Author Topic: MCT Speculation and Discussion Thread 4  (Read 605542 times)

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2440 on: 07/15/2016 03:43 AM »
Another concept I just considered, with a small BFS and a larger second stage without using SEP.  Launch with just cargo in the BFS and all propellant in the 2nd stage, the two stay together and reach orbit much like a Dragon capsule and it's trunk. 

Then both are refueled to full, TMI is conducted by firing the 2nd stage for ~2 km/s of acceleration then the BFS separates and performs the remaining boost.  This leaves the 2nd stage far short of Earth escape and it will be in an elliptical orbit which can easily allow it to land again.

Another 2nd stage without a BFS attached is also placed in orbit and refueled to make a TMI on it's own with a propulsive insertion at Mars, by utilizing a slow hohoman transfer the propellant delivery is much more efficient.  BFS then rendezvouses with it in orbit and takes on the necessary propellant for TEI.  The 2nd stage now very light now returns to Earth via a slow transfer and aero-captures at Earth. 

The 2nd stage would be capable of this kind of total DeltaV because it is almost nothing but tank so a 4-5% dry mass fraction is reasonable for it (far lower then what the BFS could achieve).  The main challenge is endurance and maintaining propellants against boil-off for that length of time, but any vehicle that waits in LEO while it is being filled up will need considerable insulation so this 2nd stage can perform the job of tanker to LEO, depot in LEO, tanker to Mars and heavy lifter to LEO for the BFS and other payloads.

I see this stage being around 60 mt in dry mass with 1200 mt of propellant capacity and equipped with solar and radiators on the surface as on the Dragon 2 capsule.
« Last Edit: 07/15/2016 03:45 AM by Impaler »

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2441 on: 07/15/2016 09:09 PM »
Some form of orbital maneuvering engine is a must, you can't use the main propulsion system for thouse kinds of maneuvers.  Draco (not super) might do the very fine maneuvers for docking and such but you will still want something stronger for a de-orbit burn.

Lets compare to shuttle, it massed around 75 mt on orbit and had OMS with 53 kN thrust.  Scale up to a likely 200+ mt mass in orbit and your looking at the thrust of 2 Super Draco engines for a de-orbit burn.  A Raptor engine would need to throttle down to 6% to give that thrust. 

Still we would be looking at about a 6 fold increase over a reasonable orbital maneuvering system so it's not free.  Their may be additional benefits to these engines, their position high on the vehicle gives them huge leverage on the vehicle useful in EDL if a flip over is needed.

Couldn't you just use a single Raptor and fire it for a shorter time period?  What type of delta V is needed for a de-orbit burn typically?

Offline philw1776

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Re: MCT Speculation and Discussion Thread 4
« Reply #2442 on: 07/15/2016 11:24 PM »
Some form of orbital maneuvering engine is a must, you can't use the main propulsion system for thouse kinds of maneuvers.  Draco (not super) might do the very fine maneuvers for docking and such but you will still want something stronger for a de-orbit burn.

Lets compare to shuttle, it massed around 75 mt on orbit and had OMS with 53 kN thrust.  Scale up to a likely 200+ mt mass in orbit and your looking at the thrust of 2 Super Draco engines for a de-orbit burn.  A Raptor engine would need to throttle down to 6% to give that thrust. 

Still we would be looking at about a 6 fold increase over a reasonable orbital maneuvering system so it's not free.  Their may be additional benefits to these engines, their position high on the vehicle gives them huge leverage on the vehicle useful in EDL if a flip over is needed.

Couldn't you just use a single Raptor and fire it for a shorter time period?  What type of delta V is needed for a de-orbit burn typically?

~100 Km/sec from LEO.  Less from low Mars orbit.

EDIT:  100 METERS/sec!!!!
« Last Edit: 07/16/2016 02:00 AM by philw1776 »
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline TomH

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Re: MCT Speculation and Discussion Thread 4
« Reply #2443 on: 07/16/2016 01:18 AM »
Some form of orbital maneuvering engine is a must, you can't use the main propulsion system for thouse kinds of maneuvers.  Draco (not super) might do the very fine maneuvers for docking and such but you will still want something stronger for a de-orbit burn.

Lets compare to shuttle, it massed around 75 mt on orbit and had OMS with 53 kN thrust.  Scale up to a likely 200+ mt mass in orbit and your looking at the thrust of 2 Super Draco engines for a de-orbit burn.  A Raptor engine would need to throttle down to 6% to give that thrust. 

Still we would be looking at about a 6 fold increase over a reasonable orbital maneuvering system so it's not free.  Their may be additional benefits to these engines, their position high on the vehicle gives them huge leverage on the vehicle useful in EDL if a flip over is needed.

