Author Topic: MCT Speculation and Discussion Thread 4  (Read 505944 times)

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2680 on: 08/28/2016 07:41 AM »
Actually we do. Elon said in an interview that the BFR is just there to compensate for the difference in the two longest poles of the mission. The ΔV to LEO is about 9.5 km/s, including gravity and aerodynamic losses. Mars-Earth is about 7km/s. BFR will provide about 3.5km/s, so if the BFS can generate 7.5 km/s, the remainders are the payloads.

But then we get back to loaded vs empty.

However, that just reiterates my point. If the BFR is designed to exactly balance the difference between Mars-Earth and Earth-LEO, then if you make the Mars launch less efficient, you need more fuel, bigger tanks, different volume trades, then you are also changing every other part of the equation.

Online guckyfan

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Re: MCT Speculation and Discussion Thread 4
« Reply #2681 on: 08/28/2016 08:36 AM »
I expect that BFR will stage even slower than Falcon 9  for efficient RTLS. Which requires BFS to do even more than the F9 upper stage. I don't see the engines of BFS fire at any angle. They do everything to improve efficiency, light weight, highly optimized engines with high ISP. They won't give any of that away for canted engines.

Which of course makes the methods of EDL and avoiding debris hurled up more interesting. If not prepared too much in advance, find a hard flat surface. Such was mentioned as a requirement for a landing site in the NASA workshop on potential landing sites on Mars.

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2682 on: 08/28/2016 09:13 AM »
However, that just reiterates my point. If the BFR is designed to exactly balance the difference between Mars-Earth and Earth-LEO, then if you make the Mars launch less efficient, you need more fuel, bigger tanks, different volume trades, then you are also changing every other part of the equation.

There are only cosine losses for the first few seconds of the Mars launch, and never for launch to LEO. The canting is adjustable.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2683 on: 08/28/2016 09:34 AM »
Rocket engine gimbal is only on the order of 10 degrees, nothing like the kind of 45 degree plus that's being proposed here has ever been done and would complicate the engines thrust structure and plumping hugely.

A secondary set of touch down engines would be ignited BEFORE shutting down the Raptor engines and in the event that they don't start you would ride the Raptor thrust all the way to the ground in a hover-slam and risk what ever damage may come as that's preferable to crashing.  To suggest that one engines is just shut off before any validation of the next engines is to practice in straw-man engineering, Musk even went into detail about how Dragon v2 will test it's engines first before diverting from a splash-down trajectory so a parachute landing can be executed in case of any engine failure and I am sure they will not forget this concept on Mars.

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2684 on: 08/28/2016 01:15 PM »
I expect that BFR will stage even slower than Falcon 9  for efficient RTLS. Which requires BFS to do even more than the F9 upper stage. I don't see the engines of BFS fire at any angle. They do everything to improve efficiency, light weight, highly optimized engines with high ISP. They won't give any of that away for canted engines.

Which of course makes the methods of EDL and avoiding debris hurled up more interesting. If not prepared too much in advance, find a hard flat surface. Such was mentioned as a requirement for a landing site in the NASA workshop on potential landing sites on Mars.

Exactly.

Simple models with the rocket equation show that there's a sweet spot where a low & slow BFR and a high delta V 2nd stage BFS works out well for (1) launch to LEO delta V (2) refueled, TMI and EDL delta V and (3) reduced payload return to the green hills of Earth delta V.

The reduced payload from Mars makes the 3 delta Vs @ payload mass required close.  The difficult engineering is going to be mostly in the BFS with conflicting requirements for exotic Mars terrain landing techniques, high speed EDL and extreme mass savings.

The BFR is a robust (for RTLS) heavy thrust augmenter likely netting <2Km/sec after gravity losses towards LEO.  It will be engineered with all the lessons SX is in process of learning from returned F9 cores so that checkout and reliable re-flight is a minimal cost operation.  A few kilos here and there to improve safety and rapid re-use hurts less on the 1st stage. 

“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline AncientU

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Re: MCT Speculation and Discussion Thread 4
« Reply #2685 on: 08/28/2016 03:35 PM »
Agree with the low and slow staging. 
The 'few extra kilos' will be more like a few extra tens of tonnes of fuel to guarantee RTLS, paying the performance hit so that relaunch can be expeditious.
BFS will achieve low Earth orbit, but with fuel mostly exhausted -- supplying something like 7.5km/s delta-v.
Refueling those huge tanks on orbit is the genius #2* here... topping off tanks again in high orbit** before a departure burn will maximize payload or minimize transit time if crewed.

* #1 was low and slow staging, then RTLS.
** hat trick
"If we shared everything [we are working on] people would think we are insane!"
-- SpaceX friend of mlindner

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #2686 on: 08/28/2016 05:40 PM »
There are only cosine losses for the first few seconds of the Mars launch, and never for launch to LEO. The canting is adjustable.

CITATION NEEDED!!!  What is your epistemology?

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #2687 on: 08/28/2016 06:54 PM »
There are only cosine losses for the first few seconds of the Mars launch, and never for launch to LEO. The canting is adjustable.

