On Monday, November 21, 2022, 03:39:50 PM PST, Jack Sarfatti email@example.com wrote:
Why will they not burn up on re-entry - because its titanium?
On Nov 22, 2022, at 12:58 AM, XXXX wrote:
Tungsten, actually, with a melting point of 3695 K.
The problem of delivering a kinetic energy impactor onto a surface target is almost the same as delivering a nuclear warhead from an ICBM. The parameter that determines how much a body entering the Earth’s atmosphere will slow down is the ballistic coefficient (B), defined as the mass of the object (M) divided by the frontal area the object presents to the oncoming airstream (A), multiplied by the drag coefficient of the object (Cd).
i.e., B = M/(A x Cd).
The momentum (proportional to M) is what keeps the impactor moving forward, and the drag (proportional to A x Cd) is what’s trying to slow it down, so the larger B is, the less the object slows down as it passes through the atmosphere. For a given size of impactor, B will be maximized when the density of the impactor material is large and when the frontal area and drag coefficient are minimized. A pencil shape made of Tungsten or depleted Uranium is pretty much the maximum practical ballistic coefficient you can get and therefore will arrive at the Earth’s surface with the maximum velocity remaining compared to its atmospheric entry velocity. Entry velocity would typically be about 7.5 km/s and impact velocity would typically be about 3 km/s. ICBM warheads typically have a somewhat lower ballistic coefficient and arrive at the surface at ≈ 1km/s. The velocity at which the kinetic energy of a mass is equal to the TNT equivalent energy is ≈ 2.7 k/s, so an impact at 3 km/s will totally atomize the impactor and an equivalent mass of the material it impacts, while an impact at ≈ 1 km/s is survivable by an appropriately hardened nuclear warhead and is why you can have ground penetrating nuclear warheads.
Even though Tungsten has the highest melting point of any element except Carbon, the plasma fireball in front of an entry vehicle at 7.5 km/s is easily hot enough vaporize any material, given enough time. The high ballistic coefficient reduces the amount of time the vehicle is exposed to the re-entry heat, but the other trick is to use an ablative material on a blunted vehicle nose to prevent the plasma fireball from coming in direct contact with the vehicle’s body. This was figured out by Harvey Allen at Ames starting in the late 1940s and is what makes ICBM warheads feasible. The rounded or blunt nose causes the shockwave to stand off some distance in front of the vehicle, so the peak convective heating never actually touches the vehicle surface. Of course radiative heating from the plasma still adds to the heat load on the vehicle so a thin layer of sacrificial material (usually Carbon-Carbon or Carbon-Phenolic composite) is placed on the front surface. The combined effects of convective and radiative heating will vaporize the outer layer of material, turning it to a gas and carrying heat away from the vehicle in the process. A well designed heat shield will evaporate away and the front surface will recede at exactly the rate at which the heat pulse would propagate back into the vehicle. This will tend to keep the back surface of the heat shield at a constant temperature that is lower than the melting point of the material the vehicle is made of. It should be possible in theory to use the Tungsten itself as an ablative material on the nose, in which case the rod would lose some of its mass on the way down but would still arrive at high velocity. However, if I were designing one of these, I wouldn’t do that, I would use a separate ablative heat shield like Carbon-Carbon or Carbon-Phenolic in order to keep the aerodynamic performance of the vehicle constant on the way down, so I could predict with some accuracy where it was going to hit.
So that’s the long-winded answer.
On Nov 21, 2022, at 5:17 PM, XXXX wrote:
Elon’s Starship will definitely permit the implementation of revolutionary military and national security systems in the near future. Those of us who design space missions for a living saw this coming at least 10 years ago when he first announced he was embarking on the path to develop a reusable Starship with ≈ 100 ton capacity. Some of us have been quietly designing the payloads to make use of this capacity. However, he will not be allowed to become a Bond villain and deploy these systems privately. And for that matter, there is no business case for him to do so.
For the foreseeable future, the US will have a monopoly on this capacity and all the usual three letter agencies and military branches have been conducting internal studies on what they would do with the capability to launch 100 tons into Low Earth Orbit for ≈ 10 million dollars. Super large aperture optics for imaging satellites. High power, large aperture Radar Ocean Reconnaissance Satellites. The ability to launch an entire constellation of spacecraft in a single go to replace satellites taken out by an adversary. Solar power satellites for expeditionary military operations. The Marines will finally get that Starship Trooper capability they’ve always wanted–the ability to drop in a few squads of Marines anyplace on the planet in less than an hour or two.
I’m not sure Rods from God make sense. The question is, what would their targets be? I think the only targets that could make sense would be launch sites for nuclear armed ICBMs and/or associated command and control centers. The problem is that a kinetic energy impactor in orbit attacking a ground target still takes about 45 minutes from the time it is released until impact. That is too slow to do a preemptive disarming strike on an adversary’s field of ICBMs and would actually be strategically destabilizing. What could make sense is implementation of the old brilliant pebbles space based interceptor swarm as proposed by Lowell Wood, et. al. A couple hundred brilliant pebbles in appropriately phased orbital planes in retrograde orbits might very well be able to neutralize North Korea’s first strike threat. I would be surprised if the Defense Threat Reduction Agency isn’t already studying this.