### Author Topic: MCT Speculation and Discussion Thread 4  (Read 626039 times)

#### Burninate

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1020 on: 11/01/2015 08:19 AM »
How much mass would we need for a canard-engine landing, like a 'Sky Crane' but without separation?

Let's take a SuperDraco.  A dry mass of 300 tons delivered to ~1km above the surface at zero velocity, needs maybe 1G capability to land on Mars after main engine cutoff.  I've seen the SuperDraco compared to a pair of AJ-10 thrusters at 100kg each, but "probably lighter, given modern construction methods".  If SuperDraco masses 100kg total and produces 73kN, a TWR of 73, then 1G hoverslam requires 41 engines plus some propellant.  Round that up to 48 engines given that they need to carry their own mass, so 300 + 48 = 348T;  I'm inclined to say terminal landing dV needs are going to be somewhere in the 30m/s to 300m/s range - adding between 1.1% and 11.5% propellant load to the dry mass assuming ~280s Isp (a blind guess as to soft vacuum performance of optimized SD).  Take the higher of those estimates and we arrive at 388 tons for a terminal retrorocket burn from canard engines, a 29.3% increase in dry mass.

At 280s Isp and 348t dry mass, 136.8m/s can be accomplished with a wet mass of 367.1t;  That means you can achieve canard engine landing for ~22.3% extra mass.
« Last Edit: 11/01/2015 10:06 AM by Burninate »

#### Burninate

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1021 on: 11/01/2015 08:46 AM »
Oy, I seem to have forgotten to account for cosine losses.  I expect I could accomplish that by simply canting all the canard engines 30 degrees off vertical, and assuming they produce (3^0.5)/2 times as much thrust, or 86.6%, with 86.6% as much Isp as well.

Maintaining the 1G acceleration capability, that corrects to ~56 engines & ~243s Isp, a dry mass of 356 tons and a wet mass of 377 tons.  25.7% mass premium over a full rear engine landing.
« Last Edit: 11/01/2015 08:51 AM by Burninate »

#### Impaler

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1022 on: 11/01/2015 09:29 AM »
Wooo, your landing mass is more like 200 mT, not 300 and your going to use Raptor engines for all your deceleration and bring yourself to a hover at 100 m from the surface.  The touchdown engines are not slamming your into the surface they are just countering the gravity on mars and giving you fine control to maneuver around any boulders.

The thrust total your projecting is more then an entire Raptor engine would produce, if we needed or wanted that much we wouldn't bother with these vernier engines so I think you've mixed up some where (several some wheres?), for example you make each engine weight 1 mT when you were adding them to the vehicles mass.

A 200 mT lander on mars has a weight of 744 kN meaning you need just 10 Super Draco engine equivalents which means 1 mT and no significant impact to the landers dry mass.  Their would be some cosine loss too but I actually expect an improved engine running on Methane to be employed improving the thrust as well as a touchdown mass of just 175 mT.  The propellant for a few seconds of touchdown (20 seconds at mars gravity would be 75 m/s) is not going to be significant propellant fraction, though obviously a significant portion was burned prior to this by the Raptor engine, that is a separate calculation unaffected by our choice of touchdown systems.

On Earth were going to be lighter by 75 mT and we would be landing on a concrete pad so we can have a little higher speed at touch down and we might very briefly throttle up right at touchdown too.  I've doubtful that Raptor can safely fire in atmosphere but if I'm wrong then it would certainly be used here when their is no danger of debris from the surface.

#### Burninate

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1023 on: 11/01/2015 09:57 AM »
Wooo, your landing mass is more like 200 mT, not 300 and your going to use Raptor engines for all your deceleration and bring yourself to a hover at 100 m from the surface.  The touchdown engines are not slamming your into the surface they are just countering the gravity on mars and giving you fine control to maneuver around any boulders.

The thrust total your projecting is more then an entire Raptor engine would produce, if we needed or wanted that much we wouldn't bother with these vernier engines so I think you've mixed up some where (several some wheres?), for example you make each engine weight 1 mT when you were adding them to the vehicles mass.

