# Re-entry Heat Shield Alternative

Why is it that Controlled Re-entry Vehicles (like the most recent Orion & Dragon) do not use a strong magnetic field during re-entry to "shield" the blunt shaped end from plasma ?

Reasoning: Charged particles (plasma) are deflected by magnetic fields.

The magnetic field will only have to be activated for a few minutes & maybe can also pivot the vehicle by controlling its magnitude ever so slightly. It is mainly used to reduce the heat load/intensity on the heat shield, control descent speed and provide maneuverability Newtons 3rd law states that the magnetic field will slow down the vehicle depending on its strength.

A quick calculation using $F=qvB$, where $v=8900\text{ m/s}$ (highest velocity on reentry), $F=255\text{ kN}$ (3g deceleration on orion crew module $8.5 \text{ tons}$), $q=10\text{ C}$ (Arbitrary) implies $B=2 \text{ Tesla}$. Which is possible to build light.The charge of plasma was chosen randomly, a more accurate charge will be very helpful!

• If you want to add supplemental information to your question, please edit it into the question; do not leave it as comments. – a CVn Dec 20 '14 at 15:54
• An interesting thread on that topic: forum.nasaspaceflight.com/index.php?topic=29912.0 The thread evidently led to some effort from Jon Goff to develop this concept: parabolicarc.com/2014/05/05/altius-space-machines – HopDavid Dec 20 '14 at 19:07
• In addition to the points Deer Hunter raised in his answer, I would add that using a powered magnet to deflect ionized-gas is more complicated and prone to failure than a heat-shield, which has no components and weighs very little (typical heat-shield material, like PICA, is very light). And it's also (relatively) cheap. So, you're suggesting a more complex, likely heavier and more expensive solution that is inherently more likely to fail. And you'd still have to have some sort of heat shield even if the magnetic deflection worked. So.. it seems like the existing solution is better? – Kirkaiya Dec 21 '14 at 0:57
• The 2 links were very helpful, seems like its already being developed. But for now PICA is the cheapest/most efficient option we have until more R&D is undertaken. – Firas Dec 21 '14 at 9:17
• – Stu Dec 22 '14 at 14:35

## 2 Answers

There are several misconceptions in your question/proposal:

1. The Orion and the Dragon as well as other reentry capsules do not fly with the cone tip forward. They fly with the blunt "bottom" side forward. (fixed in the revised question)

2. Plasma is created by the very passage of the vehicle through the air at high hypersonic velocities. At lower velocities, ionization is far less, but the resistance from the air is much more - imagine hitting a brick wall at 60 mph.

3. Magnetic field requires bulky generators and heavier batteries, which makes launches much more expensive.

4. Reentry means using the Earth's atmosphere to decelerate the craft. If you somehow remove the source of drag, you are nullifying the advantages the atmosphere provides.

However, your question has the seeds of usable engineering devices, albeit for other phases of flight or purposes:

1. Launch through the lower, densest atmosphere does benefit from drag reduction. Hence, aerospikes have been invented.

2. Designers of reentry vehicles in less civilian settings may find it necessary to reduce their radar signature, and may resort to "electromagnetic cloaking".

• There have been idle discussions of charging batteries from the plasma going by. Sort of a regenerative braking. The braking would be comparatively small, but you could in principle harvest a fair bit of electrical energy. – Mark Adler Dec 20 '14 at 15:50
• If electromagnetic shielding is feasible but for the energy requirement, and electrical energy can be harvested from the plasma, what about using the plasma to power a shield, requiring only batteries to "kick start" it? – Anthony X Dec 20 '14 at 20:29

There have been suggestions for using a heat shield that has many small holes to allow flowing water through it. This would let the energy transpire the water instead of melting the materials. But you have to carry enough water for landing in that case.