Author Topic: MCT Speculation and Discussion Thread 4  (Read 626003 times)

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2540 on: 08/07/2016 03:47 AM »
Also how do you calculate mass flow rate for Raptor?

Conservation of momentum. Mass flow (kg/s) is thrust (kg-m/s2) divided by exhaust velocity (m/s). SpaceX has indicated that the thrust is about 2.5x106 N and the exhaust velocity is some 3700 m/s, yielding a massflow of 675 kg/s per engine. At 40% throttle that's 270 kg/s per engine, so 4 engines would burn over 1000 kg/s.

Mass flow is also proportional to thrust and inversely proportional to ISP; since we know Raptor will have thrust around 300% of Merlin, and ISP around 120% of Merlin, and Merlin has a mass flow of 270 kg/s, then Raptor should have a 40% throttled mass flow of around 270*300%/120%*40%=270 kg/s.

Both calculation methods indicate that 40% throttled mass flow for Raptor should be very similar to 100% mass flow for Merlin. Running 4 of those for 60 to 120 seconds is a LOT of fuel to put in orbit, ESPECIALLY to GTO or escape trajectories. Maybe they don't need to run 4, and maybe they can throttle lower, but please show your math for how big the rocket needs to be just to launch EDL fuel before stating it's simpler to engineer and launch than a heatshield.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2541 on: 08/07/2016 08:51 AM »
Your asking me to prove it is more EFFICIENT, that is completely different from saying it is SIMPLER.  I have NEVER claimed it is more efficient, but you keep confusing the thouse concepts.

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2542 on: 08/07/2016 10:56 AM »
Your asking me to prove it is more EFFICIENT, that is completely different from saying it is SIMPLER.  I have NEVER claimed it is more efficient, but you keep confusing the thouse concepts.

It is not an either/or question.  The optimal design will balance simple and efficient.  A heat shield has many benefits, and will be employed to maximize those obvious benefits (without adding complexity and risk to the design such as an inflatable HAID) and retro-propulsion will do the rest*. 

Generous up-mass is one of the features of the architecture being proposed -- another is refueling on orbit.
This design will not throw away what has been learned about atmospheric deceleration with heat shields, but will add to that a total destruction of the mass-constrained designs of the past.

This reminds me of the argument between the small rocket plus depot versus big rocket without refueling.  If you are going to have a significant Mars campaign, you need both... big rocket plus depots/refueling.  SpaceX is proposing the largest rocket by a factor of two and refueling it (the spaceship) as soon as it is in orbit.

* Note: F9 uses exactly this approach... it decelerates with a re-entry burn to the velocity that the engine end of the booster can handle, and then allows the atmosphere to decelerate to terminal velocity, and finally does a propulsive landing.
« Last Edit: 08/07/2016 11:30 AM by AncientU »
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Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2543 on: 08/07/2016 11:28 AM »
Another simple versus efficient issue is the use of gravitational assist...  This 'free lunch' won't be ignored.  Departure will likely use it -- Oberth burns, starting from high orbit, which are efficient, instead of direct departure burns from LEO which are simpler.  Gravitational capture (assisted by retro-propulsion) may also be employed at the Mars end instead of direct entry.  Passive/'free' benefits such as this -- which are accepted spaceflight methods  (e.g., launching to the east) -- will be in the parameter space for the Mars architecture.

We'll have to wait until September 27th to see which are chosen.  My money will be on d. All of the above.
« Last Edit: 08/07/2016 11:37 AM by AncientU »
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Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2544 on: 08/07/2016 12:11 PM »
A heat shield has many benefits, and will be employed to maximize those obvious benefits (without adding complexity and risk to the design such as an inflatable HAID) and retro-propulsion will do the rest*. 

