TPS

[grooble]

Does an orbital ship need TPS to come back to earth if it can slow down whilst in orbit to a few hundred mph and just fall to the earth?<br /><br />Perhaps a ship could flip whilst in orbit and fire some kind of advanced quick acceleration ION drive and slow down.<br /><br />

 

[najab]

><i>Does an orbital ship need TPS to come back to earth if it can slow down whilst in orbit to a few hundred mph and just fall to the earth?</i><p>To slow from 17,000mph to a few hundred mph would require just about as much energy as was required to go from 0 to 17,000mph - for the Shuttle you would have to carry another ET up as payload, which obviously is impossible.</p>

 

[grooble]

Could some kind of nuclear engine suffice?

 

[najab]

Even with a (very heavy) nuclear engine, you are still talking about cancelling out 17,000mph of velocity in a very short period of time, which requires a very high thrust and hence a lot of propellant.

 

[mlorrey]

The assertion that a ship needs a high thrust to drop its velocity is a misnomer. <br /><br />http://www.astronautix.com/craft/firlider.htm<br /><br />FIRST Re-Entry Glider <br /> <br /> <br /> <br /> <br />Re-Entry Glider<br /> <br /><br />--------------------------------------------------------------------------------<br />Class: Manned. Type: Bailout. Destination: Space Station Orbit. Nation: USA. Manufacturer: Aerojet. <br />FIRST (Fabrication of Inflatable Re-entry Structures for Test) used an inflatable Rogallo wing for emergency return of space crew from orbit. <br /><br />In the early 1960’s Aerojet studied project FIRST (Fabrication of Inflatable Re-entry Structures for Test) in order to evaluate the use of inflatable Rogallo wings for emergency return from orbit. The system would be stowed in a cylindrical package, docked to the external surface of a space station. In an emergency the escaping crew member would enter the coffin-sized cylinder, seal the back hatch, and be blown free of the station. The paraglider would then inflate and deploy. The crew member would use a gas stabilisation and control system to orient the spacecraft for retro-fire, and then to keep the glider at the correct attitude for re-entry. From a 600 km circular station orbit re-entry would occur 26 minutes and 11,800 km from retro-fire at an altitude of 110 km. Computer studies indicated that minimum heating would occur at a re-entry angle of - 1 degrees, an angle of attack of 70 degrees, and a lift to drag ratio of 0.5. The resulting trajectory was found to be practical under automatic or manual control. G loads during re-entry would not exceed 2.0 G. <br /><br />The paraglider would reach transonic speeds at an altitude of 43,000 m. From here its subsonic lift-to-drag ration of 8.0 would give it a 345 km range to reach a landing point. Landing speed would be 55 kph; a flare just before touchdown would reduce the horizontal ve

 

[henryhallam]

The described system has a basically conventional reentry using the atmosphere to lose kinetic energy. It doesn't use the (ridiculous and impractical) idea of having a rocket dump all that velocity. Such a rocket WOULD need to be high-thrust because if the burn took longer than a few minutes you'd be in the soup quickly at a steep angle with most of your velocity intact - not a pleasant place to be.

 

[strandedonearth]

"The assertion that a ship needs a high thrust to drop its velocity is a misnomer. "<br /><br />True, but grooble was wondering about slowing a ship TO a few hundred mph while in orbit, to essentially fall straight down. From LEO, unless the delta-v was fairly rapid, the re-entry would occur before the vehicle decelerated to a few hundred MPH.<br /><br />So it would require a good chunk of the energy required to reach orbit in the first place. As for the reentry heating, well, after falling for 100km the vertical velocity would be about 1400kph. So I'm guessing if you could manage the delta-v, the re-entry heating should be manageable without exotic materials, although aluminum may not stand up to it and steel would be heavy.

 

[strandedonearth]

Whoops, I was rather slow making my post due to distractions.

 

[najab]

The original poster's idea was to drop (nearly) all of the orbital velocity <b>before</b> reentry, this would require a <b>very</b> high thrust engine.

 

[mlorrey]

Depends on your mass and initial altitude. LEO? yes, probably. 10,000 mi altitude? No, not so much. 23,000? Certainly not. Keep in mind that orbital velocity drops the higher you are, and that gravitational attraction is less the higher you are, so once you drop below orbital velocity you accelerated due to gravity less the higher you are. <br /><br />GEO orbital velocity is about 3 km/s, so, adding in delta v you gain on the way down, you can drop your velocity to zero while still above the atmosphere, and just drop in, however, paradoxically, if done conventionally, the more you retrofire, you continue to drop to lower orbit and accelerate while doing so, until you are in LEO at 17,000+ mph. There is a point, though, between microthrusting and megathrusting, where you can wind up where the poster wants to be. <br /><br />Essentially you want to be able to accelerate with your retrofire engines at 1-5% above whatever the local rate of gravitational acceleration is. So, you don't need engines big enough to do 3-5 gs, but you need ones big enough to do about 1.0 g acceleration at the bingo fuel point just before reaching zero velocity above the atmosphere, and throttlable down to whatever percent of G exists at the starting orbital altitude.