FS US CommentInsta-9:1-throttle Yes No Sea-level engines Yes No Slow reentry Yes No Mass penalty 4:1 1:1Entry point downrange launch siteAspect ratio slender squatMass higher lowerDrag/mass lower higher
OTOH, mid-air helicopter recovery which was practically impossible for the first stage, becomes almost natural for the second stage:
Quote from: meekGee on 03/18/2015 06:04 pmOTOH, mid-air helicopter recovery which was practically impossible for the first stage, becomes almost natural for the second stage:How do you know it is light enough to be mid air recovered?
Quote from: Jim on 03/18/2015 06:10 pmHow do you know it is light enough to be mid air recovered?An empty second stage? I'd put it at around 5 tons. Maybe 6 after modifications. Heavy helicopters can lift well over 10 tons.
How do you know it is light enough to be mid air recovered?
Quote from: meekGee on 03/18/2015 06:19 pmQuote from: Jim on 03/18/2015 06:10 pmHow do you know it is light enough to be mid air recovered?An empty second stage? I'd put it at around 5 tons. Maybe 6 after modifications. Heavy helicopters can lift well over 10 tons.What would terminal velocity of the stage be? Do you think a helicopter could catch an object falling at that speed? Would the second stage need a parachute of some sort to be slowed enough to be successfully caught?
Seems to me the 2nd stage could land on legs by parachute in the desert somewhere. Then be picked up via a large helicopter or if it had enough fuel, land like the first stage. If it slowed down enough, it could almost drop straight out of orbit and land like the first stage.
Quote from: spacenut on 03/18/2015 07:14 pmSeems to me the 2nd stage could land on legs by parachute in the desert somewhere. Then be picked up via a large helicopter or if it had enough fuel, land like the first stage. If it slowed down enough, it could almost drop straight out of orbit and land like the first stage. The problem with landing "like the first stage" is that the existing motor can't be used, so you need a new motor, or more likely a set of them.Touching down with a parachute always has horizontal velocity, so it's hard to do it on legs. Not impossible, but not straight forward.Once you're on parachute though, air-capture is very gentle, and the stage never interacts with the ground or water. It's back on base within minutes, and you're done. For all practical purposes, it's RTLS.
Quote from: meekGee on 03/18/2015 08:47 pmQuote from: spacenut on 03/18/2015 07:14 pmSeems to me the 2nd stage could land on legs by parachute in the desert somewhere. Then be picked up via a large helicopter or if it had enough fuel, land like the first stage. If it slowed down enough, it could almost drop straight out of orbit and land like the first stage. The problem with landing "like the first stage" is that the existing motor can't be used, so you need a new motor, or more likely a set of them.Touching down with a parachute always has horizontal velocity, so it's hard to do it on legs. Not impossible, but not straight forward.Once you're on parachute though, air-capture is very gentle, and the stage never interacts with the ground or water. It's back on base within minutes, and you're done. For all practical purposes, it's RTLS.Is that assuming once-around?
Several-around. The second stage is assumed to be alive, and with very little impulse you can get the ground track to overfly the launch site. Whether you want it back in 90 minutes or 6 hours is up to you.
Quote from: meekGee on 03/18/2015 10:32 pmSeveral-around. The second stage is assumed to be alive, and with very little impulse you can get the ground track to overfly the launch site. Whether you want it back in 90 minutes or 6 hours is up to you. Quite wrong. 90 minutes is not doable, especially for higher inclinations at low altitudes. Takes a very lot of impulse.And ones in GTO are going to take days to get in the right phasing for return to launch site.
1. A very rough estimate:2. For a GTO trajectory, each revolution takes much longer, and so there's more time for your correction to have an effect.
Quote from: meekGee on 03/18/2015 11:36 pm1. A very rough estimate:2. For a GTO trajectory, each revolution takes much longer, and so there's more time for your correction to have an effect.1. Not even close. Changing inclination takes much more delta V2. No, after the first orbit, the perigee over Africa.
1. This is where you need to apply more rigorous methodologies and put up better numbers.2. Yes, that's true. But again, it should be trivial to simulate the effect of 100 m/s on the location of the perigee one orbit later.
Quote from: meekGee on 03/18/2015 11:51 pm1. This is where you need to apply more rigorous methodologies and put up better numbers.2. Yes, that's true. But again, it should be trivial to simulate the effect of 100 m/s on the location of the perigee one orbit later.1. Not when totally nonsensical and plainly wrong methodologies are used.2. 100 m/s is not going to change a GTO enough to matter, because it will be over Asia one orbit later, and then the Pacific and then in the same hemisphere as the launch site 24 hours after launch.