1 Spacecraft Systems Design

Short Course

Prof. Craig Underwood

Surrey Space Centre University of Surrey

1 Jun. 2012 © University of Surrey Key References

Fortescue, P., Stark, J. and Swinerd, G. (Eds) Spacecraft Systems Engineering, John Wiley & Sons, Chichester, 2003. ISBN: 0-41-61951-5 Cruise, A.M., Bowles, J.A., Patrick, T.J. and Goodall, C.V. Principles of Space Instrument Design, Cambridge University Press, Cambridge, 1998. ISBN: 0-521- 45164-7 Griffin, M.D. and French, J.R. Space Vehicle Design, AIAA Education Series, Washington D.C., 1991. ISBN: 0-930403-90-8 Sellers, J.J. Understanding Space (3rd Ed.), Space Technology Series, McGraw- Hill, New York, 2005. ISBN:0-07-340775-5 Houston, A. and Rycroft, M. (Eds) Keys to Space, McGraw-Hill, Boston, 1999. ISBN: 0-07-029438-0 Larson, W.J. and Wertz, J.R. (Eds) Space Mission Analysis and Design (2nd Ed), Space Technology Series, Kluwer Academic Publishers, 1992. ISBN: 0-7923- 1998-2

Jun. 2011 2 © University of Surrey Introduction

Spacecraft Systems Design involves techniques from a variety of scientific and engineering disciplines. The aim of this course is to introduce the key space system design principles and techniques. In particular, the course focuses on mechanical and thermal design of space vehicles, as well as the electrical and system design of their key sub-systems. The course ends by setting a spacecraft design exercise. Delegates should leave the course with a knowledge of: · Launching, Orbits and Manoeuvres; · The Space Environment & its Effects; · Spacecraft Platform Systems & Design; · Space Mission Analysis & Design; . Applications of Small, Low-Cost Satellites

Jun. 2011 3 © University of Surrey Introduction

What is a Spacecraft?

Magellan – NASA’s Venus Space Probe, Launched 1989

4 Aug. 2011 Sellers, J.J. Understanding Space, pp. 367, McGraw-Hill Inc., 1994 © University of Surrey Introduction

Sellers, J.J. Understanding Space, pp. 369, McGraw-Hill Inc., 1994 Aug. 2011 5 © University of Surrey Introduction

6 Aug. 2011 Sellers, J.J. Understanding Space, pp. 369, McGraw-Hill Inc., 1994 © University of Surrey Elements of a Space Mission

7 Aug. 2011 7 Jun. 2012 © University of Surrey Key References

Fortescue, P., Stark, J. and Swinerd, G. (Eds) Spacecraft Systems Engineering, John Wiley & Sons, Chichester, 2003. ISBN: 0-41-61951-5 Sellers, J.J. Understanding Space (3rd Ed.), Space Technology Series, McGraw-Hill, New York, 2005. ISBN:0-07-340775-5 Houston, A. and Rycroft, M. (Eds) Keys to Space, McGraw-Hill, Boston, 1999. ISBN: 0-07-029438-0 Larson, W.J. and Wertz, J.R. (Eds) Space Mission Analysis and Design (2nd Ed), Space Technology Series, Kluwer Academic Publishers, 1992. ISBN: 0-7923-1998-2 Wertz, J.R. Spacecraft Attitude Determination and Control, Reidel, Dordrecht, 1978.

Aug. 2011 8 © University of Surrey Space Mission Design

 All space missions are born of a set of requirements – objectives to be fulfilled within certain constraints such as budget and time (the latter sometimes defined by launch windows for exploration missions.

 The requirements of the mission must be well-defined and concise.

 Systems engineering plans and integrates technical solutions within the schedule and within budget. Much of the methodology resembles that from software engineering.

 The mission analysis should describe the mission, its operations, system configuration, subsystem specifications, quality assurance and reliability.

 The specification should flow-down from systems to subsystems to components to parts at increasing levels of detail to ensure consistency.

Aug. 2011 9 © University of Surrey Space Mission Design

Space Segment, Ground Segment, Launcher Segment

Aug. 2011 Houston, A. and Rycroft, M. (Eds) Keys to Space, 10 McGraw-Hill, Boston, 1999. pp. 5-4 © University of Surrey Space Mission Design

Space Segment, Ground Segment, Launcher Segment

Aug. 2011 Houston, A. and Rycroft, M. (Eds) Keys to Space, 11 McGraw-Hill, Boston, 1999. pp. 5-5 © University of Surrey Space Mission Design

 In designing a space mission: - mission objectives must be clearly defined (requirements definition) - mission users must be defined (e.g. scientific community) - resource availability (typically budget, political considerations, and technological maturity) - design constraints (schedule, mass and power typically)

 Mission architecture must be defined on the basis of system level trade-off studies to achieve a given performance

Aug. 2011 12 © University of Surrey Space Mission Design

 From the mission requirements and constraints, an iterative design based on a trade-off analysis is developed – often altering the mission requirements if necessary.

