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EGA-341 Space Propulsion > Theory > Flashcards

Theory Flashcards

1
Q

What are the propulsion requirements for the three key functions:

Orbit Achivement

Orbit Adjustments

Disposal

A

Orbit Achievement - High thrust, High efficiency, low precision

Orbit Adjustment - High precision, efficiency not an issue

disposal - high efficiency, high dormant reliability, low precision

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2
Q

In comparison to chemical propulsion, what are the advantages of electric propulsion systems?

A

Chemical propulsion has high trust but poor exhaust velocity, high exhaust velocity can be achieved by electric propulsion

Good mass savings in terms of propellant (6% payload ratio for chemical, 60% for electric)

Electric propulsion supplies small thrust for a long time

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3
Q

State the four different types of electrothermal thrusters

A

Resistojet
Arcjet
Electrothermal Hydrazine
Microwave Electrothermal
Pulsed Electrothermal

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4
Q

In general, electric propulsion is used as the main propulsion to achieve a high
exhaust velocity (Ve) for deep space missions. Assuming expellant mass flow rate is constant during propulsion burn time and the expellant velocity is also during the
burn. The power needed for electric propulsion (Pe) in kW is expressed by the
Equation 3-1. The power system mass (Mp) is expressed by the Equation 3-2.

1 Pe = 0.5((Me/Tb)Ve^2)
2 Mp = (Mt-Msc)/1+(2TbPsp/Ve^2)

Me = 5 kg; Tb = 60 sec; Ve = 10 km/sec; Mt = 10 kg; Psp = 2 W/kg

(i) Using the given equations and parameters, calculate the power needed for
electrical propulsion. [1 mark]

(ii) Using the given equations and parameters, calculate the power system mass and spacecraft mass. [2 marks]

(iii) With the obtained masses from (d)-(ii), draw appropriate mass versus exhaust
velocity (Ve) diagram, illustrating the characteristics of expellant mass (Me),
power system mass (Mp), total bus mass, available payload mass and total
spacecraft mass (Mt) as a function of Ve.

(iv) The spacecraft is fitted with a GaAs/Ge solar cell (efficiency, 14.7%), which is to
be used for generating the required power for the electric propulsion. The solar
cell efficiency (n) is expressed by Equation 3-3.

3 n = P/Js*A

Where
P: electrical power output (in W)
JS: solar flux (= 1370 W/m2)
A: surface area (in m2)
Four surfaces of the spacecraft, each with dimension = 2000 mm x 1500 mm,
can be utilised for embedding the solar cells. Comment on the feasibility of
generating the power (Pe) obtained in (d)-(i)

A

2018 Q3d

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5
Q

Draw a diagram and explain the working principle of a solar sail

A

Solar sails use photo pressure of force on thin, lightweight reflective sheets to produce thrust; the ideal reflection of sunlight from the source.

Bow and arrow diagram 2018 Q4a

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6
Q

Tsail = 9.113E-6 * (RA/D^2)*sin^2(theta(t))
ac = 8.25/sigma

(i) With the parameters given below, calculate the total mass of the solar sail.
[Parameters]
Tsail = 0.5 N
theta(t) = average over period of time (t) = 75deg angle
D = 1 AU
sigma = 5.27

(ii) Calculate the expected sail velocity (in km/s) if the solar sail travels for 3 years.

A

i) 319.49kg

ii) 148.1km/s

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7
Q

Draw a diagram and explain the relationship of a solar array power versus dry mass of geostationary earth orbit satellites by considering the scaling for power level

A

Scaling for power level Mn = Ms*(Pn/Ps)^0.7

Array power V Spacecraft by mass
Thin line
2018 Q4c

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8
Q

What are the advantages and disadvantages of regulated bus (RB) with direct energy transfer (DET)

A

Advantages: Electronics are simple and saves weight; therefore reducing launch cost.

Disadvantages: Solar array voltage and power operating point must be adjusted and optimised to End of Life (EOL) operational conditions.

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9
Q

Draw the essential functional blocks of an EPS satellite

A

2019 Q3a

[Primary Power source] [Power Conditioning]

[Energy Storage] [Charge and Discharge Control]

Connected to both [power distribution] [Power Consumer]

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10
Q

If conversion of primary energy is performed by using the photovoltaic effect of solar cells, describe briefly how the photoelectric current is generated

A

There are 2 processes for the conversion of sunlight in electrical energy within a solar cell:

1 - The absorption of solar radiation within a light absorbing semiconductor and the associated generation of charger carriers.