Couldn't you just use a single Raptor and fire it for a shorter time period?  What type of delta V is needed for a de-orbit burn typically?

I was thinking the same thing. De-orbit burns have typically been low thrust and moderate duration from small engines because that was sufficient; in this situation you don't need a large engine to fight gravity losses. Also, typically, larger engines and tankage have already jettisoned. If, however, you already have a larger engine on the vehicle (e.g. BFS) which can do a burn that gives accurate ∆V, and you have fuel and oxidizer available (STS couldn't de-orbit with RS-25 as prop tanks were gone), use of the larger engine seems possible.

I am not suggesting a high-G burst that lasts a fraction of a second, as you are more unlikely to get the precision ∆V required for precision landing. If, however, the burn is long enough and the engine controllable enough to give precise ∆V, it seems that option is possible.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2444 on: 07/16/2016 02:41 AM »
Perhaps a shorter burn can do the job for de-orbit but that is the heaviest demand placed on an OMS, I'm sure their will be needs that are too fine and delicate for a Raptor engine and a set of maneuvering engines will exist. 

The question is will they be very small jets that are of no use in landing or would they be intentionally oversized to act as touchdown engines, I favor that approach as it solves a major issue during touchdown while allowing another system to serve double duty.

Note that the thrust from these small engines because they would have the thrust to just hover at landing with cargo could allow the vehicle when empty to make short hops like the Grasshopper test vehicle which might be an excellent means of moving the vehicle around the surface and landing it adjacent to fueling, servicing or repair infrastructure without endangering these places with a strong rocket exhaust.

Offline Nilof

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Re: MCT Speculation and Discussion Thread 4
« Reply #2445 on: 07/16/2016 06:52 PM »
One thing worth keeping in mind is that since Mars has a very low atmospheric pressure, the throttle range is essentially the vacuum throttle range, not the sea level one. So I wouldn't be surprised if the Raptor could have very deep throttling for a Mars landing, significantly deeper than the Merlin for Earth landings.
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2446 on: 07/16/2016 07:33 PM »
Whats the Vac throttle range on comparable Russian staged combustion hydro-carbon engines?

Offline KelvinZero

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Re: MCT Speculation and Discussion Thread 4
« Reply #2447 on: 07/16/2016 10:48 PM »
btw, would the weight of these small low-ISP engines reduce overall performance to orbit, or could they pay for themselves with a bit more oomph at the very beginning of liftoff?

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2448 on: 07/17/2016 12:25 AM »
Insignificant at launch from either Earth or Mars, their mass is a performance loss but the comparison is not just will the same vehicle minus these engines, if your trying to land directly on Raptor engines the vehicle might need to be strengthened or have more mass in other places. 

I've estimated 1 mt for these engines and they would draw from the vehicles main Methane/LoX tanks, though they may require a pressurization tank and a tertiary pressurizing gas supply.
« Last Edit: 07/17/2016 12:25 AM by Impaler »

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2449 on: 07/18/2016 03:40 PM »
Another concept I just considered, with a small BFS and a larger second stage without using SEP.  Launch with just cargo in the BFS and all propellant in the 2nd stage, the two stay together and reach orbit much like a Dragon capsule and it's trunk. 

Then both are refueled to full, TMI is conducted by firing the 2nd stage for ~2 km/s of acceleration then the BFS separates and performs the remaining boost.  This leaves the 2nd stage far short of Earth escape and it will be in an elliptical orbit which can easily allow it to land again.

Another 2nd stage without a BFS attached is also placed in orbit and refueled to make a TMI on it's own with a propulsive insertion at Mars, by utilizing a slow hohoman transfer the propellant delivery is much more efficient.  BFS then rendezvouses with it in orbit and takes on the necessary propellant for TEI.  The 2nd stage now very light now returns to Earth via a slow transfer and aero-captures at Earth. 

The 2nd stage would be capable of this kind of total DeltaV because it is almost nothing but tank so a 4-5% dry mass fraction is reasonable for it (far lower then what the BFS could achieve).  The main challenge is endurance and maintaining propellants against boil-off for that length of time, but any vehicle that waits in LEO while it is being filled up will need considerable insulation so this 2nd stage can perform the job of tanker to LEO, depot in LEO, tanker to Mars and heavy lifter to LEO for the BFS and other payloads.

I see this stage being around 60 mt in dry mass with 1200 mt of propellant capacity and equipped with solar and radiators on the surface as on the Dragon 2 capsule.