A HIAD is just a more efficient heatshield. It is a simple concept and it has been shown to work. This NASA video shows the application of a HIAD for the safe return of a cargo spaceship from the ISS. It is not a great leap to imagine the same technology being used for the return of an F9S2.

https://www.nasa.gov/directorates/spacetech/game_changing_development/HIAD/HEART-Desig-Concept.html

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2545 on: 08/07/2016 12:24 PM »
A heat shield has many benefits, and will be employed to maximize those obvious benefits (without adding complexity and risk to the design such as an inflatable HAID) and retro-propulsion will do the rest*. 

A HIAD is just a more efficient heatshield. It is a simple concept and it has been shown to work. This NASA video shows the application of a HIAD for the safe return of a cargo spaceship from the ISS. It is not a great leap to imagine the same technology being used for the return of an F9S2.

https://www.nasa.gov/directorates/spacetech/game_changing_development/HIAD/HEART-Desig-Concept.html

Adding a HAID to the flamey end of a F9 is not simple.  It has to deploy from the engine compartment/octaweb over hot engines between burns and then get out of the way for the landing burn. 

Efficient, maybe. Simple, NOT.
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Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2546 on: 08/07/2016 12:29 PM »
Adding a HAID to the flamey end of a F9 is not simple.  It has to deploy from the engine compartment/octaweb over hot engines between burns and then get out of the way for the landing burn. 

Efficient, maybe. Simple, NOT.

Who said to add it to the flamey end?

https://forum.nasaspaceflight.com/index.php?topic=37808.msg1565000#msg1565000

Just pack it in the interstage with some nitrogen bottles.

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2547 on: 08/07/2016 12:53 PM »
Adding a HAID to the flamey end of a F9 is not simple.  It has to deploy from the engine compartment/octaweb over hot engines between burns and then get out of the way for the landing burn. 

Efficient, maybe. Simple, NOT.

Who said to add it to the flamey end?

https://forum.nasaspaceflight.com/index.php?topic=37808.msg1565000#msg1565000

Just pack it in the interstage with some nitrogen bottles.

Oops. Sorry, did not see your application to F9S2.


A HIAD is just a more efficient heatshield. It is a simple concept and it has been shown to work. This NASA video shows the application of a HIAD for the safe return of a cargo spaceship from the ISS. It is not a great leap to imagine the same technology being used for the return of an F9S2.

https://www.nasa.gov/directorates/spacetech/game_changing_development/HIAD/HEART-Desig-Concept.html

How many of the cargo return proposals/designs are using HIADs?  Why is that?
How much cargo is NASA itself returning from the ISS?  Why is that?
NASA PowerPoint/video does not an operating system make.
« Last Edit: 08/07/2016 01:02 PM by AncientU »
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Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2548 on: 08/07/2016 01:09 PM »
How many of the cargo return proposals/designs are using HIADs?
NASA PowerPoint/video does not an operating system make.

HIAD is way beyond PowerPoint presentations. Google is of course your friend, but there have been successful tests of the technology, and it's potential applications also extend to super heavy Mars landers.

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2549 on: 08/07/2016 01:13 PM »
How many of the cargo return proposals/designs are using HIADs?
NASA PowerPoint/video does not an operating system make.

HIAD is way beyond PowerPoint presentations. Google is of course your friend, but there have been successful tests of the technology, and it's potential applications also extend to super heavy Mars landers.

How many super heavy Mars landers have used HIADs?  Why is that?

Google "If a hammer is the only tool"
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Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2550 on: 08/07/2016 01:27 PM »
How many of the cargo return proposals/designs are using HIADs?
NASA PowerPoint/video does not an operating system make.

HIAD is way beyond PowerPoint presentations. Google is of course your friend, but there have been successful tests of the technology, and it's potential applications also extend to super heavy Mars landers.

How many super heavy Mars landers have used HIADs?  Why is that?

Google "If a hammer is the only tool"

Err, either because it's a potential application, or because there haven't been any super heavy Mars landings yet, take your pick.

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2551 on: 08/07/2016 03:03 PM »
NASA has been flying to Mars for over 50 years, and landing there for 40+.
The heaviest payload so far has been under 1t.  None used HIADs.  Mars 2020 and ExoMars won't either.

