Author Topic: Skycrane future developments discussion thread  (Read 5750 times)

Offline Jim

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Re: Skycrane future developments discussion thread
« Reply #20 on: 03/01/2021 12:53 am »

First, could you squeeze more fuel in the descent stage?  There is certainly room (see below).  Then how about mass margin?   

Wrong, there is no mass margin.  And there is no room for larger tanks


Instead, JPL could use the pressurization sequence to move liquid fuel from tank to tank, changing the center of gravity, then move it back again.

No, not viable.  Too slow.  And there is no real good way to measure the mass moved and regulate gas pressures at multiple levels.

 

This would reduce the needed ejectable masses

All the extra plumbing, flow meters, regulators, etc would likely negate any savings and just increase complexity with no real benefits.

 
A bi-propellant descent stage would only need 300 kg of propellant as opposed to the 400 kg they used.  (Plus the tanks would be lighter both due to smaller size and lower pressure - 
 

That would be wrong.  A biprop system would weigh more with the extra tanks and propellant lines.  The higher density of NTO would make balancing the descent stage problematic.   Try finding room for 4 or 6 tanks.  What biprop thrusters meet the mission requirements?

Then there is the matter of roll thrusters type and plumbing .  The operating pressure of the tanks has little effect on their mass in the days of composite overwrap.


And again, your statement "This would leave plenty of margin" is unsupported.

 
Finally, their descent profile is designed to be as safe as possible, not to use the minimum fuel.   A more hoverslam-type landing would use less fuel, leaving more for post-rover-dropoff maneuvers. 
 

Wrong.  A hover slam does not save propellant.  It is used when the engines can't throttle low enough and still have a T/W>1.  It actually uses more propellant because a larger than necessary engines are used.

The hover slam does not allow for obstacle avoidance.  The descent profile is to allow for maneuvering to avoid bad landing areas.   And there is the divert maneuver, required to avoid the back shell and parachute.

No slack in the system, just somebody throwing darts at the wall.

Offline Vultur

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Re: Skycrane future developments discussion thread
« Reply #21 on: 03/01/2021 02:34 am »
SpaceX's plans are irrelevant to any future Skycrane developments.

I think your point may be retro-propulsive landings will obsolete everything else.  Maybe...maybe not...that is a possibility, however current missions like sample return are paced towards later this decade.

More that if a continuous human presence is established on Mars, separately-launched-and-landed science spacecraft might no longer be a thing (instead carrying the science payloads, rovers, etc. along with the human missions).

I agree already planned missions wouldn't necessarily be affected, but given the lead time, we should know whether the SpaceX plan is working before those missions are done (Musk has talked about Mars landing in 2025... even pushing that out two synods for "Elon time", that's still this decade).

Offline Zed_Noir

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Re: Skycrane future developments discussion thread
« Reply #22 on: 03/01/2021 03:19 am »
<snip>
SpaceX's plans are irrelevant to any future Skycrane developments.
<snip>
Even if SpaceX succeeded with landing on Mars tomorrow, nothing is in the NASA, ESA, JAXA or any other agencies pipeline for science missions that would take advantage of that.  ( I could be unaware of it , but haven't heard of anything)

Well, there is a slight possibility that SpaceX might launch a prototype EDL (Entry, Descend & Landing) test Starship to Mars in the 2022 launch window. Presuming the Starship make it through entry and drops below hypersonic speed. It could eject a number Skycrane type vehicles  along it's landing path. But again it's a slight possibility there will be a Mars bound Starship launching in 2022, never mind any exotic Starscrane type vehicles.


SpaceX itself could have basic scientific payloads in development to support their future Mars missions. Something like a Martian weather station that can also measure various radiations. SpaceX is also capable of mass producing payloads just like the Starlink comsat bus components.

Offline Kaputnik

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Re: Skycrane future developments discussion thread
« Reply #23 on: 03/01/2021 06:15 am »
I recall that there was a suggestion to replace the balance masses with a circulating liquid mercury system. Did anybody (Jim?) know how far that idea got? Obviously sounds like a lot of extra complexity for marginal gain, but perhaps a future growth path.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline libra

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Re: Skycrane future developments discussion thread
« Reply #24 on: 03/01/2021 06:54 am »
It is interesting to analyze SpaceX Mars EDL system in the light of the Braun paper I linked. The paper dates from 2006 just when Musk launched its first Falcon 1. And a section of the paper is pondering how to get from MSL 1 metric ton to Human Space Flight 100 metric ton. Quite a leap !

I wonder if Musk red this paper, probably yes. One can see how  trying to land 100 times more he ran straight into "Mars versus EDL" issues.

