NASASpaceFlight.com Forum
General Discussion => Q&A Section => Topic started by: DanClemmensen on 06/03/2022 04:41 pm
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Consider a scenario where SpaceX gets the booster (SH) and its recovery system working well, but a rapidly reusable second stage is delayed.
Is it feasible to build a very inexpensive expendable second stage to use instead? SH is supposed to be very cheap to operate even compared to Falcon 9, so a super-cheap expendable SS would allow SpaceX to continue with the F9 economic approach. In particular for Starship to work at all, the SH is RTLS so the recovery fleet is not needed.
I envision a range of stubby SSs with only enough Raptors and propellant to launch a specific max payload mass.
Questions:
--Does this work at all?
--how many SS raptors are needed to launch an F9-equivalent payload?
--how tall is this SS?
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A step sideways. Unnecessary.
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--how many SS raptors are needed to launch an F9-equivalent payload?
That doesn't make any sense. Just better to continue F9.
There is no need to change much if Starship is used as expendable. Just delete flaps and maybe simplify plumbing. It just launches heavier payloads than FH. And the Starship can be used for escape missions.
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--how many SS raptors are needed to launch an F9-equivalent payload?
That doesn't make any sense. Just better to continue F9.
There is no need to change much if Starship is used as expendable. Just delete flaps and maybe simplify plumbing. It just launches heavier payloads than FH. And the Starship can be used for escape missions.
It only makes sense as a way to gain experience with Starship. But yes, if the cheapest feasible expendable SS can carry larger payloads than an F9, then it can be used for a larger range of payloads in addition to F9 replacement. The real question: does this configuration have a lower absolute cost per launch than F9? RTLS is cheaper than a barge landing and SH turnaround is supposed to be cheaper than F9 turnaround, so can the cheapest SS get down to near the cost of an F9 second stage?
Certainly for Starlink (about half the falcon 9 payloads) an appropriately-sized expendable SS will make a lot of sense if the reusable SS is delayed.
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RTLS is cheaper than a barge landing
Is it? and by what measure?
A Falcon RTLS attempt requires the evacuation of several thousand personnel from the CCSFS industrial zone for from 75 minutes to six hours. There are reports that this has a cost in the high six figures. (There have also been reports that Space Force is working to reduce the evacuation area and possible to move the landing zone to somewhere more remote so this might be ameliorable.)
Any efforts to reduce cost should not just force the government to pay.
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RTLS is cheaper than a barge landing
Is it? and by what measure?
A Falcon RTLS attempt requires the evacuation of several thousand personnel from the CCSFS industrial zone for from 75 minutes to six hours. There are reports that this has a cost in the high six figures. (There have also been reports that Space Force is working to reduce the evacuation area and possible to move the landing zone to somewhere more remote so this might be ameliorable.)
Any efforts to reduce cost should not just force the government to pay.
Oops, I was unaware of this and I had not thought about it. However, This is easy to mitigate. Just put the barge catcher platform about 3 miles out at sea, so the travel time is shortened dramatically but no evacuation is needed.
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Consider a scenario where SpaceX gets the booster (SH) and its recovery system working well, but a rapidly reusable second stage is delayed.
Is it feasible to build a very inexpensive expendable second stage to use instead? SH is supposed to be very cheap to operate even compared to Falcon 9, so a super-cheap expendable SS would allow SpaceX to continue with the F9 economic approach. In particular for Starship to work at all, the SH is RTLS so the recovery fleet is not needed.
This might be a good approach if the recoverable Starship was found to be unfeasible but if it's only delayed, then the thing causing the delay is almost certainly the lack of sufficient opportunities to experiment with recovery.
Starship R&D takes what Starship R&D takes. Gathering recovery data on every launch is part of that R&D effort. SpaceX will need to price its launches accordingly.
To a first-order approximation, a Starship is a stainless steel tube, six Raptors, some funny aerosurfaces, and a heat shield. It's hard to imagine the Raptors being less than 25%-50% of the manufacturing cost. If they cost less than $1M each, that puts the maximum manufacturing cost of the Starship at less than $24M.