 All space missions comprise several major systems defining its architecture: - spacecraft payload which performs the function of the mission - spacecraft bus which supports the payload (housekeeping) - launcher to place the spacecraft into its required orbit - orbital trajectory which defines the ground coverage - ground system which controls the mission operations through a communications infrastructure

Aug. 2011 13 © University of Surrey Space Mission Design

 The systems architecture is the end product of the mission design process consisting of an overall system design, and covering all elements of the system with the necessary specifications to meet the stated mission objectives in an optimum way.

 The systems architecture does not enter into the design of the individual elements any further than is required to establish its functional, cost and schedule feasibility in accordance with the corresponding assumptions included in the systems plan

 The systems architecture establishes clearly the mutual dependence of the various systems elements, thus providing a complete and structured framework of interdependency formulas for the requirements and characteristics of the various elements

Aug. 2011 14 © University of Surrey Space Mission Design

Houston, A. and Rycroft, M. (Eds) Keys to Space, McGraw-Hill, Boston, 1999. pp. 5-9 Aug. 2011 15 © University of Surrey Space Mission Design

Phases of a Space Mission

Aug. 2011 Houston, A. and Rycroft, M. (Eds) Keys to Space, 16 McGraw-Hill, Boston, 1999. pp. 5-7 © University of Surrey Space Mission Design

 Space mission design is an iterative process.  All space missions have a well-defined programme of development with regular reviews with the client, e.g. ESA.  Reviews provide independent, critical assessment and provide a forum for communication.  It also ensures that documentation is clear and concise.  ESA standards are described in ECSS (European Cooperation for Space Standardisation) standards documents.

rd Aug. 2011 Sellers, J.J. Understanding Space (3 Ed.), Space 17 Technology Series, McGraw-Hill, New York, 2005. © University of Surrey pp.362 Mission and Operations Tasks

Mission Design & Manufacturing Team

Operations Team

Aug. 2011 18 Sellers, J.J. Understanding Space (3rd Ed.), Space Technology © University of Surrey Series, McGraw-Hill, New York, 2005. pp.634 Mission and Operations Tasks

Mission Definition and Design Sub Systems Design and Manufacturing

AIT

Testing

EVT

Launch

Mission Commission & Operations

Aug. 2011 Space Missions at Surrey 19 © University of Surrey Getting into Space

Thought Experiment I Suppose we took a cannon to the top of a high mountain and experimented with firing projectiles: How fast would the projectile have to go to follow path 3? Motion in a circle implies a centripetal force must be acting • here supplied by gravity. Thus, mv2/r = GMm/r2 i.e. V2 = GM / r

Thought Experiment II Suppose our mountain was 300 km high (as high as a typical Space Shuttle orbit), then:

Given the mass of the Earth, M = 5.977 x 1024 kg the radius of the orbit, r = 6.671 x 106 m and the Gravitational Constant G = 6.672 x 10-11 m3 s-2 kg-1 => V2 = 6.672 x 10-11 x 5.977 x 1024 / 6.671 x 106 i.e. V = 7.732 km s-1 ('Mach 24')

Thus, in order to achieve a low-Earth orbit, we have to be able to attain speeds of the order of 7-8 km s-1 - this presents a challenge! © University of Surrey Getting into Space

Thought Experiment III Do all orbits have to be circular? No, they can be any conic section: • Circle • Ellipse • Parabola • Hyperbola

The last two trajectories are not ‘closed’, and so a spacecraft in these orbits will escape from the Earth altogether.

Principle:

ROCKET EXHAUST GAS

ROCKET MOVING WITH: EXHAUST GASSES MOVING WITH: MASS = MR MASS = MP VELOCITY = VR VELOCITY = VP

MOMENTUM CONSERVATION

MR . VR = MP . VP

The Rocket Equation: Suppose we have a rocket of mass 'M', which increases its velocity by an amount 'dV' after burning an amount of fuel 'dM', and ejecting the exhaust at a relative velocity 've', then by conservation: M dV = ve dM. Integrating this expression over time, the total velocity change, ‘DV' after burning a mass of fuel 'm', is : DV = ve loge [(M+m)/M].

This is more usually expressed in terms of specific impulse, Isp:

DV = go Isp loge [ Minitial / Mfinal] -2 where go = 9.807 ms -the Earth's surface gravitational acceleration.

Isp is a measure of the effectiveness of the rocket and depends upon the fuel/ oxidizer combination used, and upon the design of the rocket engine -particularly its exhaust nozzle. A 'good' rocket has a high Isp.

