Author Topic: MCT Speculation and Discussion Thread 4  (Read 611128 times)

Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #2520 on: 08/05/2016 06:00 PM »
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.

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.

Thanks for the update!  I'd forgotten that the 20g load limit was for short durations myself!

     Overall, I think what I said still holds up pretty well.  With the possible exceptions and additions of broken bones during sustained 20g reentries...
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Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2521 on: 08/05/2016 06:07 PM »
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.
« Last Edit: 08/05/2016 06:14 PM by Burninate »

Offline oiorionsbelt

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Re: MCT Speculation and Discussion Thread 4
« Reply #2522 on: 08/05/2016 07:05 PM »

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.

The forums HMXHMX may not agree with you.
 Here's t/Space's CVX pivoting seat, weighing less than 20% of the shuttle seat.

Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #2523 on: 08/05/2016 07:47 PM »
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.

I considered the possibility of a craft design that in some ways, is similar to a tractor trailer system.

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.  (Single use would actually be twice use in this case. Once for Aerocapture then again for final descent)  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.

     I doubt something quite like this has ever been tried, but it doesn't really seem too far fetched.
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Offline raketa

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Re: MCT Speculation and Discussion Thread 4
« Reply #2524 on: 08/05/2016 07:59 PM »
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.

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.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2525 on: 08/05/2016 08:24 PM »
I assume you are talking about Mars EDL here. There are a lot of discussions going on here and it's getting confusing, so stating something like: "for Mars EDL" or "Earth EDL from interplanetary trajectories" would help.

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.

This requires duplicating all the avionics, power, thermal control, retro-propulsion engines and tanks, landing legs, TPS, etc from the main module to the cargo module, essentially making it a complete spacecraft. ALL of those elements would be single use, which contradicts SpaceX philosophy.

Quote
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.

Terminal velocity for a 15 meter, 100 tonne object on Mars is around 1 km/s, much too fast for "parachutes and retrorockets". Also, they won't land near each other if one pops chutes and the other uses retropropulsion, those are very different trajectories.

Retropropulsion is required on Mars for large, dense objects that want to achieve precision landing locations. Like a heavy cargo shipment that's going somewhere specific.

Offline Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2526 on: 08/05/2016 08:25 PM »
Taking a look at the Libelle G-Multiplus suit, which uses a liquid-immersion-mimicking system.  Seems to be a drastic upgrade over the pneumatic G-suits they've used in air forces, shifting manual dexterity, communication, and lastly consciousness up 3 or 4 G's.  Short-duration testing capped out at 12G fully conscious, with talking at 9G and (as a stunt) eating candy at 6G, using a trained pilot in their self-contained suit.  Their marketing graphs portray a doubling of usable acceleration in fighter aircraft, from ~7G to ~14G.

I guess high-G suits/couches are something we can work on Earthside without a ton of money.  I'm just mystified that we haven't already, given the importance of fighter aircraft.
« Last Edit: 08/05/2016 08:29 PM by Burninate »

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2527 on: 08/05/2016 09:02 PM »
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.

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.
Yes, but... it's still really hot and still heats up the vehicle through radiative transfer. Compression heating goes up with the velocity^3, so orbital entry at Mach 25 has the heating rate is 30 times more than a hot F9 S1 at Mach 8. It also takes a lot longer to slow down to Mach 3 from Mach 25 than it does from Mach 8, so the vehicle is soaking heat for at least 60 seconds (and can be 10x that) instead of around 20 seconds for an F9 S1.

Plus, it doesn't save any orbited mass with hypersonic retropropulsion. A single Raptor can easily burn 60 tonnes of fuel during a 10 G orbital entry, so the expended fuel to keep the engines running, plus the TPS needed to keep the engines from torching the vehicle, almost always add up to a lot more mass than a modern heatshield.

Bottom line: SpaceX has proposed and actually flown a number of methods for suborbital and orbital entry, but orbital retropropulsion isn't one of them based on any evidence I've ever seen.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2528 on: 08/06/2016 01:23 AM »
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.

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.

The exhaust IS the shield, run the engine fuel rich or inject some kerosene into the nozzle and you would get a sooty exhaust.  The dark soot absorbs the intense radiation coming off of the shock front and keeps it from actually reaching the ship, ablative carbon rich heat shields already exploit this principle.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2529 on: 08/06/2016 01:44 AM »
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?

