Apollo 17 astronaut Dr. Harrison "Jack" Schmitt and UW grad student Craig Schuff. (Sept. 2011)

NEEP602 Course Notes (Spring 1996)
Resources from Space

Lecture #29: One Small Step and One Big Kick!

Title: Chemical Rockets

April 3, 1996

Note: This page has not been used in our Resources from Space course for several years, and it is only infrequently maintained. Please inform me of any broken links.

Please see our most recent Resources from Space course web page, which contains, among other information, updated links to web pages on

Spacecraft Trajectories
Plasma and Electric Propulsion
Fusion Propulsion
Chemical Propulsion (by Michael Griffin, a true chemical rocket expert)

Chemical rocket overview

Some rocket-history documents on the Web include A Brief History of Rocketry (from a rather American perspective), a History of Marshall Space Flight Center, where a great deal of US rocket development occurred, and a fairly comprehensive document History of Space Exploration. Highlights include:

Year	Event
300 BC	Gunpowder-filled bamboo tubes used for fireworks in China
1045	Military rockets in use in China
1895	Konstantin Tsiolkovsky derives the fundamental rocket equation
1926	Robert Goddard launches first liquid-fueled rocket
1942	Wernher von Braun's team launches first successful A4 (V2)
1957	Sputnik launch
1958	Explorer I launch
1967	Saturn V first launch
1969	Apollo 11 Moon launch

Chemical rockets will almost certainly play a major role in space operations for the foreseeable future. Although advanced, high-exhaust-velocity rockets will eventually take over most long-range missions, chemical and nuclear-thermal rockets remain the uncontested leaders in producing the high thrust-to-weight ratios required for leaving most planetary surfaces.

Specific impulse

The impulse,

I

, given to a rocket is the thrust force integrated over the burn time. Traditionally, for the case of constant exhaust velocity (

v

_ex), the specific impulse has been used:

I

_sp = I / (m_p g₀) = v_ex / g₀, where

m

_p is the propellant mass and

g

₀ is Earth's surface gravity. In English units,

I

_sp is thus measured in seconds and is a force per weight flow. Often today, however, specific impulse is measured in the SI units meters/second [m/s], recognizing that force per mass flow is more logically satisfying. The specific impulse is then simply equal to the exhaust velocity,

I

_sp = v_ex.

Chemical rocket fuels

Two main classes of chemical-rocket fuels exist, liquid and solid. A good introductory discussion of chemical-rocket fuels can be found by clicking here. The advantages of liquid-fueled rockets are that they provide

Higher exhaust velocity (specific impulse),
Controllable thrust (throttle capability),
Restart capability, and
Termination control.

The advantages of solid-fueled rockets are that they give

Reliability (fewer moving parts),
Higher mass fractions (higher density implies lower tankage), and
Operational simplicity.

**Selected Chemical-Rocket Fuels**
Liquid propellants
Fuel	Oxidizer	Isp (s)
Hydrogen(LH2)	Oxygen (LOX)	450
Kerosene	LOX	260
Monomethyl hydrazine (MMH)	Nitrogen tetroxide (N2O4)	310
Solid propellants
Fuel	Oxidizer	Isp (s)
Powdered Al	Ammonium perchlorate	270

Rocket efficiency

When the total kinetic energy of the rocket and its exhaust are taken into consideration, the highest efficiency occurs when the exhaust is equal to the instantaneous rocket velocity, as shown in the figure at right. See Sutton for a discussion.

Ascent from planetary surfaces

	Effect
Key effects that come into play during launch from planetary surfaces are shown at right. Gravity reduces the effective acceleration, and it is generally the most important force. Atmospheric drag can create significant friction, both heating and slowing a rocket. The coriolis and centrifugal forces due to the planet's rotation are usually small effects, but the planet's rotation can often be used to give the rocket an initial velocity in a desired direction.	Gravity
	Atmospheric drag
	Coriolis force
	Centrifugal force
	Wind velocity
	Planetary rotation

Launch criteria

Various criteria are used to decide when to launch a rocket. For the Space Shuttle, these include

Launch window
Weather

Wind conditions
Lightning and cloud cover
Temperature

Engine design

	F-1 engine
Chemical-rocket engines are technological marvels, combining knowledge of physics, chemistry, materials, heat transfer, and many other fields in a complicated, integrated system. The F-1 engine used in the first stage of the Saturn V rockets that launched the Apollo missions appears at right.

