Self-Assessment Quiz: Combustion and Propellants

Twenty questions on combustion chemistry, the two performance knobs, propellant families, cryogenics, and combustion instability. Answer each before opening the key. Aim for 16 or more. Use $v_e \propto \sqrt{T_c/\mathcal{M}}$ and treat all propellant numbers as approximate.

Question 1

The energy released by combustion comes physically from:

A) friction between propellant molecules B) the difference in chemical bond energy between reactants and products C) the pressure of the turbopumps D) nuclear binding energy

Question 2

The exhaust velocity of a chemical rocket scales approximately as:

A) $T_c \cdot \mathcal{M}$ B) $\sqrt{T_c \cdot \mathcal{M}}$ C) $\sqrt{T_c / \mathcal{M}}$ D) $\mathcal{M} / T_c$

Question 3

Hydrogen gives the highest specific impulse of any chemical propellant mainly because:

A) it burns at the highest flame temperature B) its exhaust has the lowest molecular weight C) it is the cheapest fuel D) it is the easiest to store

Question 4

The adiabatic flame temperature of real propellants is capped near ~3,600 K primarily by:

A) the melting point of the nozzle B) the boiling point of the oxidizer C) dissociation of the product molecules D) the speed of the turbopump

Question 5

"Mixture ratio" (O/F) refers to:

A) the ratio of chamber to exit pressure B) the mass of oxidizer consumed per unit mass of fuel C) the ratio of exhaust velocity to flame temperature D) the fraction of propellant that is unburned

Question 6

Most rocket engines run fuel-rich of stoichiometric because:

A) it saves money on oxidizer B) excess fuel lowers the exhaust molecular weight faster than it lowers the temperature, raising $v_e$ C) stoichiometric mixtures will not ignite D) it increases the flame temperature

Question 7

A hypergolic propellant is one that:

A) must be stored below 120 K B) ignites spontaneously on contact between fuel and oxidizer C) has the highest possible specific impulse D) contains no oxidizer

Question 8

Which propellant is the hardest to store, and why?

A) RP-1, because it is toxic B) liquid methane, because it is hypergolic C) liquid hydrogen, because of its extreme cold, fast boiloff, and leakage D) N$_2$O$_4$, because it is cryogenic

Question 9

Liquid methane is called "space-storable" partly because its boiling point (112 K) is close to that of:

A) liquid hydrogen (20 K) B) liquid oxygen (90 K) C) water (373 K) D) RP-1 kerosene

Question 10

Which propellant combination is the modern choice for reusable engines because it burns cleanly (little soot)?

A) LOX / RP-1 B) LOX / LH$_2$ C) LOX / CH$_4$ D) N$_2$O$_4$ / UDMH

Question 11

Combustion instability is fundamentally a coupling between:

A) the payload and the guidance computer B) heat release and the acoustic (pressure) modes of the chamber C) the fuel tank and the atmosphere D) two adjacent stages

Question 12

The Rayleigh criterion says an oscillation grows when heat is released:

A) out of phase with the pressure oscillation B) in phase with the pressure oscillation C) only at the throat D) at constant temperature

Question 13

The most destructive form of combustion instability, coupling to the chamber's acoustic modes, is:

A) chugging (low frequency) B) buzzing (intermediate frequency) C) screech / screaming (high frequency) D) boiloff

Question 14

Injector baffles on the F-1 engine worked by:

A) cooling the propellant before injection B) physically breaking up the transverse acoustic modes across the injector face C) adding more oxidizer D) increasing the chamber pressure

Question 15 (True/False, justify)

"A propellant that burns cooler always has a lower specific impulse." True or false? Justify in one sentence.

Question 16 (True/False, justify)

"Green propellants are chosen mainly because they have higher specific impulse than hydrazine." True or false? Explain briefly.

Question 17 (True/False, justify)

"Per kilogram of fuel, hydrogen carries far more energy than kerosene, so per kilogram of propellant mixture it must also carry far more." True or false? Say why.

Question 18 (Short answer)

Explain in one or two sentences why a naive energy balance predicts a hydrogen–oxygen flame temperature near 4,700 K, but the real chamber sits near 3,300 K.

Question 19 (Short answer)

An engine has $\gamma = 1.2$, $T_c = 3{,}300\ \text{K}$, exhaust $\mathcal{M} = 0.013\ \text{kg/mol}$, and expands from $p_c = 200\ \text{bar}$ to $p_e = 0.1\ \text{bar}$. State (you may quote from the chapter) its approximate $v_e$ and $I_{sp}$, and name the propellant.

Question 20 (Short answer)

In your own words, why is combustion the reason the "tyranny of the rocket equation" bites so hard for chemical rockets — that is, why can't chemistry give arbitrarily high specific impulse?

Answer Key

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