Author Topic: Falcon-family Successor (Speculation)  (Read 1859 times)

Offline HarbingerDawn

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Falcon-family Successor (Speculation)
« on: 12/24/2015 10:10 PM »
It seems inevitable to me that there will be a successor to Falcon 9/Falcon Heavy, probably in the mid-2020s. SpaceX will need a fully reusable medium-heavy lift launcher, and Falcon won't be able to fulfill that role.

For a long time now I've had an idea in my head for what a successor vehicle to Falcon might be like, something that SpaceX might actually design. I recently gave form to this idea as a rough 3D model, as well as vehicle specifications.

The overall vehicle (picture attached) is a two-stage methalox fully reusable VTVL launch system. It is based on the existing Falcon 9 as much as possible to minimize development time, cost, and risk.

The first stage is outwardly identical to Falcon 9's, the only change being to the propellant tanks to accommodate methane instead of kerosene. I used 9 engines on the model, but 5 or 7 engines are also possibilities, depending on the capabilities of the engine (thrust, throttle range). I assumed all engines to be derived from Raptor, and thus they have the same Isp.

The second stage has the same base diameter as Falcon, and same primary propellant volume, but it flares out to a width of 5.5 meters at the top, where a heat shield is located. Also located in and around the top are Draco thrusters and hypergolic propellant tanks (neither shown). Farther down along the sides are four equally-spaced SuperDraco pods, each with two engines (identical to Crew Dragon). These are used for landing the second stage after reentry. They could possibly double as retro engines for the LV during launch abort, to aid spacecraft separation, but this is not their purpose. The stage is powered by a single vacuum-specialized engine.

The payload fairing is 5.5 meters in diameter, and overall is approximately the same size and mass as Falcon's PLF.

Here are some detailed vehicle specifications:

Stage 1
CH4 vol.: 161,578 L
O2 vol.: 227,422 L
Propellant mass: 327,775 kg
Mass at staging: 74,766 kg
Dry mass: 25,600 kg (same as F9S1 mass)
Wet mass: 353,375 kg

Stage 2
CH4 vol.: 37,879 L
O2 vol.: 53,314 L
Main prop. mass: 76,840 kg
Landing prop. mass: 1,388 kg
Mass at payload separation: 9,672 kg
Mass at reentry: 9,288 kg
Dry mass: 7,900 kg (F9S2 mass + 4,000 kg for added structure and reusability hardware)

Gross liftoff weight: 438,115 kg
Total vehicle mass at first stage separation: 160,894 kg
Engine Isp (SL/Vac): 321/363 s
Payload to LEO (fully reusable config): ~8-9,000 kg (this was a VERY rough estimate on my part, and is probably too low, I would love for someone to conduct an analysis and get a more robust answer)

All masses given above are sans payload and fairing. Assumes 15% propellant reserve for first stage and 0.5% reserve for second stage (actual value for first stage may be considerably lower, I would love for someone to analyze that).

Final note: I know that SpaceX has said nothing of a Falcon successor, and I imagine that they won't be working on such a thing for another 5-10 years, so this is obviously speculation. However, speculation can sometimes be useful, as food for thought if nothing else.

I would love to hear what input everyone has regarding this design, as well as more detailed analysis than I was able to make.
"I much prefer the sharpest criticism of a single intelligent man to the thoughtless approval of the masses."
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Offline Dante80

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Re: Falcon-family Successor (Speculation)
« Reply #1 on: 12/24/2015 10:24 PM »
Thanks a lot for your work. There have been a couple of threads regarding possible F9 derivatives/successors.

I personally don't think that SpaceX is going to retire/replace F9/FH anytime soon, they are probably going to be working on them for decades (iterating for cost reduction, re-usability and reliability). This is of course assuming that the BFR remains a Mars only relevant architecture..

Regarding your design now. Yes, it is fully re-usable, but what do you think about the performance? You would have problems with getting a medium/heavy sized commsat to GTO, and thats where most of the money is in the business currently.
« Last Edit: 12/24/2015 10:25 PM by Dante80 »

Offline HarbingerDawn

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Re: Falcon-family Successor (Speculation)
« Reply #2 on: 12/24/2015 10:33 PM »
Thanks a lot for your work. There have been a couple of threads regarding possible F9 derivatives/successors.

I personally don't think that SpaceX is going to retire/replace F9/FH anytime soon, they are probably going to be working on them for decades (iterating for cost reduction, re-usability and reliability). This is of course assuming that the BFR remains a Mars only relevant architecture..

Regarding your design now. Yes, it is fully re-usable, but what do you think about the performance? You would have problems with getting a medium/heavy sized commsat to GTO, and thats where most of the money is in the business currently.