Heat transfer

Heat transfer presents a critical problem for rocket engine designers. Various approaches have been considered, including

Radiative cooling: radiating heat to space or conducting it to the atmosphere.
Regenerative cooling: running cold propellant through the engine before exhausting it.
Boundary-layer cooling: aiming some cool propellant at the combustion chamber walls.
Transpiration cooling: diffusing coolant through porous walls.

Nozzles

Rocket nozzles are usually of an expansion-deflection design. This allows better handling of the transition from subsonic flow within the combustion chamber to supersonic flow as the propellant expands out the end of the nozzle and produces thrust. Many nozzle variations exist. The governing equation for the magnitude of the thrust, in its simplest form, is

$F = v$ _ex dm/dt + (p_ex - p_a) A_ex = v_eq dm/dt,

where $v$ _ex is the exhaust velocity, $dm/dt$ is the propellant mass flow rate, $p$ _ex is the pressure of the exhaust gases, $p$ _a is the pressure of the atmosphere, $A$ _ex is the area of the nozzle at the exit, and $v$ _eq is the equivalent exhaust velocity (that is, corrected for the pressure terms). If the pressure inside the chamber is too low, the flow will stagnate, while too high a pressure will give a turbulent exhaust--resulting in power wasted to transverse flow.

Saturn V

	Saturn V launch
Wernher von Braun's team at Marshall Space Flight Center developed the three-stage Saturn V rocket, shown at right. The Saturn V served as the workhorse of the Apollo Moon launches. Its first stage developed over 30 MN (7.5 million lbs) of thrust and burned about 14 tonnes of propellant per second for 2.5 minutes.

Space Shuttle

	Space Shuttle on launch pad
Approximate parameters for the Space Shuttle (officially, the Space Transportation System or STS) are listed in the table below. A good Worldwide Web link for further information is NASA's Space Shuttle home page.

Space Shuttle Parameters
Total length	56 m
Total height	23 m
Wingspan	24 m
Mass at liftoff	2x $10$ ⁶ kg
Orbiter dry mass	79,000--82,000 kg
Solid-rocket booster thrust, each of 2	15,000,000 N
SSME (main engine), each of 3	1,750,000 N

Single-stage to orbit (SSTO)

The Delta Clipper is a key candidate single-stage-to-orbit vehicle. The experimental version DC-X appears at right. Goals for the research program include complete reusability and minimal cost. Driving the concept of a single-stage-to-orbit vehicle is the premise that Earth launch will become affordable only when operations become similar to those of commercial airlines. NASA's main program for developing advanced launch systems is called the Long Term High Payoff Main Propulsion System Program. The DC-X program has evolved into the X-33 program.

Useful references

Texts

George P. Sutton, Rocket Propulsion Elements (Wiley-Interscience, New York, 1992).
Michael D. Griffin and James R. French, Space Vehicle Design (AIAA, Washington, DC, 1991).

Worldwide Web

The Jet Propulsion Laboratory (JPL) maintains a Web site, Basics of Space Flight, which gives a good tutorial introduction to many of the topics covered here.

The Cambridge University Press's Handbook of Space Astronomy and Astrophysics contains an excellent chapter on Aeronautics and Astronautics, with parameters for many current launch vehicles.

Space-related, government, and other potentially useful Web sites

Bookmarks

Questions

Discuss the relative advantages of liquid propellants and solid propellants.
Assuming that a rocket nozzle is designed to be most efficient part way along the rocket's flight path, explain the effects reducing efficiency before and after this point.
Consider the problem of routine access to space from Earth's surface, and discuss the relative advantages and disadvantages of

Heavy-lift launch vehicles, as epitomized by the Saturn V
Space Shuttles
Single-Stage to Orbit (X-33) vehicles

Dr. John F Santarius

Fusion Technology Institute,
University of Wisconsin-Madison
1500 Engineering Dr.
Madison, WI 53706
USA

415 Engineering Research Building
e-mail: santarius@engr.wisc.edu; ph: 608/263-1694; fax: 608/263-4499
Last modified: April 12, 1996

University of Wisconsin Fusion Technology Institute · 439 Engineering Research Building · 1500 Engineering Drive · Madison WI 53706-1609 · Telephone: (608) 263-2352 · Fax: (608) 263-4499 · Email: fti@engr.wisc.edu