Long Questions

Question 1

Why there is a surface of revolution of departure hyperbolae and circle of injection points, for a given hyperbolic excess velocity and radius of periapse?

Answer 1

• Only requirement on the orientation of the departure hyperbolae plane is that it contains the CoM of the planet and the relative velocity vector.
• Rotating the hyperbola through all the possible points sweeps out a surface of revolution on which all the possible departure hyperbolae lie.

Question 2

List the principles of operation and applications of pressure-fed systems for rocket engines.

Answer 2

• Use a pressurant gas to force propellant into the chamber.
• Whole feed system must be 1000kPa greater than the chamber pressure to ensure good feed.
• Simple and reliable but heavy.
• Often used in satellites and upper stages.

Question 3

Explain how a blowdown pressure-fed system works.

Answer 3

• Propellant tank comprises comprises a rigid vessel containing a flexible bladder and perforated axial stand pipe.
• Propellant is contained in the bladder and the pressurant fills the gap between the bladder and the tank wall (usually 50% tank volume).
• Pressurant gas squeezes the bladder and forces the propellant through the stand pipe to deliver it to the rocket engine or thrusters.

Question 4

What are the disadvantages of blowdown and regulated pressure-fed systems?

Answer 4

Blowdown:

• Pressure, thrust and flow rate vary with time.
• Specific impulse is a function of chamber pressure so it too varies over time.
• Difficult to use with bi-propellant systems.

Regulated:

• Expensive, complex and heavier.
• Propellant can overpressure.
• Storage pressure needs to be high.

Question 5

Explain how a regulated pressure-fed system works.

Answer 5

• Tank is pressurised from an independent pressurant vessel via a pressure regulator.
• The tank needs to be designed to withstand the highest possible pressure of both of these systems.

Question 6

List the main space stations.

Answer 6

• Salyut
• Mir
• Skylab
• ISS
• Tiangong
• Genesis

Question 7

Give some history on the Salyut space station.

Answer 7

First station programme by the Soviet Union. Series of 4 crewed scientific research stations and 2 crewed military reconnaissance stations over a period of 15 years from 1971.

Question 8

Give some history on the Mir space station.

Answer 8

This Soviet station was the first consistently inhabited long-term research station and was adapted but a series of long duration crews.

Question 9

Give some history on the Skylab space station.

Answer 9

First and only US station which paved the way for the ISS. Included a workshop, solar observatory and multiple docking adapter systems that allowed 3 crews to spend 84 days in space.

Question 10

Give some history on the ISS space station.

Answer 10

Built in 1998 by 5 space agencies; NASA, Roscosmos, JAXA, ESA and CSA.

Question 11

Give some history on the Tiangong space station.

Answer 11

Chinese modules for rendezvous and docking practice.

Question 12

Give some history on the Genesis space station.

Answer 12

Commercial station intended to test for hotels in space by US hotelier Bigelow.

Question 13

Give 3 different space station options for pressure and

atmospheric content and explain their implications for space station design.

Answer 13

Sea level pressure 101 kPa and composition 21% O2, 79%N2:
- This is what current space stations use.
- It means heavier structure due to mixed gas storage and
regulation.
- It means that going in to spacesuits is an issue due to the drop in pressure and therefore risk of getting decompression sickness.

Pure oxygen at 25 - 35 kPa:

• This was used in Mercury capsules and Apollo 1, but is a serious fire risk and resulted in the deaths of Apollo 1 crew on launchpad.
• Lower pressure means lighter module can be built.

Air at 70 kPa:

• People can breathe air at lower pressure as cities on Earth have this pressure.
• No one has yet tried this for a space station although pressure is dropped for preparation for EVAs.

Question 14

Describe the pork chop plot.

Answer 14

• For interplanetary missions.
• Bone pieces describe the optimum/minimum C3 for the shortest transfer time.
• Necessary when considering real bodies whose orbits are not coplanar or circular.

Question 15

What is a free return trajectory?

Answer 15

Free return is where the spacecraft is sent on a big eclipse that intersects with the path of the target. The period of the ellipse around the Sun is a whole multiple of the Earth’s orbit, thus after N years, both the spacecraft and Earth are in the same position.

Question 16

List the main functions of a spacesuit

Answer 16

1. Remove CO2
2. Supply water
3. Remove hygiene waste
4. Maintain pressure
5. Remove waste heat
6. Maintain body temperature
7. Prevent micrometeoroid damage
8. Communications
9. Mobility of body
10. Vision

Question 17

Describe the different spacesuit concepts.

Answer 17

Soft Suits - Fabric is the pressure vessel. To prevent ballooning, constant volume joints are used.

Hard Suits - Aluminium or composites for the pressure shell. Movement is allowed by rotation joints but is still limited.

