Exam 2016, questions PDF

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PRIFYSGOL ABERTAWE SWANSEA UNIVERSITY College of Engineering SEMESTER 2 MAY/JUNE 2017

EGA-341

SPACE PROPULSION AND POWER SYSTEMS YEAR 3 You may only use calculators provided by the University. Candidates may only refer to the English Dictionaries that are available at the venue. Translation dictionaries are not permitted.

Time allowed:

2 hours Answer THREE questions

Please insert any standard constants, or additional information here:-

TURN OVER

Page 1 of 7

Note: all descriptions must be concise, essential, readable and limited in size (maximum 150 words). Answers to numerical questions must show the final numerical solution with appropriate dimensions (measuring units), as well as the calculation steps that you made to reach the answer. Q1. a) What is the difference between the terms “specific impulse” and “total impulse” for a rocket motor (use the mathematical equations to answer the question)?

[2 marks]

b) Which type of the propellant would be most suitable for each of the following applications: (i)

Booster for the satellite launch vehicle

[1 mark]

(ii)

Main engine of the satellite launch vehicle

[1 mark]

(iii)

a rocket engines for the satellite manoeuvrings

[1 mark]

c) Two different solid rocket motors (SRMs) are represented on the Figure 1-1 and on the Figure 1-2. Both SRMs shown on the figures have the same size of the throat diameter “dt” (the throat does not erode during burning), and identical propellant, i.e. density of the propellant “p”, characteristic velocity of the propellant “C*”, linear coefficient in the burning rate law “b”, and exponential coefficient in the burning rate law “n” are constant, and the same for both solid rocket motors (dt = const, p = const, C* = const, b = const, n = const). In this question, you should mathematically prove by deriving the equations, whether it’s possible or not, to achieve “neutral” burning of solid rocket motor with appropriate ratios (proportions) between the propellant grain inner diameter “d”, outer diameter “D” and the propellant grain length “L”. The propellant grain configurations are as follows: (i)

The propellant grain inhibited on the outer diameter and both foreheads, burning from the inside (from the central port) – Figure 1-1

(ii)

[10 marks]

The propellant grain inhibited on the both foreheads, burning from the inside (from the central port) and from the outside – Figure 1-2

[10 marks]

Question 1. continues on the following page

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Question 1. (continued)

w

Figure 1-1: Scheme for Question Q1 c) (i)

Figure 1-2: Scheme for Question Q1 c) (ii) Remarks: 1) Neutral Burning – dP/dt = 0, dF/dt = 0, 2) Thickness of the propellant grain inhibitor is negligible Useful equations: Equation of equilibrium combustion pressure inside SRM: 1

S P   b    C*  b  At

 1 n  

Burning rate L aw:

r  b  P n - Burning rate Law in general form End of Question 1.

(TOTAL 25 MARKS) TURN OVER

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Q2. a) For a stoichiometric mixture, which bi-propellant combination of the liquid oxidizer and the liquid fuel has the highest specific impulse amongst the liquid rocket propellants? [2 marks] b) Name any two (out of six) types of the liquid rocket engine cycles?

[4 marks]

c) Which chemical substance is primarily used as oxidiser in contemporary composite solid rocket propellants? What is the chemical formula of the substance in question?

[4 marks]

d) The solid rocket motor (SRM) shown on the Figure 2-1, has two-stage end-burning propellant grain, i.e. the propellant grain with the “booster” and the “sustainer” part (stage) of the propellant grain. The first stage, or the booster stage, has constant burning surface Sb1 = Sb = const and the web thickness w1= w. The second stage, or the sustainer stage, has S the burning surface Sb2= b = const and the web thickness w2= 2w. Propellant for both 2 burning stages is same, with exponent coefficient in burning pressure law n= 0.5. Nozzle is without divergent part (At=Aexit,

 exit = 1).