Rockets and Chemistry : A good rocket has a high exhaust velocity, thus, the molecules that comprise the exhaust products should be small and light so that they can move quickly:

Water (H2O) and nitrogen (N2) are both excellent exhaust products. Also the chemical reaction that gives rise to the exhaust products should be very exothermic, so that the heat energy evolved can be converted into the kinetic energy of the exhaust products. Reactions which replace weak chemical bonds in the reactants with strong chemical bonds in the products have this property, e.g. :

N2H4 (liquid) -> N2 (gas) + H2(gas) (Hydrazine + catalyst)

Solids: Calculated Isp Ideal Isp

10 CH2/ 52 NH4Cl04/ 20 Al 347 s Liquid Monopropellants:

H2O2 192 s 245 s

N2H4 264 s 269 s Liquid Bipropellants:

N2O4/ N2H4 354 s 404 s

O2/ RP-1 (Kerosene) 380 s 461 s

O2 / H2 470 s 528 s Ions: Xe ~2000-3000 s (18 mN thrust!)

Solid Rocket Motor :

Liquid Fuelled Rocket Engine :

Hydrazine Thruster :

Ion Thruster :

The table of specific impulses for various rocket types shows that a typical launch vehicle rocket engine has an Isp between 350 - 470 s.

But exhaust velocity ve is related to the Isp through Ve = g0 Isp' so a typical rocket exhaust velocity is in the range: 3.4 - 4.6 km s-1 i.e. about half the required orbital velocity (~ 8 km s- 1).

Applying the Rocket Equation {DV = ve loge [(M+m) / M]} for 2 DV = 2 ve' gives a mass ratio { (M+m)/M } of at least e  7.4. This means that at least 88% of the mass of the rocket is fuel, with no more than 12% of the mass left over for the body, engines and payload. For a typical rocket, the mass of the engines and fuel tanks alone is 15% of mass of the rocket. Thus, it cannot carry enough fuel to get into orbit - spaceflight is impossible! © University of Surrey Achieving Orbit

Well not quite - there is a trick we can play: Staging:

• Selection of a launch system depends on the mission’s technical characteristics • Others factors are risk, cost, political or security considerations • First criterion for selection is the capability of launcher to handle payload requirements in terms of weight and dimensions and orbit parameters (altitude, inclination) • Other factors are launcher availability, infrastructure availability (launch pad, communication systems), launch rate • Customer decides on a dedicated or shared launch

• Optimisation of the spacecraft system has to be worked out • Payload mass can be augmented by using the launcher or by using its own propulsion system • GEO comm’s satellites usually use their own propulsion system as a 4th stage in order to change from geostationary transfer orbit (GTO) to geostationary orbit (GEO) • Location of a launch site is important: if close to the equator one can use to his advantage due to the high velocity derived from the earth’s rotation • Main advantage launching from equator is for geostationary orbits for which no orbit change is necessary

Kourou

• A plane change requires a lot of energy: in LEO a DV=200 m/s is needed for an inclination change of 1 deg • From an equatorial launch site any orbit inclination can be chosen • For other locations the original orbit inclination can’t be lower than the latitude of the launch site

Minimum Inclination: The inclination angle, ‘i’, of the orbit plane is given by: cos (i) = cos (a) cos (l) where 'a' is the launch azimuth angle (East = 0°), and ‘l' is the latitude of the launch site. To achieve and inclination angle less than this requires the use of an expensive plane-change manoeuvre once in space.

Earth Rotation Effect: The velocity of the rocket is augmented by the velocity of the launch site due to the rotation of the Earth. The maximum beneficial effect comes from a launch due East starting from the Equator, where the initial speed is already 0.46 km s-1 before launch! This extra speed reduces the need for propellant to be carried by geostationary satellites.

The orbit plane often needs to have some fixed relationship to the celestial sphere or to bodies such as the Sun. This dictates the time of launch, as the orientation of the initial orbit plane depends upon the celestial co-ordinates of the launch site at lift-off, and these change as the Earth rotates. The right conditions for launch exist for a period of 10 minutes or so each day - this is the launch window

http://science.howstuffworks.com/space-shuttle1.htm

India and China – the Next Space Race?

http://en.wikipedia.org/wiki/Image:Pslv_thm.jpg http://en.wikipedia.org/wiki/Image:China_%28172%29.jpg © University of Surrey Launch Vehicles

Single Stage to Orbit?

Private Enterprise?

Where Next? Back to the Moon? Mars?

Single-Stage-to-Orbit – i.e. Fly to Space?

http://www.bis.gov.uk/assets/bispar tners/ukspaceagency/images/news/ skylon-landed-reaction-engines.jpg

www.britain-in- space.co.uk/1990/images/skylonmann.jpg