Similarly, it's hard to imagine Raptors being less than 50%-75% of the manufacturing cost of a SuperHeavy, and here we can reasonably expect recovery to work in fairly short order. Even if SH is only 10x reusable to begin with, that amortizes each launch at less than $7M.
Add in $5M/launch in ops costs, and you're at $36M in cost. Figure 50% gross margin during the R&D phase and you're at a retail price of $72M. That's almost at the F9 price point even without Starship reusability.
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Consider a scenario where SpaceX gets the booster (SH) and its recovery system working well, but a rapidly reusable second stage is delayed.
Is it feasible to build a very inexpensive expendable second stage to use instead? SH is supposed to be very cheap to operate even compared to Falcon 9, so a super-cheap expendable SS would allow SpaceX to continue with the F9 economic approach. In particular for Starship to work at all, the SH is RTLS so the recovery fleet is not needed.
This might be a good approach if the recoverable Starship was found to be unfeasible but if it's only delayed, then the thing causing the delay is almost certainly the lack of sufficient opportunities to experiment with recovery.
Starship R&D takes what Starship R&D takes. Gathering recovery data on every launch is part of that R&D effort. SpaceX will need to price its launches accordingly.
To a first-order approximation, a Starship is a stainless steel tube, six Raptors, some funny aerosurfaces, and a heat shield. It's hard to imagine the Raptors being less than 25%-50% of the manufacturing cost. If they cost less than $1M each, that puts the maximum manufacturing cost of the Starship at less than $24M.
Similarly, it's hard to imagine Raptors being less than 50%-75% of the manufacturing cost of a SuperHeavy, and here we can reasonably expect recovery to work in fairly short order. Even if SH is only 10x reusable to begin with, that amortizes each launch at less than $7M.
Add in $5M/launch in ops costs, and you're at $36M in cost. Figure 50% gross margin during the R&D phase and you're at a retail price of $72M. That's almost at the F9 price point even without Starship reusability.
Which translates to significantly lower cost per kg to orbit of course.
But most critically, it still maintains two of the REAL paradigm shifting benefits of the thing:
1) Ability to do on orbit refueling. Although expensive, it’d still be feasible.
2) Utterly massive payload lift capacity & fairing volume, which I think even with rapid SS reusability is the most paradigm shifting feature of the entire platform.
For the most part, launch costs themselves aren’t the primary cost drivers of huge government & private sector mega projects. Rather, it’s the human labor costs required to design & custom build bespoke payloads out of exotic ultra light materials & them origami the crap out of them to fit into a traditional sized fairing. Even if SS exists solely as a one & done hardware item, the lift capacity, fairing volume, and on orbit refueling capability is going to enable a HUGE amount of tonnage to be placed in deep space as ultra capable probes/landers, on planetary/satellite bodies to support human exploration, at key Lagrange points to deliver far bigger and/or far cheaper space telescopes compare to JWST, etc.
As someone said in the “Why are people fans of StarShip & Not SLS?” Q&A thread:
Even if SS/SH can only deliver on a fraction of its theoretical capabilities, it’ll STILL represent an improvement over ever we currently have by at least an order of magnitude.
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If the cheap expendable upper stage is only needed for a short period of time. It might be attractive to rework the sea level Merlins from retired Falcon 9s to a more vacuum optimized iteration as the stage engines. Think a trio of modified Merlins (about 2700 kN total) in a shorten expendable Starship variant should be adequate for most missions. Just need to adjust the tankage for a different propellants ratio and make bigger payload fairings that can landed in the Ocean.
The Merlin engine rework is mostly adding an exhaust nozzle extension to increase the expansion ratio. Probably with the ratio somewhere between the current sea level and vacuum variants of the Merlin.