Figure 2-1: Scheme of propellant grain for Question 2 In this question, you should calculate the following ratios:

P1 P2

(i)

the pressure ratio between booster and sustainer stage

(ii)

the burning time ratio between booster and sustainer stage

(iii)

the thrust ratio between booster and sustainer stage

[5 marks]

t1 t2

[5 marks]

F1 . F2

[5 marks]

Useful equations: 1

Equation of equilibrium combustion pressure inside SRM: Burning rate Law: Thrust:

r  bPn-

S P   b   C * b  At

 1 n  

Burning rate Law in general form

F  C F  P  At

Specific impulse: I sp 

F A p  pa  CF  C *  uexit  exit ( p exit  p a )  u exit  C *   ( exit )   m m pc (TOTAL 25 MARKS)

End of Question 2.

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Q3. (a)

What is electron bombardment and explain how is it implemented in a gridded ion thruster? [5 marks]

(b)

How does electromagnetic propulsion (EMP) differ from the electrothermal propulsion system? [2 marks]

(c)

Briefly describe, what is an ion and ionisation? [3 marks]

(d)

Briefly describe, what is the working principle of resistojet thruster? [3 marks]

(e)

The characteristic of the ratio of propellant mass (mp) to satellite mass (ms) as a function of v is expressed by the Equation 3-1. For a typical monopropellant system, effective exhaust velocity (ve) = 210 m/s and velocity increment (v) is found to be 125 m/s. ……. Equation 3-1

(i)

Calculate the propellant mass (mp) when the satellite mass (ms) is 2100 kg. [2 marks]

(ii) With the calculated value of mp from (i) and the derived mp/ms ratio, suggest a way forward to achieve a higher velocity increment (v) for the satellite. [5 marks] (f)

In general, the Arcjet (N2H4) thruster is used for a combination propulsion system on satellites. It has the disadvantage of having chemical erosion problems, which can be intensified at higher specific impulses. Describe what is meant by the term ‘chemical erosion’. [5 marks]

End of Question 3.

(TOTAL 25 MARKS)

TURN OVER

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Q4. (a)

Briefly describe the working principle of a fuel cell.

[3 marks]

(b)

In terms of solar array performance, how does the silicon based solar array cell differ from the TJ-GaAS cell? [4 marks]

(c)

With reference to Figure 4-1, estimate the maximum power that can be generated. [2 marks] 100

Power (W)

Current (A)

80

60

40

20

I-V curve Power curve

0

0 0

20

40

60

80

100

120

140

Voltage (V)

Figure 4-1. I-V characteristic and the associated electrical power generated

(d)

Solar sail propulsion can be estimated by the following expression (Equation 4-1). 𝑅𝐴

𝑇𝑠𝑎𝑖𝑙 = 9.113 × 10−6 𝐷2 𝑠𝑖𝑛2 (𝜃(𝑡))

….. Equation 4-1

With the given parameters, calculate the characteristic acceleration (ac) of the solar sail if ac is expressed by the Equation 4-2. 𝑎𝑐 =

8.25 2 , in mm/s (1AU, assuming 90% efficiency) ….. Equation 4-2 𝜎

[Parameters] Tsail = 2N R = 0.86 Sail tilt angle (average over period of time (t) = 45 deg angle) D = 1 AU Total mass of the solar sail = 225 kg  = sail loading (total mass/sail area (A), expressed in g/m2) [5 marks] Question 4. continues on the following page TURN OVER Page 6 of 7

Question 4. continued (e)

List and briefly describe the three life-determining parameters of a satellite battery. [6 marks]

(f)

Figure 4-2 shows the example of electrical power system inside the International Space Station. It is equipped with a battery charge discharge unit (BCDU), which is a combination of battery charge regulator (BCR) and battery discharge regulator (BDR) devices. What are the advantages and disadvantages of having BCR and BDR?

[5 marks]

Figure 4-2. International Space Station (power system)

End of Question 4.

(TOTAL 25 MARKS)

END OF PAPER

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