CITATION NEEDED!!!  What is your epistemology?

It was his theory, so his post IS the citation.

Online Burninate

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Re: MCT Speculation and Discussion Thread 4
« Reply #2688 on: 08/28/2016 10:24 PM »
Rocket engine gimbal is only on the order of 10 degrees, nothing like the kind of 45 degree plus that's being proposed here has ever been done and would complicate the engines thrust structure and plumping hugely.

A secondary set of touch down engines would be ignited BEFORE shutting down the Raptor engines and in the event that they don't start you would ride the Raptor thrust all the way to the ground in a hover-slam and risk what ever damage may come as that's preferable to crashing.  To suggest that one engines is just shut off before any validation of the next engines is to practice in straw-man engineering, Musk even went into detail about how Dragon v2 will test it's engines first before diverting from a splash-down trajectory so a parachute landing can be executed in case of any engine failure and I am sure they will not forget this concept on Mars.

While I tend to agree with your reasoning for the most part, the numbers aren't quite so bad.  The Space Shuttle Orbiter's main engines were IIRC offset from each other's center of thrust line by 10 or 15 degrees, and had an additional +-10.5 degree gimbal range on top of that, to deal with the very large changes in center of mass and the aerodynamic asymmetry during the ascent of the Shuttle system.

Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #2689 on: 08/28/2016 11:43 PM »
The decelerator designs are generally inherently stable in either direction once they have commenced their descent into the atmosphere. The hypersonic heating rate is proportional to velocity cubed, and to the inverse of the square root of the atmospheric density divided by the leading edge radius.

HeatingRate = 1.83e-4 * Math.Pow(speed, 3) * Math.Sqrt(atmosphericDensity / HeatingRadius());

So, the larger the leading edge radius (i.e. the blunter the body), the lower the heating rate. Although perhaps counter-intuitive, this is why the decelerator is best positioned at the front of the vehicle. I've attached a few design alternatives below.

Thank you, OneSpeed, for your very informative reply!

Offline Oersted

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Re: MCT Speculation and Discussion Thread 4
« Reply #2690 on: 08/28/2016 11:55 PM »
I would hardly consider a set of pressure fed engines firing on touch-down to be a 'miracle', it's a normal and standard part of the Soyuz capsule landing after all.

Soyuz uses small solid-fuel retro rockets. More like explosives, actually.

Offline raketa

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Re: MCT Speculation and Discussion Thread 4
« Reply #2691 on: 08/29/2016 01:08 AM »
I expect that BFR will stage even slower than Falcon 9  for efficient RTLS. Which requires BFS to do even more than the F9 upper stage. I don't see the engines of BFS fire at any angle. They do everything to improve efficiency, light weight, highly optimized engines with high ISP. They won't give any of that away for canted engines.

Which of course makes the methods of EDL and avoiding debris hurled up more interesting. If not prepared too much in advance, find a hard flat surface. Such was mentioned as a requirement for a landing site in the NASA workshop on potential landing sites on Mars.
I think you are right BFS will give same or less staging speed then Falcon 9.

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2692 on: 08/29/2016 03:56 AM »
There are only cosine losses for the first few seconds of the Mars launch, and never for launch to LEO. The canting is adjustable.

CITATION NEEDED!!!  What is your epistemology?

My last response was one of several in a chain, and some of those responses did include brief discussion of the issues. If you would like to know more about why extreme canting could be useful for SRP, you could do a whole lot worse than start with the attached Parametric Study of Peripheral Nozzle Configurations for Supersonic Retropropulsion by Bakhtian and Aftosmis.
« Last Edit: 08/29/2016 12:30 PM by OneSpeed »

Offline CuddlyRocket

Re: MCT Speculation and Discussion Thread 4
« Reply #2693 on: 08/29/2016 05:37 AM »
Rocket engine gimbal is only on the order of 10 degrees, nothing like the kind of 45 degree plus that's being proposed here has ever been done ...

There's always a first time!

Quote
... and would complicate the engines thrust structure and plumping hugely.

But has to be compared with complications etc of having a separate set of engines for landing etc.

SpaceX needs to make an assessment of what exactly are the dangers of landing, and especially taking off again, on a natural Mars surface. Perhaps it may be worth their while doing some experiments on those lines? If the dangers are too great, then they'll need another solution.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2694 on: 08/29/2016 10:53 AM »
Ok lets make that comparison, touch down engines would be as simple as a dozen super draco engines around the nose of the vehicle.  That is sufficient thrust to hover the likely landing mass of around 100 tons after the main propulsion system brings the vehicle to zero ground velocity at some safe height.  These engines would mass less then a ton and consume just slightly more propellant then the Raptor engines would to do the same job, the lower touch down speed should allow for a much lighter landing gear system as well which is nothing to sneeze at as landing gear is historically 10 percent of landed mass.

