined, characterization of each stage can proceed:

• Structural elements• Tankage (and insulation if required)• Attitude control system• GNC (guidance, navigation, and control)

16

Mass Budget -- 2:

• Payload shroud (eject when out of atmosphere)• Interstage assembly (divide between lower and upper)• Ullage systems (stage separation and propellant

settling for upper stage ignition)• Destruct system

17

Mass Budget -- 3:

• Propulsion system:• Motors : Nozzles behave differently at sea level

and in vacuum so compromises needed in design • Pumps• Plumbing• Gimbals

18

Mass Budget -- 4:

• Multiple iterations required• Interactions/relationships• Reusability implies:

More robust (and massive) structure, andThermal fatigue and shock issues for motors

19

Illustrative Interaction -- 1: A new definition: Net structural mass fraction

NFS or NMF = ( Ms - Mm ) / Mp

Ratio of structural mass less motor to propellant mass – also called net mass fraction or NMF

20

Illustrative Interaction -- 2:

21

Variations:

Multiple motors:• Shut down and drop some motors part way

through burn• Atlas missilePropellant Transfer• Shift propellant from tanks that can be dropped• SpaceX Falcon Heavy? 22

An Alternative to Propellant Transfer:

• Instead of similar core and 2 side boosters with fuel transfer to core, have similar tankage with more motors supplied from side boosters and fewer from central core

• Eject side boosters and motors when fuel depleted 23

Cost Considerations: Dr. Dietrich E. Koelle: Transcost• Statistical-analytical model for cost estimation

and economical optimization of launch vehicles• Parametric cost estimation: Method of

estimating cost per unit mass• See Launch Vehicle Business Workshop .ppt

24

Political and Regulatory Considerations:

To be discussed

25