2 - Seperation of the electrons and holes under the influence of the electric field across a semi-conductor junction , thus generating an electromotive force and photocurrent

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11
Q

A solar cell is characterised by the following 5 parameters. Describe each

Isc
Voc
Imp
Vmp
Fill factor

A

Isc - Short circuit current (V=0; R = 0)
Voc - open circuit voltage (I=0; R=infinity)
Imp = Maximum power point current (current at max solar cell output power)
Vmp = Maximum power point voltage (Voltage at maximum solar cell output power)
Fill Factor = (IscVoc)/(ImpVmp)

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12
Q

What is a solar sail and explain its physical principle for generating sailing propulsion

A

A solar sail is made of large, thin mirrors and are a form of spacecraft propulsion using solar radiation pressure.

The solar radiation exerts a pressure on the sail due to reflection.

The momentum of a photon is given by p = E/c. A sail will have an overall specular reflection efficiency of around 90%

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13
Q

Do 2019 Q3d

A

2019 Q3d

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14
Q

What type of propulsion subsystems are generally used for the following tasks

i) Apogee injection
ii) Orbit Control
iii) Attitude Control
iv) Additional Tasks

A

i) Apogee engine with a thrust level of 400 to 600N (Bipropellant)
Engine is activated in the apogee to move the satellite into the required circular orbit

ii) Thrust levels of 10 to 22N (monopropellant)
Maintenance of required position
Injection into a graveyard orbit

iii) Thrust levels of 10 to 22N (monopropellant or bipropellant)
Orientation of the satellite for pointing antennas towards the earth

iv) Thrust levels in the order of some millinewtons to about 1N (electric propulsion)
Providing large Dv, precise course corrections during flights lasting several years.

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15
Q

What are the three major categories of electric propulsion? Give one example for each of the three catagories

A

Electrostatic propulsion - Ion Bombardment, Hall effect

Electromagnetic propulsion - MPD, variable specific impulse plasma thruster

Electrothermal propulsion - Resistojet, Arcjet

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16
Q

Advantages and disadvantages of an electric propulsion system compared to a chemical propulsion system

A

Advantages: High Isp
High payload ratio
Less propellant mass

Disadvantages: Low thrust
Must be qualified for much higher thrust durations

17
Q

Advantages and disadvantages of an electric propulsion system compared to a chemical propulsion system

A

Advantages: High Isp
High payload ratio
Less propellant mass

Disadvantages: Low thrust
Must be qualified for much higher thrust durations

17
Q

Advantages and disadvantages of an electric propulsion system compared to a chemical propulsion system

A

Advantages: High Isp
High payload ratio
Less propellant mass

Disadvantages: Low thrust
Must be qualified for much higher thrust durations

18
Q

For missions to comets, electric propulsion is used as the main propulsion to
achieve a high velocity increment (v). The characteristic of the ratio of propellant
mass (mp) to satellite mass (ms) as a function of v and effective exhaust velocity
(ve) can be represented by the basic rocket equation:

mp/ms = 1-e^(-dv/ve)

Typical achievable v for various propulsion systems is known (on the basis of
mp/ms = 65%):
- Cold gas system 74 m/s
- Monopropellant system 233 m/s
- Bipropellant system 340 m/s
- Electrical system 3140 m/s

Using the Equation 4-1 and considering the above achievable v, draw mp/ms vs v
curves for the various propulsion systems. Assume that the v varies between 5
and 2000 m/s, and ve is constant (i.e. once ve is calculated when mp/ms = 65%, this
ve value remains the same irrespective of v values).

A

Vary dv from 5 to 2000m/s

Ve values

  • Cold gas system 70.488 m/s
  • Monopropellant system 221.942 m/s
  • Bipropellant system 323.864 m/s
  • Electrical system 2990.983 m/s
19
Q

Electrothermal thrusters use ionisable gases as the propellant. Arcjet thruster, for example, used Hydrazine. What is the advantage and disadvantage of hydrazine when compared to hydrogen?

A

+ Can be used for a dual combination system on a satellite. Can be easily stored

  • Chemical erosion problems at higher Isp. Heat transfer problems at nozzle and chamber
20
Q

What is plasma?