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If the cheap expendable upper stage is only needed for a short period of time. It might be attractive to rework the sea level Merlins from retired Falcon 9s to a more vacuum optimized iteration as the stage engines. Think a trio of modified Merlins (about 2700 kN total) in a shorten expendable Starship variant should be adequate for most missions. Just need to adjust the tankage for a different propellants ratio and make bigger payload fairings that can landed in the Ocean.
The Merlin engine rework is mostly adding an exhaust nozzle extension to increase the expansion ratio. Probably with the ratio somewhere between the current sea level and vacuum variants of the Merlin.
Pretty much every proposal for a product variant that I've seen during my professional life that got into real trouble started with the word "just". <shudder>
Adjusting the tankage for a different propellant ratio (and propellant type, don't forget) changes centers, moments, resonances, plumbing, cabling, baffle placement and number, temperatures, pressures, control software, guidance software, GSE, processes, procedures and on and on and on. All this needs to be designed, reviewed, built, tested and certified. People need to perform these jobs so there will be transfers, relocations, hiring and other personnel issues. Then this whole thing will have to be managed, including being documented to various government agencies' satisfaction (IRS, FCC, FAA, etc.). To paraphrase Yoda, "There is no 'just'".
Adding all this to an engine type change, as well as a fairing change, makes me agree with those who question the real utility of this over using an alternate existing operational rocket while pushing through to find and fix the problem.
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Starship R&D takes what Starship R&D takes. Gathering recovery data on every launch is part of that R&D effort. SpaceX will need to price its launches accordingly.
To a first-order approximation, a Starship is a stainless steel tube, six Raptors, some funny aerosurfaces, and a heat shield. It's hard to imagine the Raptors being less than 25%-50% of the manufacturing cost. If they cost less than $1M each, that puts the maximum manufacturing cost of the Starship at less than $24M.
This is a crazy way to estimate costs. It's very easy to imagine the thermal protection system being 90% of the cost. At the very least there is no relationship between the cost of thermal protection and the cost of a motor. Figuring out how to build one cheaply does not help with the other.
Silica tiles may turn out to be a dead end and they have to use barelyobtainium or they may need unobtainium, which they can't have. As you say, the R&D effort costs what it cost, and, as I say, it gives you what it gives you, not always what you started looking for or need. Then SpaceX has to decide if they can live with it.
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Starship R&D takes what Starship R&D takes. Gathering recovery data on every launch is part of that R&D effort. SpaceX will need to price its launches accordingly.
To a first-order approximation, a Starship is a stainless steel tube, six Raptors, some funny aerosurfaces, and a heat shield. It's hard to imagine the Raptors being less than 25%-50% of the manufacturing cost. If they cost less than $1M each, that puts the maximum manufacturing cost of the Starship at less than $24M.
This is a crazy way to estimate costs. It's very easy to imagine the thermal protection system being 90% of the cost. At the very least there is no relationship between the cost of thermal protection and the cost of a motor. Figuring out how to build one cheaply does not help with the other.
Silica tiles may turn out to be a dead end and they have to use barelyobtainium or they may need unobtainium, which they can't have. As you say, the R&D effort costs what it cost, and, as I say, it gives you what it gives you, not always what you started looking for or need. Then SpaceX has to decide if they can live with it.
It's usually a pretty good way to estimate costs for expendable stages--and even reusable first stages. My understanding is that engines comprise 50%-75% of the costs for most types of rocket stages.
TPS is indeed a wildcard here, which is one of the reasons I assumed that the Starship's six engines comprised only 25% of the cost instead of 50%-75%. But the premise of the thread is that Starship reuse is either extensively delayed or downright unfeasible. In that environment, how much of the TPS cost do you want to allocate to marginal cost vs. R&D?
If things get really bad and SpaceX can't think of trying anything new on each launch, SpaceX will start launching Starlinks and 3rd-party cargoes on something that's a lot closer to lunar Starship than it is to vanilla Starship, in which case my $24M/Starship manufacturing cost is probably too high. Meanwhile, you've got TPS and elonerons that will only get added when there's something to learn. I expect (hope) that they're added for every flight so that there can be an EDL experiment with every Starship. But that's pretty much irrelevant to how SpaceX would price those missions.