A 200 mT lander on mars has a weight of 744 kN meaning you need just 10 Super Draco engine equivalents which means 1 mT and no significant impact to the landers dry mass.  Their would be some cosine loss too but I actually expect an improved engine running on Methane to be employed improving the thrust as well as a touchdown mass of just 175 mT.  The propellant for a few seconds of touchdown (20 seconds at mars gravity would be 75 m/s) is not going to be significant propellant fraction, though obviously a significant portion was burned prior to this by the Raptor engine, that is a separate calculation unaffected by our choice of touchdown systems.

On Earth were going to be lighter by 75 mT and we would be landing on a concrete pad so we can have a little higher speed at touch down and we might very briefly throttle up right at touchdown too.  I've doubtful that Raptor can safely fire in atmosphere but if I'm wrong then it would certainly be used here when their is no danger of debris from the surface.
Ahh, I knew that sounded like a lot.  Thanks for spotting the error - 10 times too much mass allocated to engines.  That should bring the extra mass margin down below +10%.

If you're trying to avoid unprepared landing site excavation issues, you need space for the exhaust plume to spread out a bit.  I picked ~1km for a very rough & arbitrary figure.  At 100m, the exhaust plume is barely larger than the vehicle fairing diameter.  Shut the main engines down at 1km AGL after doing the full job of entry & descent, and they won't destroy the landing pad.  Then drop for a bit (now's a nice time to correct to vertical and unfurl the legs), & start controlled thrusting on the sideways-canted canard engines;  Any terrain damage they do will be well away from the place the legs impact the ground.  They still need plenty of thrust, however.  If they were only capable of precisely Mars gravity acceleration, they could hover at this 1km point, but not descend (because they could never correct for that additional velocity);  Pure gravity loss.  To minimize gravity loss they need substantially larger thrust in order to accomplish an efficient suicide burn.

EDIT: To clear up your confusion, a supplementary set of engines towards the top of the vehicle ("Canard engines") are intended to solve the problems raised in posts like this http://forum.nasaspaceflight.com/index.php?topic=37466.msg1372150#msg1372150 .  As a secondary point, they might be used if a nested MAV design turns out to be needed because of low ISRU mass payoffs.
« Last Edit: 11/01/2015 11:07 AM by Burninate »

#### AncientU

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1024 on: 11/01/2015 12:03 PM »
The set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines.  Scaling these engines up and fueling them with methlox should provide the fine control needed for landing.  Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.  This technology/approach could be useful for a reusable lander that explores undeveloped sites.
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#### Burninate

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1025 on: 11/01/2015 12:13 PM »
The set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines.  Scaling these engines up and fueling them with methlox should provide the fine control needed for landing.  Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.  This technology/approach could be useful for a reusable lander that explores undeveloped sites.

I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them.  Liftoff thrust requirements are a large multiple of landing thrust requirements.

#### AncientU

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1026 on: 11/01/2015 12:25 PM »
The set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines.  Scaling these engines up and fueling them with methlox should provide the fine control needed for landing.  Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.  This technology/approach could be useful for a reusable lander that explores undeveloped sites.

I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them.  Liftoff thrust requirements are a large multiple of landing thrust requirements.

If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing.  Why are lift-off thrust requirements 'a large multiple' of landing?  You only need to clear the ground by 100m (and maybe move laterally a bit)...
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#### Paul451

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1027 on: 11/01/2015 01:42 PM »
Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.
I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them.  Liftoff thrust requirements are a large multiple of landing thrust requirements.
If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing.

To reach TEI from Mars surface, you're looking at around 400 tonnes of fuel (depending on dry-mass). Additionally, launch needs to be at 2-3 g to reduce gravity losses. So minimum 2*9.8*500 = 9.3MN. So over 130 Super-Dracos just to reach 100m. (Hell, 25 just to hover.)

#### Burninate

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1028 on: 11/01/2015 01:49 PM »
The set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines.  Scaling these engines up and fueling them with methlox should provide the fine control needed for landing.  Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.  This technology/approach could be useful for a reusable lander that explores undeveloped sites.