Hybrid - A combination of the above.

Mechanical Counter Pressure - Instead of pressurised gas it uses layers of stretchy material to shrink-wrap the body. MIT are developing a ‘biosuit’ with this principle.

Question 18

Describe the heating mechanisms which occur on reentry.

Answer 18

Atmospheric Heating:

• Shock wave ahead of the body.
• Viscous dissipation in boundary layer and wake.

Vehicle Heating:

• Friction (air behind shock wave has significant velocity).
• Conduction (air behind shock wave is hot).
• Radiation (from white hot gases in shock wave).

Question 19

What is the best cooling method for Solid Rocket Booster motors?

Answer 19

Ablative - SRB motors use cast propellant which works like
ablative material and burns from the inside out. So the casing is not affected until the end of the burn when the propellant is nearly completely consumed.

Question 20

What is the best cooling method for an N2 and O2 fuelled Shuttle Main Engine?

Answer 20

Regenerative Cooling -This is best on big engines as very efficient. Only works with liquid propellants.

Question 21

What is the best cooling method for a bi-propellant ‘Reaction Control System’ for in-orbit attitude control?

Answer 21

No cooling - necessary for in-orbit systems as they are exposed to the cold of space (radiative cooling).

Question 22

What are the assumptions for Keplerian orbits?

Answer 22

• Newtonian gravitation
• Point masses
• Two bodies only
• No other perturbation forces apart from gravity

Question 23

List the perturbations that meed to be considered in non-Keplerian orbits.

Answer 23

• Three-body solutions
• Non-sphericity
• Solar radiation pressure
• Atmospheric drag
• Luni-Solar perturbations

Question 24

What do the Jx terms represent?

Answer 24

The deviations of the geopotential from perfect sphericity.

Question 25

List the order of strength of the perturbation forces.

Answer 25

1. Earth’s gravity
2. J2
3. Lunar gravity
4. Solar gravity
5. Solar radiation pressure
6. Atmospheric drag (curve)

Question 26

Explain the significance of the local solar time or ‘LTAN’ for a geosynchronous orbit.

Answer 26

The local solar time or LTAN (local time of ascending node) gives an indication of the
Sun angle. If we choose a ‘dawn-dusk’ orbit the satellite will always have its solar panels in sunlight. If you choose 10:30am then the angle of the Sun on the Earth may be perfect for camera images or, for example, you might want to measure the pollution over Beijing
at noon.

Question 27

Explain the design considerations for rocket combustion chambers which use liquid propellant.

Answer 27

• Volume has to be large enough for mixing, evaporation and complete combustion.
• Volume varies based on reaction times, species and temperature/pressure.
• Weight is a premium, it is a function of pressure and configuration.
• Manufacturing is important so standard thicknesses and shapes should be used.
• Impact on vehicle length and width.
• Pressure drop for accelerating propellants should be minimised.
• Conductive copper alloys move heat away.
• Cooling can be achieved through gas film, regenerative, ablative, heat sinks and radiation means.

Question 28

What extra difficulties might the zero g environment cause in the design of propulsion tanks?

Answer 28

• How to manage the fluids in zero g with the implications that this has upon surface tension and gas bubbles.
• Special propellant management devices or surface tension tanks are needed.
• Bladders can be used to force the fluids to
  collect in a certain place or pressure differentials.

Question 29

Describe briefly the 3 schools of thought on how a reusable ‘Single Stage to Orbit’ launcher could be achieved and their relation to the rocket equation.

Answer 29

Structure Optimists - address the mass ratio and believe structures can be made so lightweight that pure rocket systems are viable SSTO.

Engine Optimists - address the effective exhaust velocity (= Specific Impulse) and believe advanced Scramjet engines can enable direct flight to orbit with typical aircraft mass ratios rather than rocket type mass ratios.

Systems Optimists - believe that the correct balance of existing technology applied to both the engine (with some kind of air-breathing but not scramjets) and the mass ratio can produce a system solution to a viable reusable SSTO.

Question 30

Propose the direction of future space suit design.

Answer 30

Mars Mark III - Hybrid so it is more mobile but still has a high operating pressure which makes it a ‘zero-prebreathe’ suit.

MIT Biosuit - Mechanical Counter Pressure which is much lighter and more flexible than current suits.

I-Suit - An all soft suit with walking boots for planetary exploration and EVAs.

Question 31

Explain how a space station might use a GPS system and trilateration to establish its position in orbit.

Answer 31

The receiver would have to be on the ISS. Assuming the space station is in LEO then it would be below the US GPS network of satellites and can use them just as a receiver on Earth would, except that it is travelling at 7km/s around the Earth so it must be in sight of a minimum of 4 satellites at any one time to eliminate clock errors.