I think SpaceX is likely to build a tanker stage similar to what you're suggesting. They are going to be launching a LOT of fuel, and a lightweight stage can put more payload up. And since it would never carry significant downmass, its very low ballistic coefficient makes nose-first entry from LEO much easier. No need for hypersonic retropropulsion, just float down to a low terminal velocity, and swan-dive to land.

But, sending it to Mars seems pretty inefficient long term. They are going to want that stage at Earth paying for itself by ferrying up payloads. The prop tanks on BFS will also be in the 4% mass fraction range, so having tanks big enough for 6km/s isn't a major mass penalty. Massive ISRU refueling is a significant long pole in the architecture anyway, sending TEI fuel from Earth might somewhat reduce that short-term but adds a lot of cost and complexity. And the BFS needs to be big enough to shuttle a significant fuel payload to LMO if it's to be used as a LMO shuttle eventually.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2450 on: 07/19/2016 02:43 AM »
BFS will certainly cost more then the 2nd stage I'm suggesting, so any job that can be performed by the cheaper stage should be given to it rather then the BFS.  And remember everything is predicated on propellant depots which occupy vehicle time too.  Sending one 2nd stage off to Mars for a synod to do a refueling in order to bring back a crew is not that significant in the course of the many many launches necessary to make the whole mission work.

The difference in ISPP needed between these scenario is quite massive, direct return will likely require around 8 km/s because the vehicle needs to go fast and have propellant for Earth landing.  Which means 2-3x as much propellant on Mars surface vs the minimal trip to LMO.  If massive ISRU is the long pole then we want an architecture that makes that pole as short as possible, not one that simply assumes that is will be done and makes no attempt to conserve.

Lastly the known 25 mt launch capability of the BFS from Mars means that it can do the future job of surface to orbit tanker by substituting propellant in place of that cargo.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2451 on: 07/19/2016 07:54 PM »
Sending prop from Earth to LMO reduces ISPP by 2x to 3x, but increases launch requirements from Earth by the same 2x to 3x (by my calculations ISPP goes down from 500t to 200t, but Earth launches increase from 4 to 9). And there's a mission-critical Mars orbital rendezvous, and lot more separation events and LEO rendezvous.

So you're retiring some risk from one part of the architecture (ISRU) by imposing more risk on other parts (mission complexity). Granted, LEO rendezvous and Earth launch are at a much higher TRL than ISRU. Until ISRU is proven it certainly makes sense to send as much fuel as possible from Earth, but not in the long term.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2452 on: 07/19/2016 09:30 PM »
Yes this 2nd stage to Mars orbit for refueling is only intended as an early mission enabler when the ISPP surface production is first being established and presents a significant constraint on return capability and an untested element.  All ISPP infrastructure is going to remain in place once deployed so their will be an infrastructure build up that allows a gradual transition to more and more earth return propellant being sourced at Mars.

In the long term I favor rapid mars surface to orbit cycling of the BFS, bringing down cargo from orbit and dropping off a small propellant surplus in a Mars depot each time.  Then when at conjunction the Mars propellant depot is drained to allow a fast return for crew.  The 10:1 cargo:crew ratio means just a small propellant offload on each cargo retrieval (25 mt) can supply a generous return burn.

Note that all of the rendezvous at Mars involve the same vehicle and the same propellant transfer task that is part of the Earth side orbital fueling process which is expected to be as common as launch as most launches are tankers.  Rendezvous is completely routine today and is one of the the safest most reliable parts of space travel, to date I believe the one instance of Soyuz striking Mir is the only anomaly EVER to occur in the history of rendezvous.  I see no reason why we should consider Mars orbital rendezvous even remotely dangerous considering the other far more dangerous elements in the mission like EDL and launch.

Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #2453 on: 07/19/2016 10:12 PM »
It is not "considered dangerous",  it elevates the risks for mission success. It's a statistics and systems analysis thing and prior performance of the general principle of docking is probably only marginally relevant to a specific system, in this context. Three docking events present more risk than two identical docking events, even if you eventually manage to pull both flawlessly, 100 times each.
« Last Edit: 07/19/2016 10:13 PM by mfck »

Offline the_other_Doug

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Re: MCT Speculation and Discussion Thread 4
« Reply #2454 on: 07/19/2016 10:18 PM »
It is not "considered dangerous",  it elevates the risks for mission success. It's a statistics and systems analysis thing and prior performance of the general principle of docking is probably only marginally relevant to a specific system, in this context. Three docking events present more risk than two identical docking events, even if you eventually manage to pull both flawlessly, 100 times each.