I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them.  Liftoff thrust requirements are a large multiple of landing thrust requirements.

If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing.  Why are lift-off thrust requirements 'a large multiple' of landing?  You only need to clear the ground by 100m (and maybe move laterally a bit)...

Because of all that methalox!  At landing the vehicle is near the penultimate dry mass.  At liftoff it's at around 3.35x the dry mass (in the case of 380s Isp & LMO refueling at ~4.5km/s dV), or 6.55x the dry mass (in the case of 380s Isp & no LMO refueling with Hohmann transfer home at ~7km/s dV) or more (in fast transit cases without LMO refueling).

This translates directly into proportionately higher thrust.  This higher thrust figure is achievable, but the sheer weight of the engines required adds quite a bit to the vehicle.

Admittedly, you need lower acceleration at liftoff than at landing;  I need to do further math on this.
« Last Edit: 11/01/2015 01:54 PM by Burninate »

#### AncientU

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1029 on: 11/01/2015 02:26 PM »
The set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines.  Scaling these engines up and fueling them with methlox should provide the fine control needed for landing.  Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.  This technology/approach could be useful for a reusable lander that explores undeveloped sites.

I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them.  Liftoff thrust requirements are a large multiple of landing thrust requirements.

If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing.  Why are lift-off thrust requirements 'a large multiple' of landing?  You only need to clear the ground by 100m (and maybe move laterally a bit)...

Because of all that methalox!  At landing the vehicle is near the penultimate dry mass.  At liftoff it's at around 3.35x the dry mass (in the case of 380s Isp & LMO refueling at ~4.5km/s dV), or 6.55x the dry mass (in the case of 380s Isp & no LMO refueling with Hohmann transfer home at ~7km/s dV) or more (in fast transit cases without LMO refueling).

This translates directly into proportionately higher thrust.  This higher thrust figure is achievable, but the sheer weight of the engines required adds quite a bit to the vehicle.

Admittedly, you need lower acceleration at liftoff than at landing;  I need to do further math on this.

Not true... a 100mT payload plus the vehicle dry weight (~25mT?) at landing -- at lift-off, 25mT plus 4x propellant gives same mass as at landing. (Sorry for the gross approximations.)  This gets you back to LMO where you fuel-n-go for Hohmann transfer home.  If vehicle is a lander, it refuels in LMO and prepares for another descent to the surface.

The 'sheer weight' of a set of 8 superdracos is less than 1mT if I recall correctly.
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#### AncientU

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1030 on: 11/01/2015 02:39 PM »
Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.
I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them.  Liftoff thrust requirements are a large multiple of landing thrust requirements.
If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing.

To reach TEI from Mars surface, you're looking at around 400 tonnes of fuel (depending on dry-mass). Additionally, launch needs to be at 2-3 g to reduce gravity losses. So minimum 2*9.8*500 = 9.3MN. So over 130 Super-Dracos just to reach 100m. (Hell, 25 just to hover.)

Isn't 'g' equal to 9.8*0.38 on Mars?  And why does launch have to be at 2-3 Martian gs -- only 1.2g is typical from Earth's surface?

You need to lift the craft off the ground to 100m or so (at an increment above 0.38g) before the Raptor firing to return to LMO (scenario we are discussing).  Even if the 25 to hover is accepted, which I don't, this is just a 3x increase in superdraco power... not a show-stopper (superdracos were a 200x scale-up from dracos).
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#### stoker5432

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1031 on: 11/01/2015 03:05 PM »
But it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.

It might be needed for the first MCT. It's not required to get people or cargo to Mars after that. It's an optional safety feature not a requirement.

#### dror

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1032 on: 11/01/2015 03:58 PM »
Musk is borrowing somewhat from Mars Direct (or is it Semi-Direct?) where an already-fueled ascent vehicle is fueled up on the surface. I don't see a good reason not to have a fueled up vehicle ready when they arrive.

