Midget Space Shuttle Drops Back To Earth

[pErvinalia]

The re-entry has to be so steep or the craft would "bounce" off the atmosphere and not re-enter.

[cronus]

The re-entry has to be so steep or the craft would "bounce" off the atmosphere and not re-enter.

Useful for a Stuka style nosedive onto a target those back wings. Easier to come out of the dive and fly straight towards a airfield or a nuclear reactor. You can see it's two craft in one. That's so it can fly slow to land, or fast to strike. Nothing else explains the configuration. Those back wings designed for a roll out of a deep dive.

[pErvinalia]

Is that its launch configuration?

[cronus]

Is that its launch configuration?

Yeah, you can see why 'merica is bankrupt the way they waste space...as if it don't cost nothing.

[Clinton Huxley]

It's payload was an even smaller space shuttle, and inside that was.......

[Hermit]

you can see why 'merica is bankrupt the way they waste space...as if it don't cost nothing.

Something-or-other fallacy.

Space is not a finite resource. And you can't buy it.

[cronus]

you can see why 'merica is bankrupt the way they waste space...as if it don't cost nothing.

Something-or-other fallacy.

Space is not a finite resource. And you can't buy it.

Cost of getting there...forgot to placate the pedants again, oops.

[mistermack]

Good old wikipedia knows everything. The vehicle actually stays cooler, if the drag is higher, because of an air cushion and shock-wave that forms. :

In the United States, H. Julian Allen and A. J. Eggers, Jr. of the National Advisory Committee for Aeronautics (NACA) made the counterintuitive discovery in 1951[5] that a blunt shape (high drag) made the most effective heat shield. From simple engineering principles, Allen and Eggers showed that the heat load experienced by an entry vehicle was inversely proportional to the drag coefficient, i.e. the greater the drag, the less the heat load. If the reentry vehicle is made blunt, air cannot "get out of the way" quickly enough, and acts as an air cushion to push the shock wave and heated shock layer forward (away from the vehicle). Since most of the hot gases are no longer in direct contact with the vehicle, the heat energy would stay in the shocked gas and simply move around the vehicle to later dissipate into the atmosphere.

The Allen and Eggers discovery, though initially treated as a military secret, was eventually published in 1958.[6]

[Hermit]

you can see why 'merica is bankrupt the way they waste space...as if it don't cost nothing.

Something-or-other fallacy.

Space is not a finite resource. And you can't buy it.

Cost of getting there...forgot to placate the pedants again, oops.

[Calilasseia]

Yeh, that's all perfectly feasible.

You would think though, that you could do all those tests on the space station and in the lab back on Earth. You can expose the tiles outside the space station for a couple of years, and then bring them home for testing under more controlled conditions.
But someone might argue that it's more realistic in a real re-entry.

I've often wondered why re-entry has to be so violent. Why can't they make a much more gradual descent, at much lower temperatures and drag forces?
I'm sure that there ARE good reasons, or else they would do it that way.
But I can't picture what the problems would be.

The big problem with this idea, can be summed up simply. Slowing down from 18,000 mph to, say, less than 100 mph, before making contact with the atmosphere, is going to take fuel. Lots of it. Fuel that would be far better deployed launching another mission, and which, if carried aloft into space, increases your launch payload substantially, making it necessary to use even more extravagant quantities of fuel to launch the craft in the first place.

On the other hand, using the atmosphere to provide free braking, costs far less, reduces your payload, and reduces the fuel costs of the entire mission.

Another problem that arises, when trying to aim for a low-speed re-entry, is that your spacecraft starts falling toward Earth the moment its speed falls below the orbital speed. Which means you have all the fun of ensuring that your speed reduction down to the desired low speed is complete, before you make contact with the atmosphere. Now since a typical low Earth orbit speed is around 18,000 mph, and it takes powerful rockets 10 minutes or so to accelerate a spacecraft to that speed from launch, then it's safe to assume that the deceleration process will take a similar amount of time. But during that deceleration process, Earth's gravity will be accelerating the spacecraft toward Earth with an acceleration rate of 1g. 10 minutes' acceleration at 1g in a vacuum, without any atmosphere to slow down the vehicle, will result in the spacecraft acquiring a speed of close to 6,000 metres per second in the Earthward direction, or somewhere a little short of 14,000 mph. Oops.

Which means burning up even more fuel to counteract this radial acquisition of speed, whilst simultaneously reducing the tangential speed. The engineering problems arising from trying to do this are formidable. Even if you succeed in lifting the fuel required into orbit along with the rest of your payload, which will be a hideously expensive affair, you then have the problem of trying to reduce tangential speed over a 10 minute period, whilst simultaneously providing, for that 10 minute period, a radial acceleration directly opposite and equal to that of Earth's gravity, to stop your spacecraft becoming a man made meteorite. Either you have to install separate horizontal and vertical thrusters for the purpose, and keep both functioning for that critical 10 minute period, or, you have to perform precise orientation control for the duration to enable one engine to perform both tasks.

This is why it's a lot easier to let the atmosphere take the strain. The atmosphere handles all of this for you, without you needing to do anything other than stop your craft becoming hot enough to melt. Ablative heat shields perform this latter task quite successfully, in combination with the requisite aerodynamic solutions, ensuring that the super-hot plasmas generated during re-entry flow around your spacecraft at a distance, instead of dumping all of that heat directly into the spacecraft, which would be bad news both for the spacecraft and any occupants.

