Quote from: Danny Dot on 08/05/2016 01:42 pmQuote from: Impaler on 08/05/2016 09:39 amDon't assume the EDL takes as long as a manned capsules entry, deceleration can be VERY fast if your coming in steep it just makes for very high g-forces and a tank can tolerate that.The crew can tolerate a 20G entry.No, frankly. They can't. Check your sources. We're talking about sustained G-loading that lasts tens of seconds at near-peak intensity, presumably with considerable additional vibration from the turbulence. Per Wikipedia, "Only the most motivated volunteers were capable" of reaching this 20G level in testing for 10s duration with a steady centrifuge, and more than that is suggested to result in injury and/or LOC.Call it 10G peak eyeballs-in and assume some of the crew will black out or come down with concussions. For existing spacecraft like Soyuz, 8.2G is an emergency condition encountered only during a ballistic reentry, after something goes wrong with separation, and the Shuttle gets more like 3G.Eyeballs-in may not always be perfectly achievable either. A reusable lander has to decelerate at a variety of orientations, and it may not be weight-feasible to have a pivoting chair. 20G's applied for a very short time at a slight angle to eyeballs-in will cause a red-out or a black-out by pushing enough G's at a person in the wrong direction, where they have low tolerance.I'm not even trying to take into account deconditioning; A known unknown. But there's too many reasons to be conservative with G-loading already. If your mission needs 20G reentry, redesign your mission.
Quote from: Impaler on 08/05/2016 09:39 amDon't assume the EDL takes as long as a manned capsules entry, deceleration can be VERY fast if your coming in steep it just makes for very high g-forces and a tank can tolerate that.The crew can tolerate a 20G entry.
Don't assume the EDL takes as long as a manned capsules entry, deceleration can be VERY fast if your coming in steep it just makes for very high g-forces and a tank can tolerate that.
Eyeballs-in may not always be perfectly achievable either. A reusable lander has to decelerate at a variety of orientations, and it may not be weight-feasible to have a pivoting chair. 20G's applied for a very short time at a slight angle to eyeballs-in will cause a red-out or a black-out by pushing enough G's at a person in the wrong direction, where they have low tolerance.
10G is marginal but probably safely achievable assuming you screen/test passengers on the ground before missions. If there's lots of vibration or not a lot of weight to spend on anti-G accommodations or deconditioning is a major stressor, even that's chancy.In cases where we're reentering from high orbit (definitely Mars fast transits, maybe also Mars slow transits and GTO) without advanced techniques like MAC, it may be advantageous (the additional radiation and uncertainty may be outweighed by the advantages in spacecraft design tradespace) to do a two-stage entry, with an aerocapture to an intermediate elliptical orbit near LEO (or a suborbit with one end below LEO), then a control burn to target the landing zone, then a final entry the rest of the way.
Quote from: Impaler on 08/05/2016 09:39 amDon't assume the EDL takes as long as a manned capsules entry, deceleration can be VERY fast if your coming in steep it just makes for very high g-forces and a tank can tolerate that.It still has to be shielded; it will be exposed to at least 60 seconds of plasma blast hotter than Raptor exhaust temps. It's going to be WAY hotter than a Falcon 9 S1 entry, which is already pretty toasty.
A separate cargo module attached to the main MCT craft that would have a heavy, "single use" TPS shield for the aerocapture, which is afterwards detached and either automatically landed at the primary landing site, or remote piloted down... The main craft would then descend and land nearby the cargo module. The advantage here is that a cargo module could take higher G loads and more punishment than the fragile human cargo.
The alternative idea I had was to Aerocapture, then reenter with both craft and cargo module through the "7 minutes of hell" then detach the two, cargo module coming down via parachute with a Russian style retrorocket cushioning in the final few meters, and the main craft descending via retropropulsion. This latter idea keeps both craft and cargo near to each other, while protecting the main craft's TPS for the reentry at Earth.
Quote from: envy887 on 08/05/2016 04:29 pmQuote from: Impaler on 08/05/2016 09:39 amDon't assume the EDL takes as long as a manned capsules entry, deceleration can be VERY fast if your coming in steep it just makes for very high g-forces and a tank can tolerate that.It still has to be shielded; it will be exposed to at least 60 seconds of plasma blast hotter than Raptor exhaust temps. It's going to be WAY hotter than a Falcon 9 S1 entry, which is already pretty toasty.My understanding is, that proper rocket thrust could cause that heated and compressed air is moved from engine compartment in front of the vehicle and plasma slide around vehicle, this what I try describe on picture.I think, this is what Spacex found out during his retropropulsion events.
I'd imagine MCT would land automatically rather than human-piloted, so blackouts from G-forces may not be relevant. You'd need to keep below the level where any lasting harm is done, of course, but brief unconsciousness itself might not matter.
OK, while computers and software have advanced a lot since Apollo 11, I'm still not quite sure I'd want to put my complete faith and my life in the trust of a machine that could easily glitch out because of a stray cosmic ray.
Interesting idea. Is there any evidence that they run the F9 S1 fuel rich during entry?Are you proposing that this would work for manned interplanetary entries as well? How much fuel would have to be burned for a 13 km/s entry, and what decelerations are reasonable for manned entries? How many engines are what throttle settings?
and propulsion will only be able to take out 1 km/sec of that.
Quote from: the_other_Doug on 08/06/2016 07:05 pmand propulsion will only be able to take out 1 km/sec of that. I have been wondering what that exactly means. Is that 1km/s the landing burn or do they need to brake 1km/s before they can enter the atmosphere and the landing burn will be separate? I had anticipated at the time he means the latter but may be completely wrong.
Propellent cost in orbit is significantly different than propellent cost at stage 1 separation; and propellent cost to GTO or escape is even more. Even the largest estimates for BFR only have a GTO payload in the 50t range, and on a second stage the entry propellent trades for payload at a 1:1 ratio. 4 Raptors will burn through over 1000 kg/s of propellent, even when throttled to 40%. A 10G orbital entry needs about 60 seconds to lose 6000 m/s and thus needs about 60 tonnes of fuel for that maneuver; an entry from GTO needs to lose almost 9000 m/s over 90 second, and needs nearly 90 tonnes of fuel; and entry from 12+ km/s interplanetary velocities needs over 120t of fuel for a burn of more than 120 seconds.When an ablative heatshield for return from GTO or interplanetary velocities is only about 15% of dry mass, why would it possibly make sense to carry more than 100% of the vehicle's dry mass (and more than its payload capacity to GTO) in fuel for the same purpose?
Quote from: envy887 on 08/06/2016 02:27 pmWhen an ablative heatshield for return from GTO or interplanetary velocities is only about 15% of dry mass, why would it possibly make sense to carry more than 100% of the vehicle's dry mass (and more than its payload capacity to GTO) in fuel for the same purpose?Same reason the F9 first stage uses propulsion rather then heat-shields to re-enter, orientation demands base first entry and it is simpler to both engineer and operate which means minimal cost.
When an ablative heatshield for return from GTO or interplanetary velocities is only about 15% of dry mass, why would it possibly make sense to carry more than 100% of the vehicle's dry mass (and more than its payload capacity to GTO) in fuel for the same purpose?