I see no reason why to land the fuel production equipment at all.
It can do the same job on orbit.

you have said it yourself:
http://forum.nasaspaceflight.com/index.php?topic=17984.msg620589#msg620589
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#### Lars-J

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1033 on: 11/01/2015 06:02 PM »
Perhaps, but that kind of coverage does not exist. Yes. And even when it does, it tells you nothing about the relative strength of the surface. It could be the Martian equivalent of quicksand for all we know.

But it is still irrelevant. MCT will need to be able to land on unprepared terrain, it will be necessary to allow of off-nominal EDL and abort scenarios. So it will need a sturdy gear, and you seem reluctant for some reason to admit that.

You should have watched the NASA workshop for selecting Mars landing sites. They do know a lot. They have identified landing spots with a hard surface that allow for safe landing. They do have a lot of coverage for different kinds of observation already and the teams can request more observations for each of the proposed 40 landing sites. They can do very thorough orbital survey for multiple data once they have narrowed down to few potential sites.

I did watch many of those workshops. And I also know that observations from the above is not as comprehensive as one might think, unless one also has ground observations to validate them. And if you have any reference of how they can judge the hardness of a surface (brittle and compressible vs hard as granite), then please link to the presentation where they show that.

MCT will certainly not need to be designed for landing on any not suitable off target landing sites so don't try to include such requirements into your mass budget.

If you are willing to claim that no off-nominal landing will ever happen, nor be planned for, then go ahead. I'm not part of the crowd that cries for a useless abort system, but I do think a sturdier landing gear with some extra margin is mass well spent. Extra margin to allow landing on marginal sites during an emergency/off-nominal, NOT any site on Mars.

#### guckyfan

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1034 on: 11/01/2015 06:12 PM »
I did watch many of those workshops. And I also know that observations from the above is not as comprehensive as one might think, unless one also has ground observations to validate them. And if you have any reference of how they can judge the hardness of a surface (brittle and compressible vs hard as granite), then please link to the presentation where they show that.

Available hard surfaces for landing were mentioned in many presentations. And emphasized in general discussion. Of course you can chose not to believe them.

If you are willing to claim that no off-nominal landing will ever happen, nor be planned for, then go ahead. I'm not part of the crowd that cries for a useless abort system, but I do think a sturdier landing gear with some extra margin is mass well spent. Extra margin to allow landing on marginal sites during an emergency/off-nominal, NOT any site on Mars.

Of course off-nominal landing can and will happen. It is not to be expected that MCT will survive those undamaged. It is enough that the passengers have a chance to survive. When landing off-nominal that MCT will never fly again. Landing gear will always have margin, no need to add anything.

#### Burninate

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1035 on: 11/01/2015 07:03 PM »
The set of eight superdracos on Dragon 2 could land 100mT on Mars with margin to spare, once velocity is reduced to near zero by the Raptor engines.  Scaling these engines up and fueling them with methlox should provide the fine control needed for landing.  Could be used to lift-off the surface, too, before the large, centerline engine(s) are started.  This technology/approach could be useful for a reusable lander that explores undeveloped sites.

I would *really like* to employ them in an integrated single-vehicle system for liftoff, but that would also mean a hell of a lot more of them.  Liftoff thrust requirements are a large multiple of landing thrust requirements.

If you've off-loaded 100mT of cargo and taken on equivalent fuel, the lift-off problem is same as landing.  Why are lift-off thrust requirements 'a large multiple' of landing?  You only need to clear the ground by 100m (and maybe move laterally a bit)...

Because of all that methalox!  At landing the vehicle is near the penultimate dry mass.  At liftoff it's at around 3.35x the dry mass (in the case of 380s Isp & LMO refueling at ~4.5km/s dV), or 6.55x the dry mass (in the case of 380s Isp & no LMO refueling with Hohmann transfer home at ~7km/s dV) or more (in fast transit cases without LMO refueling).

This translates directly into proportionately higher thrust.  This higher thrust figure is achievable, but the sheer weight of the engines required adds quite a bit to the vehicle.

Admittedly, you need lower acceleration at liftoff than at landing;  I need to do further math on this.