Of course, the problem with re-entry is that even with the requisite aerodynamic solutions in place, some heat is going to reach the front end of the spacecraft, and so it's wise to put something at that front end, that's going to keep that heat from frying the occupants. Which is where ablative heat shields come in. Their job is to dissipate yet more of that heat by sacrificing themselves, atomic layer by atomic layer.

Incidentally, working out the best solution for a given re-entry not only depends upon the speed at which the re-entry vehicle makes first contact with the atmosphere, but upon the atmospheric constituents of the planet that the re-entry vehicle is landing on. This is because different gas mixtures have, shall we say, interesting characteristics once subject to shock plasma heating, and the dynamics undergo some interesting changes as one moves into new speed regimes. Mars, for example, is a relatively benign case - the atmosphere is 97% carbon dioxide, and the re-entry speeds are relatively low for any craft detaching from a Mars orbiter. What made the "six minutes of terror" all the more troublesome for the Mars Curiosity mission, was that the craft wasn't detaching from the mothership whilst in orbit around Mars, but on final approach to orbit, and was therefore travelling at around 13,000 mph when it made first contact with the atmosphere. A craft detaching from a Mars orbiter would detach at a considerably lower speed: for example, the current Mars Reconnaissance Orbiter has a mean orbital altitude above the surface of around 300 km, and feeding this into the formula for orbital speed:

v = (GM/r)½

gives us, for Mars, an orbital speed of 3,405 metres per second for the MRO, or around 7,600 mph. (When performing this calculation, don't forget that r is measured relative to the centre of the planet being orbited, and so you have to add the radius of the planet to the orbital distance above the surface, to obtain the correct value of r to feed into the formula).

Now at those relatively low speeds (which are about 35% of those for an Earth re-entry vehicle), the plasma dynamics are somewhat simpler, and the gas plasma model used to determine how much heat will be generated upon re-entry will be simpler to solve, because [1] the heat being generated is lower, and [2] the Martian atmosphere can effectively be treated as one chemical species (namely CO2), with lower likelihood of non-equilibrium dissociation. For Earth, on the other hand, the gas plasma model has to take account of a greater number of chemical species, a multiplicity of dissociation and reaction equations, including non-equilibrium disssociations, and that's just to calculate the Gibbs Free Energy being liberated in the plasma reactions. That's before we move on to the business of actual heat transfer in shocked hypersonic flows, which brings us to the wonderful world of the Navier-Stokes Equations once more. For re-entry from low Earth orbit, you need all the complexity of a non-equilibrium real gas model, into which you feed the data for five principal chemical species and 17 dissociation equations. To deal with higher speed re-entries, such as the Apollo lunar missions, you need to move on to a 12-species model, by which time you're spending money on serious computer power to solve the system of equations, which in addition to being complicated, has a number of undesirable features from the standpoint of numerical solution (JimC will know what I mean when I talk about a stiff system of equations).

Now take all of the above complications, and apply them, with additional factors, to the Galileo probe that was sent to Jupiter. This included a re-entry vehicle intended to be sent into the Jovian atmosphere. The problem being that thanks to the orbital dynamics of the mission, the re-entry probe would be hitting the thick atmosphere of Jupiter at a whopping 47,800 metres per second - over four times the escape velocity for Earth. That's a little short of 107,000 mph in case you're wondering, about 8½ times faster than the Space Shuttle. Toss into the mix, that the designers had to work with a gas model including some previously unencountered species (e.g., ammonia and methane), and that these were being subject to plasma shocking a whole order of magnitude greater than anything ever covered in a re-entry gas model before, and it's not difficult to imagine the fun and games the Galileo mission designers had, constructing a heat shield to survive that atmospheric entry. When the probe arrived, and began Jovian atmospheric entry, the transition from 107,000 mph to subsonic speed took jst two minutes, with peak deceleration measured at a whopping 230g. Which, apart from the conditions now known to be present on Jupiter, explains why we're not going to be sending humans on a Jovian atmospheric entry mission any time soon - I dont know anyone who can survive decelerations equivalent to 230 times Earth gravity, and that sort of deceleration will probably turn most living organisms into mush.

Of course, the ultimate solution would be force shielding, combined with antigravity as an engineering reality, of the sort imagined by James Blish in his Cities In Flight books. If this sort of solution was available, re-entry as we know it would cease to be a problem, because one could simply park a spaceship of any size in orbit, let the antigravity system keep it in orbit whilst decelerating, then slowly decrease the effect of the antigravity system for a nice, slow, gentle descent. Unfortunately, P. M. S. Blackett, contrary to the storyline of the Blish books, rendered this solution unfeasible, at least as described therein. An Alcubierre warp drive isn't going to be any easier to build, and would probably be hideously unsafe to operate in the vicinity of a planet anyway.

There's you're little helping of Calilasseia for the evening.

[mistermack]

Cali, I didn't suggest slowing down the vehicle using fuel and rocket power. I was suggesting that the slowing down should be over a longer period. ie, spending more time in the thin atmsphere, and dropping speed more slowly, like using the brakes on your car more gently...

But I think you missed this post ; http://rationalia.com/forum/viewtopic.p ... 5#p1583343 where I answered my own question. (courtesy of wikepedia)

It's counter-intuitive, but a violent high speed high drag re-entry actually sets up a shock-wave that protects the vehicle from most of the heat.

So a more gradual re-entry like I was suggesting would result in more heat reaching the vehicle, not less.