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Expendable SS will only need VAC Raptors but how many?.
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Expendable SS will only need VAC Raptors but how many?.
Sent from my SM-A528B using Tapatalk
My initial question assumed a few variants, depending on payload mass. From 1 to three. beyond that, use an expendable version of a "real" Starship. So, my question now becomes what is the payload mass to LEO for each of these variants? can the single-Rvac variant get to LEO at all? It's basically a legless starhopper.
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.....
Pretty much every proposal for a product variant that I've seen during my professional life that got into real trouble started with the word "just". <shudder>
Adjusting the tankage for a different propellant ratio (and propellant type, don't forget) changes centers, moments, resonances, plumbing, cabling, baffle placement and number, temperatures, pressures, control software, guidance software, GSE, processes, procedures and on and on and on. All this needs to be designed, reviewed, built, tested and certified. People need to perform these jobs so there will be transfers, relocations, hiring and other personnel issues. Then this whole thing will have to be managed, including being documented to various government agencies' satisfaction (IRS, FCC, FAA, etc.). To paraphrase Yoda, "There is no 'just'".
Adding all this to an engine type change, as well as a fairing change, makes me agree with those who question the real utility of this over using an alternate existing operational rocket while pushing through to find and fix the problem.
What you posted as the work required for a drastic upper stage changeover is similar to what SpaceX does with each Starship iteration. It seems to me.
Conventional payload fairing for expendable Starship variants was expected as an option. Musk himself mention this for the Starkicker variant in a tweet.
Engine change is from the Raptor Vacuum with low or no flight heritage to the Merlin Vacuum with over 150 flights with no mishaps.
SpaceX will have to consider some option to replace the Starship for deploying Starlink v2 satcoms while waiting for the Starship to get fixed. There is no existing or near future alternate launcher for Starlink v2 deployment.
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My initial question assumed a few variants, depending on payload mass. From 1 to three. beyond that, use an expendable version of a "real" Starship. So, my question now becomes what is the payload mass to LEO for each of these variants? can the single-Rvac variant get to LEO at all? It's basically a legless starhopper.
First, I can't see using Merlins anywhere near a Starship launch pad; it's too big a GSE change.
Second, every time you reduce the gross mass of the second stage on a SuperHeavy, you have to maintain the same approximate MECO speed for the SH. Otherwise, you have to redesign its TPS, which pretty much defeats the purpose of what you're trying to do here. You can do that by de-engining it and only partially filling the tanks, but then you're on the path to producing an F9 with a particularly terrible structural mass fraction.
That might be worth doing if Raptors had some massive life cycle advantage over Merlins, but I suspect that the advantage isn't massive enough simply to use F9's and have done with it.
Stripping off the TPS and elonerons from Starship is a slam-dunk if they prove to be useless or if the experimentation rate is much lower than the launch rate. You might be able to go from 3 RVacs and 3 RSLs to 3RVacs and 1 RSL, but you still need gimbaling. But dramatically downsizing the second stage simply doesn't work without corresponding downsizing of the SH, and I just don't see that happening.
A Starship stage is inherently more expensive than an F9S2, because it has at least 4-5x the engine cost built into it. But it will also send 5x the payload to LEO, so it's still a perfectly competitive system. Long before we see massive single payloads for the Starship payload bay, we'll see lots of copies of vanilla payloads in a single launch--whether Starship is reusable or not. If SpaceX is smart, they're thinking really hard about making payload integration as quick and cheap as possible, to encourage the proliferation of big constellations. An expendable Starship works just fine in that kind of market.
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My initial question assumed a few variants, depending on payload mass. From 1 to three. beyond that, use an expendable version of a "real" Starship. So, my question now becomes what is the payload mass to LEO for each of these variants? can the single-Rvac variant get to LEO at all? It's basically a legless starhopper.