Not true... a 100mT payload plus the vehicle dry weight (~25mT?) at landing -- at lift-off, 25mT plus 4x propellant gives same mass as at landing. (Sorry for the gross approximations.)  This gets you back to LMO where you fuel-n-go for Hohmann transfer home.  If vehicle is a lander, it refuels in LMO and prepares for another descent to the surface.

The 'sheer weight' of a set of 8 superdracos is less than 1mT if I recall correctly.

Vehicle structural dry weight of 25mT is, in my judgement, about an order of magnitude too low for a reusable lander with integrated habitat that brings 100mT payload to the surface.  Yes, there's a question about whether ISRU gear is inside or outside the '100mT useful cargo' box, but the weight of the vehicle alone is at 100mT-200mT in most other people's scenarios, I just favor raising it to 300mT-500mT with my own special sauce.
« Last Edit: 11/01/2015 07:26 PM by Burninate »

#### Impaler

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1036 on: 11/01/2015 08:32 PM »

Ahh, I knew that sounded like a lot.  Thanks for spotting the error - 10 times too much mass allocated to engines.  That should bring the extra mass margin down below +10%.

If you're trying to avoid unprepared landing site excavation issues, you need space for the exhaust plume to spread out a bit.  I picked ~1km for a very rough & arbitrary figure.  At 100m, the exhaust plume is barely larger than the vehicle fairing diameter.  Shut the main engines down at 1km AGL after doing the full job of entry & descent, and they won't destroy the landing pad.  Then drop for a bit (now's a nice time to correct to vertical and unfurl the legs), & start controlled thrusting on the sideways-canted canard engines;  Any terrain damage they do will be well away from the place the legs impact the ground.  They still need plenty of thrust, however.  If they were only capable of precisely Mars gravity acceleration, they could hover at this 1km point, but not descend (because they could never correct for that additional velocity);  Pure gravity loss.  To minimize gravity loss they need substantially larger thrust in order to accomplish an efficient suicide burn.

EDIT: To clear up your confusion, a supplementary set of engines towards the top of the vehicle ("Canard engines") are intended to solve the problems raised in posts like this http://forum.nasaspaceflight.com/index.php?topic=37466.msg1372150#msg1372150 .  As a secondary point, they might be used if a nested MAV design turns out to be needed because of low ISRU mass payoffs.

I think 1 km is far too high an elevation to start worrying about plumes impinging the ground.  A large rocket lifting off the ground is not still bathing the launch pad in flames when it is 1 km up, rather it looks to be mostly over within 1-2 times the height of the launch tower.

Remember our goal as you point out is to avoid making craters in the ground and making dangerous ejecta which might impact the vehicle, at 100 m height their should be no danger to the vehicle even if some sand and dust are being swept up on the surface.  Under the near vacuum conditions of mars the plume will also spread MUCH wider then the vehicles base, just as the plume of a rocket expands markedly as it rises, it will never resemble the 'welding torch' look of a rocket at liftoff on Earth.

Your right that we would need slightly more then 1 G so that we can actually reach a true zero speed or something close too it.  Apollo lander was designed for 3 m/s vertical touch-down speed.  If started at that speed when Raptors cut out then we used propulsion exactly equaling mars gravity we would descend for 30 seconds at 3 m/s.

Also it occurs to me that one of the most significant differences between a landing system designed for hard landing pads and regolith is going to be the foot-pad area necessary.  On soft soil much larger pads will be needed to avoid penetrating to deeply into the ground.  We need to know (or estimate) the bearing strength of the surface which is something we really need to know for any effective ground vehicle design.  In addition you need to be able to handle a modest amount of slope or to accommodate differential soil strength which results in uneven amounts of leg penetration.

Here is a paper looking at the landing system for a hopper vehicle which would have some harsh un-improved landing site goals.  http://solarsystem.nasa.gov/docs/Gullotta_Resettable%20Landing%20Gear%20for%20Mars%20Hopper.pdf

#### Burninate

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1037 on: 11/01/2015 09:31 PM »
Well that's hilariously awful.  Per your link, current leg design for all landers to date actually crushes the footpads to absorb impact energy.