One of the next landings there (most likely) will be a spacecraft that was designed (and is being built) for human transportation to/from LEO by a company that has been flying to space for under 10 years.  It will be their first trip to Mars.  Landed mass will be about 10t, and use a fixed heat shield and supersonic retro-propulsion.  Several more such spacecraft (or their derivatives) will follow in early 2020s.

Another round of Lunar landings is in the making... no HIADs there for certain.  Landing on Europa... no HIAD.

Every spacecraft that has returned 10t or more from space to date has used heat shields and/or supersonic retro-propulsion.  Retro-propulsion alone has returned five 20+ tonne vehicles with another 70 or so planned for the immediate future.  Plans will soon be released for returning 100t vehicles from space near Earth using supersonic retro-propulsion only and delivering >>100t to Mars' surface using heat shields and supersonic retro-propulsion.

I'm not seeing a lot of real applications in the next decade or two for HIADs.  I suspect they will go the way of supersonic parachutes for applications that require tens of tonnes of landed mass.
« Last Edit: 08/07/2016 03:07 PM by AncientU »
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2552 on: 08/07/2016 05:24 PM »
2020 and exomars aren't going to be significantly larger than MSL.

This is a dumb way to argue. Please bring better game.
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Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2553 on: 08/07/2016 05:50 PM »
2020 and exomars aren't going to be significantly larger than MSL.

This is a dumb way to argue. Please bring better game.

Fine. 
What evidence is there that MCT will use a HIAD?
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Offline Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2554 on: 08/07/2016 05:56 PM »
2020 and exomars aren't going to be significantly larger than MSL.

This is a dumb way to argue. Please bring better game.

Fine. 
What evidence is there that MCT will use a HIAD?
There's some indirect evidence that it may be one of several options they'll consider.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2555 on: 08/07/2016 07:11 PM »
2020 and exomars aren't going to be significantly larger than MSL.

This is a dumb way to argue. Please bring better game.

Fine. 
What evidence is there that MCT will use a HIAD?
There's some indirect evidence that it may be one of several options they'll consider.

I don't disagree that it will be or already was considered; there are many possibilities that will be discarded as not optimum or unworkable.
Certainly no evidence that it will be used on Red Dragon, which would be the obvious opportunity to test it...
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Online lamontagne

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Re: MCT Speculation and Discussion Thread 4
« Reply #2556 on: 08/07/2016 07:17 PM »
I thought supersonic retropropulsion was particularly useful for Mars, as there was a 'hole' in the flight path between aerodynamic braking and retropropulsive landing since the atmosphere is not thick enough.
Perhaps this in not required for the Earth, with its thicker atmosphere?  Aerodynamic braking might be enough, with retropropulsion reserved for the landing?




Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2557 on: 08/07/2016 07:31 PM »
Your asking me to prove it is more EFFICIENT, that is completely different from saying it is SIMPLER.  I have NEVER claimed it is more efficient, but you keep confusing the thouse concepts.

I'm merely asking for evidence that's it's physically possible, not proof that it's optimal. It does appear possible for a stage that only enters from LEO or suborbital trajectories.

GTO and BLEO orbits would require a 3rd stage with a lot of Delta V, but I suppose that's not an issue is you assume BFS is a third stage and SEP tugs will fill some of those roles eventually.

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2558 on: 08/07/2016 07:49 PM »
Your asking me to prove it is more EFFICIENT, that is completely different from saying it is SIMPLER.  I have NEVER claimed it is more efficient, but you keep confusing the thouse concepts.

I'm merely asking for evidence that's it's physically possible, not proof that it's optimal. It does appear possible for a stage that only enters from LEO or suborbital trajectories.

GTO and BLEO orbits would require a 3rd stage with a lot of Delta V, but I suppose that's not an issue is you assume BFS is a third stage and SEP tugs will fill some of those roles eventually.

Or refueling of a second stage.
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Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2559 on: 08/07/2016 08:07 PM »
Refueling is fine for BLEO, particularly human launches, but seems like a lot of work for a GTO commsat launch.

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