Yes, but you have to also evaluate, and weight, each of your events to see if increasing their number actually significantly increases the risk of mission failure (or worse).

For example, if adding a third docking event means you can eliminate an extendable-pipe refueling event that is considered inherently riskier than the docking event, well, you weight each option and determine which one actually reduces overall risk more.

Sometimes, increasing events of low to moderate risk, even a lot, to reduce or eliminate events of much higher risk, or of much greater impact to the mission or crew safety, leads you to increase some events, and increasing risk of those events failing, to gain redundant capability and avoid single-point failures that can ensure not just mission failure, but loss of crew as well...
-Doug  (With my shield, not yet upon it)

Offline mfck

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Re: MCT Speculation and Discussion Thread 4
« Reply #2455 on: 07/19/2016 10:25 PM »
Agreed, it's all trades. Impaler made it sound like there's an irrational phobia of docking events :) I just don't think there is one.

Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2456 on: 07/20/2016 02:31 AM »
mfck you had it right, I am saying their is an irrational fear of rendezvouses in mission planning.  I think this goes back to the Apollo's fight over Lunar Orbital Rendezvous (LOR) architecture which was massively resisted and only done when it became obvious that the NOVA rocket and direct assent was impossible. 

Their was good reason to be worried about LOR durring the design of Apollo because Gemini had not even flown yet, but after the numerous Gemini-Agena missions most of the risk had been retired before Apollo left the pad, other parts of the mission were far more dangerous and likely to kill the crew then a failure to rendezvous.  But people have internalized this notion that each rendezvous event has a high failure probability but this is clearly falsified by experience.

Their have been ZERO failures of American vehicles to dock and were getting more experienced on every ISS mission.  In a fuel transfer scenario the challenges are reduced because you don't have to create an air-tight seal between habitable spaces, you just need to connect hoses which can easily have redundant ports, redundant hoses and be manipulated via EVA if any automation fails.  The true failure risk is only if ALL the contingency options fail and the mission is marooned in LMO unable to make TEI.

NASA assesses the chance of a crewed Orion capsule with a contingency EVA option failing to dock with a lunar assent vehicle at just 1 in 546.  https://www.nasa.gov/pdf/140639main_ESAS_08.pdf (Page 13)

The BFS will have even more resources to bring to the docking, particularly TIME because the crew is already IN the habitat that will return to Earth they have months on the consumables clock, the only timeline is making Earth departure which is flexible due to the ~1 month that Earth and Mars are at conjunction, a few weeks delay in departure would be accommodated by lengthening the time of flight of return.  You can even launch from Mars surface a few weeks early on purpose to provide ample time for rendezvous while still making TEI as scheduled.

So I would estimate a failure chance of the BFS to achieve refueling in Mars orbit at around 1:1000 and not at all something worth of concern.  Nor is it even a development cost driver because BFS and tankers will have propellant transfer as a BASELINE capability.  Their is cost in extra launch in sending a tanker from Earth but that should be weighed against the ISPP leverage ratio and risk of failure.
« Last Edit: 07/20/2016 02:52 AM by Impaler »

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2457 on: 07/20/2016 12:15 PM »
Impaler, I'm probably being dense here, but I fail to see the rationale in doing Mars orbit refueling for the BFS return, versus just fully fueling the BFS on Mars surface for a direct return to Earth.  ISRU fuel production is already needed to leave Mars surface, so why not just fully fuel the beast while it's on the surface and launch back to Earth.  I think delta V wise a direct return is actually somewhat better than first going to Mars orbit then doing a TEI burn, right?

Is the thrust of your position that it is not feasible to have a vehicle do direct return from Mars surface to the Earth?  I thought I'd seen that the delta V for this is only on the order of 5-6 kms, so that should be well within modern capabilities, yes?  If so, this would seem to be the much simpler way to go compared to refueling in Mars orbit, with fuel sent form Earth surface.  (Note, I don't dispute at all your points about the well established feasibility of docking events.) 

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2458 on: 07/20/2016 01:27 PM »
Mars surface to Earth surface is a minimum of about 5.8 km/s, for optimal launch alignments and 8+ month transits. 6.5 to 7.0 km/s is more realistic for real launch sites, average Earth-Mars alignments, and faster transits.

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2459 on: 07/20/2016 01:48 PM »
So even at the upper 7 kms figure, that still implies just a 6.5 mass ratio, right?  I'd think this is quite do-able with present technology.

It's just easier for me to wrap my mind around this, versus the dozens of required BFR launches to get some return fuel prepositioned in LMO.  And until direct return is shown to be unworkable, I'd think it should be the default assumption for return mode (Occam's razor and all that).

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