Making an engine gimbal a huge amount will basically require a completely second thrust structure to keep the inward thrust vectors from crushing the vehicle, thrust structures are usually more massive then the actual engines they hold.  Propellant lines need to use bellows to allow them to flex but they are like springs, the bend radius is large so to get a greater deflection the engine needs to be at the end of a longer and wider thrust structure, the mass of all this would quickly become prohibitive.

And as for the shuttle main engine achieving 10 degrees of movement please look at some cut-aways and see how deeply all the mess of plumbing goes in the vehicle just to do that much.  The shuttle engine had to be designed with that level of gimbaling, so unless SpaceX had the whole mars EDL system figured out before starting engine development it's very unlikely that Raptor would ever be capable of more then the typical single digit gimbaling of a standard rocket engine.

Offline Paul451

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Re: MCT Speculation and Discussion Thread 4
« Reply #2695 on: 08/29/2016 11:12 AM »
Ok lets make that comparison, touch down engines would be as simple as a dozen super draco engines around the nose of the vehicle. [...]

None of which helps for the re-launch.

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2696 on: 08/29/2016 01:13 PM »
That is sufficient thrust to hover the likely landing mass of around 100 tons after the main propulsion system brings the vehicle to zero ground velocity at some safe height.

I assume you mean weight, the mass would be at least 250mT, and much higher if the ship was to carry enough fuel to return directly to LMO or Earth. MCT is a fuel rich architecture, allowing for many mission profiles.

These engines would mass less then a ton and consume just slightly more propellant then the Raptor engines would to do the same job, the lower touch down speed should allow for a much lighter landing gear system as well which is nothing to sneeze at as landing gear is historically 10 percent of landed mass.

The landing velocity would be close to zero in either scenario.

Making an engine gimbal a huge amount will basically require a completely second thrust structure to keep the inward thrust vectors from crushing the vehicle, thrust structures are usually more massive then the actual engines they hold.  Propellant lines need to use bellows to allow them to flex but they are like springs, the bend radius is large so to get a greater deflection the engine needs to be at the end of a longer and wider thrust structure, the mass of all this would quickly become prohibitive.
And as for the shuttle main engine achieving 10 degrees of movement please look at some cut-aways and see how deeply all the mess of plumbing goes in the vehicle just to do that much.  The shuttle engine had to be designed with that level of gimbaling, so unless SpaceX had the whole mars EDL system figured out before starting engine development it's very unlikely that Raptor would ever be capable of more then the typical single digit gimbaling of a standard rocket engine.

Who said anything about gimbaling more than a few degrees? The idea is to allow the canting of the entire engine mount in a single degree of freedom. Canting is required anyway by SRP, so there is no extra engineering required for landing.

MCT missions will require landing a mass more than two orders of magnitude greater than any Mars mission so far. It amazes me that you assume that everything about MCT will be something that has been done before. SpaceX starts with what works, then iterates to improve the state of the art. When no solution exists, it innovates brilliantly. I expect no less from the MCT.
« Last Edit: 08/29/2016 01:25 PM by OneSpeed »

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2697 on: 08/29/2016 01:14 PM »
Ok lets make that comparison, touch down engines would be as simple as a dozen super draco engines around the nose of the vehicle. [...]

None of which helps for the re-launch.

And would be unnecessary mass once a hard concrete landing/launching pad is in place, which will probably be ASAP.

Would deployable thrust deflectors make sense? Rather than gimballing the engines, just redirect the exhaust away from the vehicle at 30 or 45 degrees. I'm thinking an ablative-coated panel set just below the engines (which I think will be mounded on the sidewalls), that pops out into the exhaust stream.

Online envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2698 on: 08/29/2016 01:28 PM »
...The idea is to allow the canting of the entire engine mount in a single degree of freedom.
...

I think you're drawing unnecessary distinctions between multiple static engine positions (e.g. 0 degrees +/- 5 degrees and 45 degrees +/- 5 degrees) and a large variable dynamic range (e.g. -5 to 50 degrees) of engine positions.

If you have 2 (nearly) static positions, you still have to feed pressurized propellant to the turbopumps at both of them, which requires incredibly flexible plumbing. And you still require a thrust structure that can take the load of the engines in VERY different directions.

Both of these are sizable engineering problems with either multiple near-static positions, or with a very wide dynamic range.

Quote
Canting is required anyway by SRP, so there is no extra engineering required for landing.

The minimum feasible solution for landing is certainly different than the minimum feasible solution for SRP, so there will definitely be "extra" engineering required to bridge the gap between those two requirements. As I understand, SRP is viable at around 15 to 20 degrees of canting, while landing/relaunching on unprepared soil is not.

Offline Arb

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Re: MCT Speculation and Discussion Thread 4
« Reply #2699 on: 08/29/2016 01:40 PM »
It could be that one of the roles of the Red Dragon missions is ground confirmation of a flat clear area of rock to use as an initial BFS landing site.

Someone mentioned that such an area was one of the requirements of the recent NASA landing site selection conference. So they are known to exist.

Would make this discussion moot.

Legs need to be very strong, don't they. What's the weight on Mars of the fuel needed to launch direct to Earth?

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