Question bank — Green propellants — LMP-103S, AF-M315E (vs hydrazine)

False. They are low-toxicity and low-vapour, but ADN and HAN are energetic oxidisers that can deflagrate, and HAN is corrosive. "Green" means reduced handling hazard, not inert.

False. For a fixed tank volume higher density packs more propellant mass, so total impulse rises as $\rho I_{sp}$. Density is an asset, not a penalty.

False. Greens need a much hotter catalyst preheat (high light-off temperature), so hydrazine's easy instant cold-start still wins for power-limited or fast-response missions.

False. Its dominant hazard is a toxic, carcinogenic vapour cloud from its high vapour pressure — that is why crews wear SCAPE suits. The green win is that salt ions barely evaporate.

False. The seconds come from dividing $v_e$ by $g_0$; $I_{sp}$ means "seconds that 1 kg of fuel can support 1 kg of weight." It is a unit-system-independent efficiency, unrelated to burn duration.

True. The $\mathrm{N}{\mathrm{O}}_2$ (dinitramide) and $\mathrm{N}{\mathrm{O}}_3$ (nitrate) groups supply oxygen internally, so the fuel is oxidised without needing air — that is why they work as monopropellants. See Oxidisers — nitrate & dinitramide chemistry.

False. Its exhaust is actually heavier ($M$ larger). It wins because it runs much hotter ($T_c$), and in $\sqrt{T_c/M}$ the temperature gain outweighs the mass penalty.

False. They are concentrated aqueous solutions / low-melting salt mixes; the water helps keep them liquid and stable. "Ionic" refers to the non-volatile salt ions doing the safety work — see Ionic liquids.

Wrong operation. $I_{sp}=v_e/g_0$, so you multiply: $v_e=I_{sp}{g}_0=266\times 9.81\approx 2609\text{m/s}$. Dividing gives a nonsensically slow exhaust.

Incomplete. A fixed tank holds $\rho V$ kg, so the tank-level metric is ==$\rho I_{sp}$== (density-impulse), not $I_{sp}$ alone. A denser, slightly-lower-$I_{sp}$ fuel can still win.

Missing the ammonia step. It first goes $3{\mathrm{N}}_2{\mathrm{H}}_4\to 4\mathrm{N}{\mathrm{H}}_3+{\mathrm{N}}_2$; the partial cracking $2\mathrm{N}{\mathrm{H}}_3\to {\mathrm{N}}_2+3{\mathrm{H}}_2$ then raises $T_c$ and lowers $M$. See Thermochemistry & enthalpy of decomposition.

The ratio must sit inside a natural log: $\Delta v=v_e\ln (m_0/m_f)$. Without the log, spending fuel would give unbounded $\Delta v$, which is unphysical — see Rocket equation (Tsiolkovsky).

No — they are monopropellants. The oxidiser (nitrate/dinitramide) and fuel are already blended in one liquid; a separate oxidiser tank defeats the whole point.

The product list says nothing about $T_c$. Green blends actually run hotter than hydrazine; benign products and high temperature are independent facts.

Dividing thrust (a force) by a weight-flow ($\dot{m}{g}_0$) cancels mass and length units, leaving seconds — the same number whether you started in kg-m or lb-ft.

Harm requires exposure; a non-volatile salt cannot form an inhalable cloud, so even a moderately hazardous salt is far safer to handle than a volatile poison like hydrazine.

For a given thermal energy per unit mass, heavier molecules move slower ($v\propto 1/\sqrt{M}$), so $v_e$ and hence $I_{sp}$ drop — this is why hydrazine's light ${\mathrm{H}}_2/{\mathrm{N}}_2$ exhaust is its one advantage.

Their salts have a higher light-off (activation) temperature for decomposition, so the catalyst bed must be electrically warmed more before the reaction self-sustains — see Catalysis.

Small satellites are volume-limited, not mass-limited, so total impulse scales with how much propellant mass fits the tank ($\rho$) times its efficiency ($I_{sp}$).

In $\sqrt{T_c/M}$ the flame temperature enters as strongly as molar mass; AF-M315E's temperature ratio more than compensates for its heavier products, so the square-root still favours it.

Extra inert water absorbs heat and lowers ==$T_c$== while adding mass to the exhaust, so $\sqrt{T_c/M}$ and $I_{sp}$ fall — there is a safety-vs-performance trade-off.

It fails to light off (no self-sustaining decomposition), so the thruster produces little or no thrust — this cold-start limit is exactly where hydrazine still wins.

The bracket $\left[1-(p_e/p_c{)}^{(\gamma -1)/\gamma }\right]$ becomes zero, so $v_e=0$ — no expansion means no acceleration; the gas must expand through the nozzle to gain speed.

$\ln (m_0/m_f)\to \ln 1=0$, so $\Delta v\to 0$ — burning almost no fuel gives almost no velocity change, as expected.

The denser one — it reaches the same total impulse in a smaller tank, freeing volume for payload, even though the mass-level performance is tied.

The chemistry fixes the products; you cannot choose $M$ freely, and lighter exhaust (more ${\mathrm{H}}_2$) usually comes bundled with lower $T_c$, so the two levers fight each other.