Offline Vultur

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Re: MCT Speculation and Discussion Thread 4
« Reply #2530 on: 08/06/2016 01:53 AM »
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.

Offline JasonAW3

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Re: MCT Speculation and Discussion Thread 4
« Reply #2531 on: 08/06/2016 03:57 AM »
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.

      Murphy's in the details, and I'd just as soon avoid giving him the chance of really messing up the day we land people on Mars.
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Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2532 on: 08/06/2016 05:10 AM »

      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.

I think I would not want to sit in a vertically landing spacecraft when there is the possibility of a pilot messing up.

Yes I am aware of Apollo 11. But BFR is not going to be like the moon lander.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2533 on: 08/06/2016 05:24 AM »
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?

I'm not proposing it for manned or interplanetary entries, just for re-entry of a conventional 2nd stage from LEO, possibly from a GTO trajectory as well.  The intent is not to be maximally efficient in propellant but to be operationally simple and be a maneuver that can be attempted on a 2nd stage with the minimum possible modifications.

I see a BFR as a 2 stage vehicle like F9, the 2nd stage carrying ~1200 mt of propellant and having 6 Raptor Vac engines and one central sea-level engine, fire 3 of the Raptors at low thrust outward and the center engine too.  At 10 G's your looking at around 1 minute for peak entry heating.

Current F9 first stages will fire 3 engines for boost-back for 50 seconds, then 3 engines again for 20 seconds during entry, then the single central engine for 30 seconds at landing, a total equivalent to 27 seconds worth of full thrust.  The whole initially launch up to MECO is 141 seconds, so all these re-entry maneuvers are using ~1/6th of the total propellant available at launch.  So something of comparable propellant cost should be a viable.

Note that the scenario I foresee is for the 2nd stage to do one orbit of the Earth and then re-enter, that means the boost back burn is eliminated and all propellant is available to entry followed by the very small  landing burn.  If the stage experiences twice the deceleration rate of the F9 from LEO orbital velocity and reaches the same speed at the end of the burn (250 m/s) then it should take 70 seconds, assuming half power that's equal to 35 seconds of full thrust which compares very well to the current burn time of 372 seconds on the current 2nd stage.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2534 on: 08/06/2016 02:27 PM »
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?

Offline the_other_Doug

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Re: MCT Speculation and Discussion Thread 4
« Reply #2535 on: 08/06/2016 07:05 PM »
Also recall what Elon tweeted a short while back (I'm pretty sure it was an Elon tweet), that when the BFS arrives at Mars it will need to shed about 13 km/sec of speed, and propulsion will only be able to take out 1 km/sec of that.  The remainder will need to be taken out via aerodynamic braking.

It won't be possible for BFS to do the entire entry propulsively, is my take-away on that...
-Doug  (With my shield, not yet upon it)

Offline guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2536 on: 08/06/2016 07:34 PM »
and 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.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2537 on: 08/06/2016 09:35 PM »
and 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.

13 km/s is consistent with Earth entry after a 6-month Earth return or a Mars entry after a 3 month transit. 1 km/s is about the terminal velocity of a 15m 100t object in Mars atmosphere. I don't see any reason to spend fuel slowing down before hitting the atmosphere, so I absolutely agree with the interpretation that the 1 km/s has to be the landing burn.

Perhaps a better way to phrase it would be "the atmosphere can brake all but 1 km/s of that".

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2538 on: 08/07/2016 12:34 AM »
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?

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.

Also how do you calculate mass flow rate for Raptor?

Offline OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2539 on: 08/07/2016 02:36 AM »
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?

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.

Are you sure you are comparing apples here?

The fastest F9S1 sub-orbital entry is about 2.2km/s. A super-synchronous GTO transfer orbit could re-enter as fast as 10.3km/s. Assuming the same altitude, the difference in heating would be 10.3/2.2^3 = 102.6, or two orders of magnitude higher.

If JCSAT-14 took "max damage" at 2.2km/s, then what sort of damage would F9S2 take at 10.3 km/s? Would it not require major (total?) redesign to mitigate that damage?

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