A

4th state of matter. It has some of the properties of a gas but if affected by electric and magnetic fields.

21
Q

What is the Lorentz Force?

A

A combination of electric and magnetic forces on a point charge due to electric fields.
F = q(E+v*B)

22
Q

Determine the value of the propellant temperature sensitivity coefficient (sigma_p) if the pressure inside the combustion chamber changes for 10% (positive change) when initial temperature changes for 40% (positive change)?

A

Example 1

23
Q

For a rocket where L2 = 2*L1

Calculate:
a) The burning time and pressure inside the combustion chamber if the initial temperature of the propellant is 288K
b) when the initial propellant temperature is 323k

sigma 1 = sigma 2 = 0.002
n = n1 = n2 = 0.5
b01 = 2*b02
t1 = 3s
P02 = 40bar

A

Example 2

24
Q

The solid rocket motor with propellant grain composed of two types of propellant – Type A and
Type B (propellant characteristics are given in Table Q3-1) in ratio of 70% of propellant A and 30%
of propellant B, has constant thrust of F = 15000 N, combustion pressure pc = 150 bar, and active
burning time of tend = 6 s. The nozzle has full expansion pexit = pambient = 1 bar. For the previously
described rocket motor calculate:
(a) Pressure integral ∫ ⋅=
(b) Total Impulse ∫ ⋅=
(c) Gas constant for the combustion products Rmix = ii Rg ⋅∑
(d) Combustion temperature T0
(e) Specific heats Cp and Cv for the combustion products R = Cp-Cv
(f) Specific heat ratio for the combustion products
κ = Cp/Cv
(g) Specific impulse
h) Propellant mass

T0A = 2500k
Ka = 1.3
rhoA = 1800
RA = 340

T0B = 3000
Kb = 1.2
rhoB = 1600
RB = 330

A

Example 3

25
Q

For the solid rocket motor defined by propellant grain represented on Figure Q4-1, nad
diagrams: Pressure vs. Time diagram - Figure Q4-2, Thrust vs. Time diagram - Figure Q4-
3, throat diameter dt = 18 mm, mass of the solid rocket motor before the firing test Mbefore =
18 kg and after firing test Mafter = 16.25 kg, calculate

Check example 4

A

Example 4

26
Q

A sounding rocket has the following characteristics:
* initial mass = 250 kg;
* mass after solid rocket motor burn-out =180 kg;
* payload + structure without solid rocket motor = 160 kg;
* solid rocket motor burning time = 3.5 s;
* average solid rocket motor specific impulse = 2400 m/s.
Determine the Rocket’s mass ratio, propellant mass fractions (based on rocket
and propulsion system), propellant flow rate, thrust, thrust/weight ratio,
accelerations and total impulse.

. Propellant mass (mp) [2 mark]
ii. Propulsion system initial mass (m0ps) [2 mark]
iii. Mass ratio of vehicle = mb / m0 [1 mark]
iv. Propellant mass fraction (propulsion system) = mp / m0ps [1 mark]
v. Propellant mass fraction (rocket) = mp / m0 [1 mark]
vi. Propellant mass flow rate m [2 mark]
vii. Thrust (F) = Isp · m [1 mark]
viii. Initial thrust/weight ratio = F / W0 [2 mark]
ix. Final thrust/weight ratio = F / Wb [2 mark]
x. Initial acceleration a0 = F / mo [1 mark]
xi. Final acceleration ab = F / mb [1 mark]
xii. Total impulse (Itot) = F · tb [1 mark]
(TOTAL 25 MARKS)
Turn Over

A

Example 5

27
Q

he following measurements were made during solid propellant rocket motor test at
sea level conditions:
* burning time (tend) = 45 s;
* SRM mass before test (mo) =1250 kg;
* SRM mass after test (mb) = 350 kg;
* average thrust (F) = 65 kN;
* chamber pressure(P0) = 7 MPa;
* nozzle exit pressure (Pext) = 70 kPa;
* nozzle throat diameter (dt) = 0.08 m;
* nozzle exit diameter (dext) = 0.21 m.
(a) Determine m& , uext, C*, CF and ISP at sea level,
(b) and F and ISP at 1000 m (Pa = 89.877 kPa),
(c) and F and ISP at 25000 m (pa = 2.511 kPa) altitudes.
Assume a constant thrust and mass flow rate.

A

Example 6

EGA-341 Space Propulsion (3 decks)

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