The leg actuation & damper is an interesting question;  Pneumatics are out in a vacuum, per the link.  I have seen assertions that we would just operate with a consummable pneumatic/hydraulic reserve, but they are not quantified and I'm skeptical about the ability to hold leg position for an 18 month deployment.  In my (completely unresearched) musings on the subject I resorted to mechanical linkages strung with actuated wire winches.  Those electric winches perform damping in convenient radial motion rather than the linear solenoid depicted.  As a bonus, you can use a gearbox for leverage against constant stresses, and still have shock absorption (even shock absorption in a large range of spring constants) from a tensioning spring wheel.  All solid, vacuum-safe, microgravity-safe materials, all usable without consumables.
« Last Edit: 11/01/2015 09:42 PM by Burninate »

#### meekGee

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1038 on: 11/01/2015 09:32 PM »

...  I expect SpaceX to sell normal commercial flights on BFR for a good long time before it is use for mars ...

... As NASA is the only conceivable customer for a first mission they need to be courted to create a mission utilizing SpaceX as the primary contractor....

I'm catching up on this thread so apologies for necro-quoting, but is this a widely shared opinion?  Basically that by the time SpaceX is ready with BFR, they will not be ready with a mission for it?

Or did you mean that NASA is the only conceivable customer other than SpaceX itself?

I can't be 100% sure that the first BFR mission will head to Mars, but I'm pretty sure there won't be a "good long time" (years?) in which BFR is used commercially before it used for MCTs to Mars.   I can see how maybe during off-season they are used for commercial purposes since why not, but they are built for a purpose, and I expect the Mars program to be pretty efficient in that things will mature by the time they are needed.
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#### Burninate

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##### Re: MCT Speculation and Discussion Thread 4
« Reply #1039 on: 11/01/2015 09:38 PM »

Ahh, I knew that sounded like a lot.  Thanks for spotting the error - 10 times too much mass allocated to engines.  That should bring the extra mass margin down below +10%.

If you're trying to avoid unprepared landing site excavation issues, you need space for the exhaust plume to spread out a bit.  I picked ~1km for a very rough & arbitrary figure.  At 100m, the exhaust plume is barely larger than the vehicle fairing diameter.  Shut the main engines down at 1km AGL after doing the full job of entry & descent, and they won't destroy the landing pad.  Then drop for a bit (now's a nice time to correct to vertical and unfurl the legs), & start controlled thrusting on the sideways-canted canard engines;  Any terrain damage they do will be well away from the place the legs impact the ground.  They still need plenty of thrust, however.  If they were only capable of precisely Mars gravity acceleration, they could hover at this 1km point, but not descend (because they could never correct for that additional velocity);  Pure gravity loss.  To minimize gravity loss they need substantially larger thrust in order to accomplish an efficient suicide burn.

EDIT: To clear up your confusion, a supplementary set of engines towards the top of the vehicle ("Canard engines") are intended to solve the problems raised in posts like this http://forum.nasaspaceflight.com/index.php?topic=37466.msg1372150#msg1372150 .  As a secondary point, they might be used if a nested MAV design turns out to be needed because of low ISRU mass payoffs.

I think 1 km is far too high an elevation to start worrying about plumes impinging the ground.  A large rocket lifting off the ground is not still bathing the launch pad in flames when it is 1 km up, rather it looks to be mostly over within 1-2 times the height of the launch tower.

Remember our goal as you point out is to avoid making craters in the ground and making dangerous ejecta which might impact the vehicle, at 100 m height their should be no danger to the vehicle even if some sand and dust are being swept up on the surface.  Under the near vacuum conditions of mars the plume will also spread MUCH wider then the vehicles base, just as the plume of a rocket expands markedly as it rises, it will never resemble the 'welding torch' look of a rocket at liftoff on Earth.

Ahh, but you have to remember: "Bathed in flames" is not the standard.  You can only see flames where the exhaust gasses exist as a hot plasma, but they do not cease to exist farther down the plume as they cool down.  Massflow per unit surface area is the standard.  You could dig a big ditch with an 8400kN cold gas thruster too.

Either way: 1km is not prohibitive, so lesser figures are also achievable.

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