We’ve been discussing the upcoming IXV reentry mission and one of the points that we’ve noted is the need for incredible accuracy in the craft’s navigation. We’ve also heard it oft repeated that if the ‘reentry angle is too shallow, the spacecraft will bounce off the surface of Earth’s atmosphere like a stone skimming the water of a pond.’
We asked Michael Khan, at ESA’s Mission Analysis Office at ESOC, for the details on the challenges of reentry.
The image of a stone skipping off the surface of a pond is appealing for its simplicity, but the reality is a bit more complex, though no less dramatic.
The actual facts about re-entry are as follows.
For a spacecraft designed to re-enter the Earth’s atmosphere or enter a planetary atmosphere such as that of Mars, the conditions at entry, mainly the velocity with respect to the ambient gas and the entry angle with respect to the local horizon, have to be within certain well-defined limits. These limits are what the spacecraft has been designed to withstand.
Clearly, the velocity is a major driving parameter. If it is too high, the thermal loads and braking forces will quickly overwhelm the heat shield and structure. However, for entry from low-Earth orbit, the velocity is unlikely to be any different than expected [typically around 8 km/s, though slightly less at 7.5 km/s for IXV, which will fly on a suborbital trajectory – Ed.].
The entry angle is quite another matter. If the spacecraft has not been navigated very accurately, such that the entry angle deviates only slightly from its nominal target value (so it remains within the so called ‘entry corridor’), things can go badly wrong.
If the entry angle is too steep, deceleration forces (the braking effect due to atmospheric friction) will become too large and the spacecraft can break up. Additionally, the steeper the entry angle, the higher the heat flux. This is a measure for the amount of thermal energy absorbed by the heat shield every second. If the heat flux is higher than what the heat shield material can take, the heat shield will fail – most likely it will burn through.
Conversely, if the entry angle is a bit too shallow, other unpleasant things can happen.
Firstly, the deceleration then will be too low, so the spacecraft will travel much farther than it is supposed to. It might end up landing on land or even in rugged terrain (which is disastrous if it was designed to land only in water), on inhabited regions or in busy shipping lanes. Also, although the heat flux – like the deceleration – would be lower than expected, there may still be thermal problems, because the heat shield will be exposed to the flux for a much longer time, so the total heat load may be a lot larger. At some point, all of the protective insulation will have been burnt away, or heat might begin to seep through the shield and temperatures inside the spacecraft might become too high.
If the entry angle is much too shallow, the spacecraft will not ‘bounce off the atmosphere like a flat stone skipping off the water surface of a pond’. A spacecraft generates little or no lift, and the outer reaches of the atmosphere are very tenuous. What will happen is that the spacecraft does enter the atmosphere at orbital velocity, but because it does not enter the denser atmosphere layers, it will not undergo much braking. It will therefore not lose enough of its velocity, and then it will simply continue on its orbit. As this orbit is slightly elliptic, the spacecraft will start gaining altitude again, go out into space and then re-enter the atmosphere after an hour (or more), as its trajectory again leads down – but then it will be at a completely different location than planned and its second entry will certainly no longer be within the ‘corridor’!
As with all atmospheric entries, accurate guidance, navigation and control is everything!