Combustion of Hydrazinium Nitroformate (HNF), N2H5C(NO2)3, has been modeled and the results compared with experimental observations including steady regression rate (pressure, initial temperature, and radiant flux sensitivities), surface temperature, condensed phase temperature profile, and linear frequency response to radiation. The underlying philosophy of the approach is to compare with a wide range of experimental conditions as much as possible. The results indicate that HNF condensed-phase decomposition is overall slightly exothermic (increasing with pressure) with activation energy similar to that for proton transfer between hydrazinium and nitroformate ions. The gas-phase flame is highly exothermic (also slightly increasing with pressure) with low activation energy, suggesting the gas-phase process is more like a low energy-barrier (low Eg), chain-carrier process than a high-energy thermal process, similar to what has been suggested for NC/NG double-base propellant and for HMX. For both of these materials, and now for HNF, the low-Eg model has been shown to be in good agreement with experimental data for both steady and unsteady burning rate. The model also demonstrates the ability to capture an important manifestation of nonlinear combustion. The prediction of a strong initial pressurization spike in a low-L* (high dP/dt) end-burning (no crossflow) motor with HNF propellant confirms our recent hypothesis that initial pressurization spikes may be due to nonlinear dynamic combustion in addition to (or instead of) erosive burning. An analytical solution for the stability boundary of L* instability without the assumption of constant chamber temperature has been derived. Results showed that variable chamber temperature has a slight de-stabilizing effect on L* instability quantitatively but not qualitatively compared with the results obtained from constant chamber temperature. The results also predict a chuffing phenomenon if L* is sufficiently small. The mechanism responsible for chuffing needs further investigation.