There and Back Again

[larrison]

Your At the edge of the atmosphere in a ten person craft. Thanks to modern building materials (carbon fiber, aluminized glass, ect) the weight of the craft is less than half of the current space shuttle. The first stage ( a double winged, heavy lift, high altitude jet) drops you and you fire your engines. You must reach orbital velocity using only 70% of your fuel. The remain fuel is for the deorbit burn. Your ship reenters the atmosphere at a less dangerous speed say 8,000 mph.<br /><br /><br />Now, how feesible is this and can anyone speculate on semi hard data.

 

[mlorrey]

Well, if you are burning a lot of fuel to deorbit to a low speed, you are going to be passing through your own exhaust plume, which will heat up your vehicle just as if you were reentering at high speed through hot plasma. So it is a bit of a wasted effort. You still need a thick thermal protection system.<br /><br />It's better to minimize your wing/surface loading by having as much surface area/cross-sectional area presented upon reentry per kg of vehicle mass as possible. With fewer kg/m^2, the vehicle slows down faster in the thinner upper atmosphere, with less plasma heating, and you can use more conventional materials. As an example, General Electric developed and tested a space station escape module in the early 70's that used a deployable Rogallo wing (like a hang glider) with an inflatable leading edge and fabric that was steel wire fabric (like car airbags) with embedded silicone. This vehicle reentered at a -1 degree angle and never exceeded the temp limits of the fabric. A brilliant design and example of how reentry temp is a function of wing/surface loading, not reentry velocity.

 

[spacester]

This is an interesting proposal, I haven’t run the numbers, it is definitely a non-trivial solution. Also, ‘larrison’ is a very interesting user_id – once upon a time many years ago there was a space advocate with much wisdom named ‘wade larrison’ – any relation? Also, this is similar to a proposal of mine I trot out every few months.<br /><br />OK for starters I don’t think this is feasible with an aircraft as a first stage. If we modify the proposal to use a conventional booster rocket, the question becomes ‘how much dV do we need from the booster to get significantly lower re-entry velocities?’ I have a spreadsheet that lets me take a whack at this question, and then we can apply the results to an aircraft as first stage.<br /><br />My proposal is to re-prop on orbit and use that to come in cool instead of hot. You still need to be able to come in hot, but if you come in cool, maintenance becomes trivial. The proposal here would allow you to come in cool every time as long as your engines work – you’re not depending on rendezvous, docking and fluid transfer as with my proposal.<br /><br />We need to work backwards from an assumed re-entry velocity. Let’s cut it in half for the sake of a first calc. Let’s say we somehow are in a very low circular orbit of 100 miles – ‘the edge of space’. Our velocity is 8.003 km/s = 17900 miles per hour. We want to cut this in half and come in cool, so we need to hit the brakes for a dV of 4.0 km/s.<br /><br />If we hit the brakes all at once, we’re going to drop like a rock – we are way below orbital velocity for that altitude. So we are going to hit the brakes gradually; I’m not going to do that calc, we just need to know that we want 4.0 km/s of dV capability at that point in time.<br /><br />How did we get to that low circular orbit? If we compare to a medium-high LEO of 600 km (372 miles) circular, we can calculate an orbital energy difference of 8.240 km/s. If we have the capability to hit the brakes in the atmosphere, we ought to be <div class="Discussion_UserSignature"> </div>

 

[chriscdc]

Well assuming you don't try and fire your engine in the atmosphere, which would be rather missing the point, your exhaust is going to have a very high dv LH2+LOx can produce exhaust of 4.5km/s. So you are not really going to catch up with it again.

 

[mlorrey]

AFAIKR, orbital velocity is something like 5.1 km/s, or about 26,100 ft/s, significantly in excess of the 6,000 ft/s exhaust velocity of LH2/LOX. While you are braking, you are initially giving your exhaust a 6,000 ft/sec boost over your velocity, it quickly loses that as it hits ionospheric plasma: the hydrogen ions that make up the microvacuum of LEO (LEO is not a perfect vacuum by any stretch). The lower your orbit, the more there is, the atmosphere just doesn't stop at a defined boundary. Being in orbit is merely being high enough that you can't measure an indicated air speed, which is different from there being no gas at all. A spacecraft stays in orbit because it is dense and has a lot of momentum. Exhaust gasses are not dense and cannot maintain momentum against gas of equal or slightly lesser density.<br /><br />As your exhaust plume expands after it leaves your nozzle, in accordance to the gas law, it eventually drops to near the ambient pressure of your surrounding space, at which point its momentum is totally cancelled by space plasma, and it becomes a contribution to the space plasma that, if it is in your trajectory, is going to heat you up when you hit it. While it is far far less dense than atmosphere would be at reentry, you spend a lot more time passing through this lower density hot plasma.<br /><br />If you are high enough in orbit, this isn't an issue, but in LEO its an issue that is highly dependent upon your altitude.<br /><br />My proposed alternative is like that used by GE in my prior post: very shallow reentry with large surface area, to slowly skip-dive multiple times. The shuttle comes in hot and heavy cause its a brick with stripes. JP Aerospace's proposals are in line with my own ideas on this.

 

[spacefire]

gosh...who say you have to fire your exhaust straight forward? if you fire slightly high, you will also decrease your potential energy state a little while slowing down. <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>

 

[kingjuggs]

has anyone heard if NASA is going to get serious about space mining?

 

[chriscdc]

Sorry but I don't get<br /><br />'Exhaust gasses are not dense and cannot maintain momentum against gas of equal or slightly lesser density'<br /><br />Your dealing with high energy particles, in a very low density medium. I can understand that the particles will lose momentum due to electrostatic interaction with the molecules in the plasma, but they would not lose the momentum if the number of partciles per volume of exhaust is equal to the particles per volume of plasma.

 

[larrison]

Sorry, no relation to wade.<br />I was thinking of a deorbit burn high enough that the "atmosphere" has no measurable effect on the craft. Also since the over all mass of the craft is less the remaining fuel (be it 30% or 10%) would have a much more profound effect on velocity change. I am also thinking of a high heat, part inflatable parachute for the absorbtion of most of the rest of the energy. The craft has no heavy heat shielding equipment.<br /> Also I really wanted a runway launched first stage. I thought about a ship that would be both infront of and behind the orbiter. Two wing sections connected by tubes that the orbiter would be attatched to. I realize it would be similar to Burt Rutans double winged troop lift vehicle he designad for the military. The landing gear wouldn't retract into the body but rather the body would close around it once airborn.

 

[mlorrey]

HTO requires excessive amounts of mass to get off the ground, unless you are using some sort of rail launch system to give it 400-800 kph and a ski jump to translate much of that velocity to the vertical, or else being air launched by a conventional transport. Even the winged GTX requires vertical launch to meet mass fractions.<br />If your 'ship' is the air launch transport, fine, you can use just about any design capable of getting your orbiter mass up to 40-50k feet and 500-600 mph. This will save you about 5% on your mass fraction calculations, which is very significant, but shouldn't be used as a buffer to float outlandish schemes.<br /><br />As someone once said, "The earth is 2/3 covered in water, and the rest covered in launch vehicle studies." The problem today isn't a lack of vehicle options, its a lack of will and leadership in choosing one that is realistically viable, sticking to it, and putting in sufficient development capital to make it a profitable endeavour. NASA is a total loss in the leadership category, and the major contractors won't do anything without the scent of free money under their noses, while venture capitalists won't jump in so long as NASA derides all private schemes and the majors float vaporware proposals about how they plan on doing it right if only they were given enough taxpayer "Zero Interest Principal", as they like to call it.