SpaceX has said that they want to have a fully reusable LV, that is their goal. It is becoming increasingly apparent that they will not achieve second stage reuseability with Falcon 9, it's pretty much a shut case at this point. That, combined with the advances in engine technology they're making and the improved performance attainable with methane, suggests that they will need a successor eventually. Keep in mind that this won't be "soon". They probably won't even announce such a thing until after BFR is flying regularly.

I'm not so sure about it not being able to fly a good-sized bird to GTO. As I said in my post, my estimates of payload capacity are very rough and probably conservative. Most likely its capability is in the same class as F9. I would like for someone to confirm that however.
"I much prefer the sharpest criticism of a single intelligent man to the thoughtless approval of the masses."
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Offline nadreck

Re: Falcon-family Successor (Speculation)
« Reply #3 on: 12/25/2015 12:13 AM »
IF there were going to be an intermediate family between F9/FH and BFR/MCT then the following stepwise development and dimensions might make sense:

Once FH is fully operational and once the Raptor is tested (and priority would have to be put on the vacuum Raptor model first) then create a 5.5 meter diameter, Raptor powered FH upper stage that masses about 2.2 times the existing upper stage.  This could develop along two lines: reuse line for LEO/GTO work, and a leaner one with no provision for re-entry that has approximately the same mass fractions as the existing S2 but with 10% higher ISP and, given its 2.2 times mass, it can readily put some heavy masses into GSO or LTO/MTO (>20t).

The next priority would be to build a matching 5.5 meter first stage that was Raptor powered once the upper stage was in service. The most important goals on this stage would be to take everything learned to date from the Falcon S1 cores and make the new family booster core truly rapidly reusable from the start.

HOWEVER the reasons why none of this will be done is that the FH/F9 will meet all commercial and government requirements for the next 10 - 15 years and Elon will have the BFR/MCT by the end of that, and it will incorporate all the technologies needed and provide the same economics of the intermediate vehicle. I am sure there will be a need for that class of vehicle once Elon starts using it for Mars exploitation, but I am not sure that there is really before then and I am not sure that there is any real market for SLS.
It is all well and good to quote those things that made it past your confirmation bias that other people wrote, but this is a discussion board damnit! Let us know what you think! And why!

Offline deltaV

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Re: Falcon-family Successor (Speculation)
« Reply #4 on: 12/25/2015 04:01 AM »
It's very hard to reuse an upper stage during GTO missions without hurting payload too much to be worth it. A reusable upper stage would therefore only help Falcon's LEO missions, and even there the payload penalty of reusability would probably force many missions to use Falcon Heavy, which may not save much money compared to an expendable second stage on Falcon 9. Making the upper stage reusable would also annoy the customers that want SpaceX to settle on a design already so they can work out the bugs and get it reliable. Basically the current Falcon upper stage is good enough to accomplish SpaceX's goal for it, namely pay the bills while they develop Mars stuff. I think they'll be too busy designing BFR, MCT and Mars surface gear for the next 20-30 years to have engineering resources to spare on a new Falcon upper stage with a mediocre expected ROI.

Edit: it looks like I posted this in the wrong thread. But my opinion on a totally new Falcon class vehicle is the same as stated above for a new upper stage: not worth the trouble.
« Last Edit: 12/25/2015 06:13 AM by deltaV »

Online Lars-J

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Falcon-family Successor (Speculation)
« Reply #5 on: 12/25/2015 06:04 AM »
If you want something fully reusable that is a follow-up to F9, you're going to have to make it bigger. It would be more economical to have 9 raptors on 6-7m diameter stage, a wider F9 stage 1 that is powered by 9 Raptors. Make a fully reusable upper stage (same diameter) with a cargo bay, that stage can place a payload in GTO, and re-enter after one orbit. Yes it will be massive compared to current GTO launchers, but it could have sufficient margins to enable rapid reuse. And thus it would be cheaper.
(I attached an image showing it that I made a while back)
« Last Edit: 12/25/2015 06:14 AM by Lars-J »

Offline guckyfan

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Re: Falcon-family Successor (Speculation)
« Reply #6 on: 12/25/2015 07:45 AM »
If you want something fully reusable that is a follow-up to F9, you're going to have to make it bigger. It would be more economical to have 9 raptors on 6-7m diameter stage, a wider F9 stage 1 that is powered by 9 Raptors. Make a fully reusable upper stage (same diameter) with a cargo bay, that stage can place a payload in GTO, and re-enter after one orbit. Yes it will be massive compared to current GTO launchers, but it could have sufficient margins to enable rapid reuse. And thus it would be cheaper.

I see it the same way, especially the larger diameter. Except that I believe 7 Raptor on the first stage would be enough and I would love to see a configuration with 7 equidistant engines. Good for servicing the engines.

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