... every time you reduce the gross mass of the second stage on a SuperHeavy, you have to maintain the same approximate MECO speed for the SH. Otherwise, you have to redesign its TPS, which pretty much defeats the purpose of what you're trying to do here. You can do that by de-engining it and only partially filling the tanks, but then you're on the path to producing an F9 with a particularly terrible structural mass fraction.
Is SH already near its MECO speed limit?
OK, that means the "stubby Starship" is a waste of time and resources, so The "cheapest feasible expendable Starship" is a full-sized Starship carrying a full load. SpaceX can launch full loads of Starlink V2.0 on Starship prototypes that can also do EDL testing and if they don't have a new variant to test they can use a slightly cheaper expendable, but they cannot completely retire Falcon 9 until they really do have a reusable SS.
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My initial question assumed a few variants, depending on payload mass. From 1 to three. beyond that, use an expendable version of a "real" Starship. So, my question now becomes what is the payload mass to LEO for each of these variants? can the single-Rvac variant get to LEO at all? It's basically a legless starhopper.
... every time you reduce the gross mass of the second stage on a SuperHeavy, you have to maintain the same approximate MECO speed for the SH. Otherwise, you have to redesign its TPS, which pretty much defeats the purpose of what you're trying to do here. You can do that by de-engining it and only partially filling the tanks, but then you're on the path to producing an F9 with a particularly terrible structural mass fraction.
Is SH already near its MECO speed limit?
OK, that means the "stubby Starship" is a waste of time and resources, so The "cheapest feasible expendable Starship" is a full-sized Starship carrying a full load. SpaceX can launch full loads of Starlink V2.0 on Starship prototypes that can also do EDL testing and if they don't have a new variant to test they can use a slightly cheaper expendable, but they cannot completely retire Falcon 9 until they really do have a reusable SS.
I think a better way to put it is that SuperHeavy requires a second stage with a gross mass that's somewhere between 1300t and 1500t. That's a payload of somewhere between 0t and 200t.
But maybe I was a bit too hasty:
There are ways that you can increase SuperHeavy's MECO speed without changing its thermal characteristics, but they all require a fairly different conops. Right now, the plan of record is RTLS with no entry burn. SpaceX could go to downrange recovery and an entry burn¹ and somewhat increase the MECO speed. But you have to scrub that entry speed down to what SH was designed for, which becomes increasingly expensive.
I'd guess that a SuperHeavy ASDS isn't exactly the top priority right now, but we're hypothesizing a world where a reusable Starship isn't in the cards anytime soon, so accelerating development of a suitable ASDS is a possibility.
My model for F9 RTLS and ASDS is very bad, and based only on a few early data points from the launch videos. That said, if you plug a SuperHeavy into that with an FHE-sized gross mass (115t S2 + 63t payload), you can get boostback and an entry burn to close. But this obviously overlooks the fact that F9 was designed around an entry burn and SH isn't. That's a whopping big difference.
To do this right, you need a decent model. It would have to account for:
1) You need a guess at the design SH entry speed. (F9 is about 1650m/s these days.)
2) A different T/W if you put a smaller stage on top.
3) A deeper/wider throttle bucket to keep max q from getting out of hand.
4) Gravity loss as a function of that T/W and throttle bucket.
5) And, last but hardly least, good estimates of downrange velocity at MECO and vertical velocity at the point where the SH passed through entry interface (i.e., the point pretty close to where you need to do an entry burn to get the speed down to the point where the SH would hit the atmosphere at no higher speed than its current design calls for).
As a first approximation, if you can kludge together a gravity turn that results in a 0-10º horizon angle at MECO, you'd be in the ballpark. But this seems beyond the level of a simple spreadsheet; you'd have to write some code. It's not horrible code, but it'll require that you sit down and wring your hands about the design for a bit.
That should give you a range of gross masses that can get to the proper horizon angle and altitude while still allowing the SH to be recovered, each with a downrange staging speed. From there, you can pick a few points on the curve that look like sweet spots.
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¹This all assumes that Raptors can do three restarts in rapid succession. I don't see why this wouldn't be true, but the current conops only requires two.