**General comments**

I covered the derivation of the Rocket equation, which relates principal figures of merit for rockets (structural efficiency and Isp) to rocket performance (velocity, height, etc.). This is described in Chapter V of the notes. Please note however, that the equation is derived in a different (but equally valid) manner in the notes. We did two PRS questions (PRS #1, PRS#2)

Next lecture we will go back to aircraft engines and develop relations that connect principal design paramenters (like compressure pressure ratio, turbine inlet temperature, and flight Mach number) to the principal figures of merit for air-breathing engine performance(thrust and efficiency). Please review Chapter VIII of the notes.**Good call on the Wednesday
donuts****.**
(1 student)

** Donuts are great, except
over Passover!!!** (1 student)

** Thanks for the heads-up on
the homework, it kind of makes me want to start early.** (1 student)

** Just curious, are you from
the NY or NJ area?.** (1 student) Yes, I grew up in Warren, NJ,
about 15 miles outside of NYC.

**Responses to 'Muddiest Part of the Lecture Cards'**

(36 respondents, 53 students in class)

1)* The integrals seem a little
shady to me, but I guess the results look nice. *
(1 student)

2) * Why did you drop out the
drag term in the duo equation?* (1 student) Only in order to simplify
the analysis. Typically, drag would be included and would be expressed as a
function of altitude and flight velocity (e.g. Cd*1/2ru^2).
This complicates the integral.

3)* Are rockets continually
accelerating throughout or is there a point perhaps due to increased drag that
it stops accelerating?* (1 student) I think they keep accelerating,
but there may be special cases where this is not the case.

4) * Can you make a brief argument
about what "forces" mean in the integral momentum equation, I am not
sure whether they are forces on the flow or forces on the body?*
(1 student). The forces (usually the left hand side of the integral momentum
equation) are all external forces that act on the mass within the control volume.
These can be pressure forces, body forces, shear forces, and external forces.
The exact nature of the forces can change depending on where the control volume
lines are drawn. For example, consider a situation where pressure is acting
on a body and these are opposed by a reaction force, say within a strut, that
holds the body in place. If the control volume is drawn around the surface of
the body, the forces are pressure forces. If the control volume is drawn through
the strut, the external force is the reaction force in the strut.

5)* I was just curious about
rockets, what is the difference between boosters and stages? At burn out, doesn't
that mean the fuel/oxidizer is all used up? Why isn't the stage released right
away?* (1 student)

6) * Will the spreadsheet you
mentioned in class (for the homework problem) be useful for other problems?* (1 student) No. You will get only one homework problem like this, but it is
almost as easy to put it into a spreadsheet (easier in my opinion) and it then
allows you to play with it to better understand the sensitivity of rocket performance
to the various design parameters. This is strictly however, and "exercise
for the reader" as they say. It is not required that you do this. I just
thought you might be interested in doing it if you are into rockets.

7) * So where is the equation for the final height?
Also, instead of hcoast, maybe you should call it hballistic
since this was the terminology used in 8.01 and the water rocket?*
(1 student). The equation is in the notes.
I agree, "ballistic" is better.

8) * Which way does the fuel
burn?* (1 student). Solid propellant rockets use a variety
of propellant geometries to tailor the propelllant burn rate to meet mission
needs. For more discussion see Hill and Peterson,

9)* What does Isp= ?*
(1 student).

10) * In the rocket equation,
isn't g a function of t?.* (1 student) Yes it is, however, for
many applications (low earth orbits for example) it does not vary a great deal
between the surface of the earth and the orbit and thus can be considered constant.
I set it equal to a constant to simlify the analysis. In a more advanced treatment,
this would be written as a function of height.

11) * What are typical rocket accelerations?*
(1 student). It depends on the application (e.g. manned launches typically have
lower g-loadings that instrumentation and warhead payloads which can be designed
to withstand greater acceleration), but I think typical values are 5-10g's.

12)
* No mud* (21 students).