Space Review: Two letters in Space Review debate whether the risks associated with the hydrazine fuel tank were fully understood by the public and the US Defense Department (DoD) before the US Navy shot down a disabled US spy satellite (see Broken spy satellite hit by US missile). The US was concerned that the fuel tank might survive reentry into the atmosphere and contaminate a wide area with the toxic hydrazine fuel.
Andrew Higgins of McGill University in Montreal, Canada, suggests that critics of the decision to shoot down the satellite, have not fully grasped hydrazine's burn rate at the pressure contained in the tank or calculated the tank's reentry survivability.
In independent computer simulations of the reentry of the USA 193 satellite, Geoff Forden of MIT and Higgins, found that the maximum deceleration of the tank would have been about 8 to 10 g’s. "This is similar to the g-loading the fully fueled tank is designed to withstand upon launch," says Higgins. "Thus, it is unlikely that a similar loading would have destroyed the tank on reentry."
Yousaf Butt of the Center for Astrophysics at Harvard University points out that questions should be raised about the overall quality of the DoD's reentry simulation models, because the DoD did not predict the hydrazine explosion that occurred during the interception.
"It would serve NASA/NRO/DoD well to immediately publicize the unclassified portions of their studies so that the US public can ascertain whether the putative public health concern of the hydrazine surviving reentry was indeed well-founded," says Butt. "As technical details of hydrazine tanks are freely available online, it is difficult to comprehend what is so classified about these studies."
Related links
Broken spy satellite hit by US missile (Physics Today Online)
North Canada, Pacific and Atlantic Oceans likely path of spy satellite debris (Physics Today Online)
More doubts surface over Pentagon’s explanation for shooting down spy satellite (Physics Today Online)
Re-entry models, including those of Geoff Forden of MIT (which I have seen), predict a range of re-entry peak decelerations: anywhere from ~10g to ~50g depending on the angle of incidence. The value of 25g I used is very middle of the road and is backed-up by standard texts on the subject. Furthermore, the high ballistic coef. of a fully loaded tank would lead one to assume it would suffer both higher peak heating, and peak deceleration.
The question of the burn rate mentioned by Prof. Higgins is relevant to a vapor fire proceeding to the liquid. However, if you look at the definitive text on this subject you will see that liquid hydrazine itself can explode/ deflagrate/ undergo thermal runaway:AIAA’s Special Project Report SP-084-1999 “Fire, Explosion, Compatibility, and Safety Hazards of Hypergols - Hydrazine”, states:
"The detonation of neat liquid hydrazine has not been observed… Although there have been reported explosions of liquid hydrazine, at present none of these observations is thought to be the result of liquid detonation but rather they are attributed to the presence of multiple phases in the liquid hydrazine or other mechanisms."
In any case, as the empirical evidence of even frozen hydrazine being able to burn in vacuum for "tens of seconds" (as in the interception) suggests that the hydrazine would not have survived re-entry.
The tank was tumbling and open at the fuel-lines such that the re-entry heat could be directly transferred to the interior hydrazine.
Even if it did make it down and you assume a 5m kill-radius this gives a probability of being injured or dying of
An upcoming commentary by the American Medical Associations should be read by all interested.