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Unformatted text preview: AAE 439 Final Exam
Fall, 2000 Open Book & Notes
Attempt All Problems  1. A liquid rocket engine/propulsion system is to be employed to accelerate a space
probe weighing 1000 lb to a AV of 10,000 f/s. The engine has an expansion ratio
'of 50 and the propeIlants produce a C* of 6000 f/s at “(:12 Assuming the
propellant mass fraction of the propulsion system is 0.8 determine i) The propellant and inert masses associated with the propulsion system. ii) The nozzle manufacturer proposes to put a skirt on the end of the nozzle to
increase the expansion ratio to 150. This change will add 100 lb of inert
weight to the existing design. The manufacturer argues that this change will
increase the AV capabilities of the propulsion system — a desirable feature for
this application. Should you implement this change? i.e.r is the nozzle
manufacturer correct? 2. A ground launched interceptor utilizes a cylindrical fuel tank 30 inches in diameter
and 100 inches long. The tank has a ullage pressure of 30 psia and a maximum
internal pressure of 120 psig. The vehicle has a maximum acceleration of 20 g’s
and the fuel tank supports 1000 lb in upper stage propulsion, interstage and
fairings. The tank is to be built from aluminum 6061 alloy which has the
following properties: Ultimate tensile strength = 42 Ksi
Yield strength (tension) = 36 Ksi
Modulus = 10x106 Psi
Density = 0.098 lb/in3 i) Determine the minimum acceptable wall thickness for this tank. ii) Assuming constant wall thickness throughout, estimate the weight of the
tank skin. I . 3. Consider the solid rocket ballistic test motor shown in the sketch below. The small
motor uses a cylindrical grain with a center perforation in order to achieve a nearly
neutral burn. i) Determine the LID ratio which will provide identical chamber pressures at
' ignition and just prior to burnout of the propellant grain. ii) Assume we want a total of 0.5 Kg of propellant in the test motor and we
desire a chamber pressure of 3 MPa for the test. Determine the dimensions
L, D, and Dt to produce these results. The propellant characteristics are:
Burn Rate = 1 CW5 , Density = 1700 Kg/m3 c* = 1500 m/s, 7: 1.2, Hint: if you can’t get an answer in Part ii), you may assume LID : 2 to work
this part of the problem. ree—L———>1 4. In our discussion of hydrazine performance, we noted that the decomposition
reaction could be written in terms of the fraction of NH3 (ammonia) dissociation,
x: 3N2H4 —)4(1—X)NH3 +(l +2x)N2 +6XH2 For optimal performance (Isp), we desire x 50.2. Suppose we ran a test of the
thruster at 500 psi and determined that x E 0.3 for this condition. Can you predict
(i.e. calculate) the chamber pressure required to reduce x to the desired value?
You may assume that a negligible variation in ﬂame temperatureoccurs over this
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, , , . Rocket Launch Project Grading Form As I had mentioned in class, your grade for the project will be determined based on the
' reviews of fellow team members working the project. On this form, please evaluate all
other members on your team except yourself. Please consider the following items in assessing your grade: I) Did he/she attend all the meetings/functions required by the team? ii) Did he/she provide inputs in a timely fashion? iii) Did he/she provide high—quality information for use by other team
members?  iv) Did he/she provide adequate leadership/organization of team activities?
(team leaderassessment only) v) Did he/she come up with innovative approaches to solving the problems
posed? ~ ' Grade each member on a 100 point scale. As a general guideline, only isaac Newton
(or an amazing facsimile thereof) should receive 100 points. Students doing an
average job might range from 6070 points; gross omissions or neglect of duties would
correspond to scores below 50 points. Above average and exceptional work is left for
the remaining range from 7099 points (remember, 99 is very Newtonlike). Team Member Name I Score
Team Member Name Score Team Member Name Score Team Member Name Score ...
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