At the beginning (Red Dragon ?) Musk wanted to go 100% propulsive, just like on the Moon - and a bit like Falcon 9, using retropropulsive rocketry (note: the paper mentions supersonic retro-propulsion, nearly a decade before Falcon 9 2013-2016 gradual mastering of it !)
Doing as if Mars atmosphere was thin enough, it simply did not existed. But Mars atmosphere did not agree, and would readily burn any such lander. Alternatively, it would burden it with an enormous amount of propellant.
In both cases: Mars versus Musk: 1-0 for Mars.
Even Musk had to concede to Mars, and accept some hypersonic glide. In turn, this led to the present SN-8 / SN-9 flight profile - with the skydive manoeuver, and hypersonic entry / glide, Shuttle style. Except there is no Shuttle Landing Strip on Mars, so back to a vertical landing...

Mars EDL is hard, because Mars is so damn unforgiving.

Offline Don2

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Re: Skycrane future developments discussion thread
« Reply #25 on: 03/01/2021 09:04 am »
How difficult would it be to put an 8.4m fairing on a Falcon Heavy or a Vulcan rocket? It seems that Mars landed missions could benefit from this, as would future space telescopes.

This would work for things that need the extra width but don't need the mass capability of SLS.

Online ugordan

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Re: Skycrane future developments discussion thread
« Reply #26 on: 03/01/2021 01:06 pm »
Once again, I don't think these changes are a good idea.  But there is certainly slack in the system - the descent stage had reliability in mind, not the utmost possible performance.

Is that "slack", though? Isn't the tradeoff between reliability and performance a constant struggle and one which the engineers always have to decide on?

You can take a higher-performing, but more risky system (say biprop), add redundant components to it which will increase reliability, but also increase mass hence reducing its performance gain over a simpler, but inherently more reliable system. Rarely can you have your cake AND eat it.

Offline baldusi

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Re: Skycrane future developments discussion thread
« Reply #27 on: 03/01/2021 03:38 pm »
Finally, their descent profile is designed to be as safe as possible, not to use the minimum fuel.   A more hoverslam-type landing would use less fuel, leaving more for post-rover-dropoff maneuvers. 
 

Wrong.  A hover slam does not save propellant.  It is used when the engines can't throttle low enough and still have a T/W>1.  It actually uses more propellant because a larger than necessary engines are used.

[...]

A Hoverslam might not be desirable for the SkyCrane system, but assuming you already have the thrust sizing, it does reduce propellant requirements because it reduces the gravity losses. It works specially well if you are at terminal velocity. So, it does makes sense for Falcon 9, and does reduces propellant usage for that case. But is completely inapplicable for SkyCrane.

Offline lrk

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Re: Skycrane future developments discussion thread
« Reply #28 on: 04/15/2021 04:59 pm »
At the beginning (Red Dragon ?) Musk wanted to go 100% propulsive, just like on the Moon - and a bit like Falcon 9, using retropropulsive rocketry (note: the paper mentions supersonic retro-propulsion, nearly a decade before Falcon 9 2013-2016 gradual mastering of it !)
Doing as if Mars atmosphere was thin enough, it simply did not existed. But Mars atmosphere did not agree, and would readily burn any such lander. Alternatively, it would burden it with an enormous amount of propellant.
In both cases: Mars versus Musk: 1-0 for Mars.
Even Musk had to concede to Mars, and accept some hypersonic glide. In turn, this led to the present SN-8 / SN-9 flight profile - with the skydive manoeuver, and hypersonic entry / glide, Shuttle style. Except there is no Shuttle Landing Strip on Mars, so back to a vertical landing...

Mars EDL is hard, because Mars is so damn unforgiving.

That's not at all accurate - Red Dragon would have performed a lifting aerodynamic entry and used its launch abort engines for the final deceleration, just like MSL/Mars 2020 and others.  A fully propulsive Mars EDL would require an impractical amount of propellant. 

Red Dragon definitely could have worked, but would have scaled up poorly - the Starship design does do more of the deceleration aerodynamically, enabling a better payload fraction.  Additionally Red Dragon was canceled after propulsive landing was deleted from Dragon 2, as modifying Dragon for Mars entry would have required even more one-off engineering effort that could be better spent on developing the next system. 

Offline baldusi

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Re: Skycrane future developments discussion thread
« Reply #29 on: 04/25/2021 01:55 pm »
Little note regarding Red Dragon: they inverted the angle of attack, so they used the aerodynamic forces pointing down. This allowed them to bleed more energy. How? Because to get more time of atmosphere drag, they needed shallow entry angles, but at those speed they would skip over the atmosphere. So, by inverting the angle of attack they actually avoided skipping and could bleed more speed.
I do wonder if they will use a similar trick with Starship.

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