Source
r/SpaceXLounge•
merlinfire

Explain Falcon Heavy to me, RE: capabilities

So I was looking at FH's stated launch capability to LEO

Per Wikipedia, Saturn V could life ~150t to LEO

Per Wikipedia, FH can lift (in fully expendable launch profile) about 70t to LEO, guessing somewhere between 50-60t in a reusable profile.

Now granted in the intervening ~50 years a lot of advances have come. The FH has better, more efficient engines (I think?), is made of newer and lighter materials, has more miniaturized electronics that weigh less resulting in lighter computers and avionics, etc. So in terms of capability I'm guessing 150 "1969 tons" would be equivalent to somewhat less (130? 120? 100?) of "2018 tons", but surely not 70 tons.

Which leads me to the following conclusion: either Saturn V could have done crewed Mars missions and just didn't, or Falcon Heavy can't.

To me it seems like, in its current form, Falcon heavy would only really be suitable for robotic or pre-supply Mars missions, or at best, a manned flyby.

Am I wrong? I want to be wrong.

Posted Dec 28, 2017, 5:52 PM UTC

451
SwGustav•

it is not meant to be a mars rocket, at most it was supposed to send unmanned dragon 2 probe into direct mars trajectory; it's not even meant to be a saturn 5 tier rocket

there's no such thing as "1969" or "2018" tonnes either

17
SwGustav•

also, saturn 5 gets its payload number by having a mostly fueled third stage after achieving LEO counted; without third stage, payload to LEO is 75 tonnes, and it's pretty impressive that FH comes very close despite being much smaller and not even intending to do that

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merlinfireOP•

that is an excellent point. the FH is about 13% shorter and about 50% lighter while lifting only about 6% less to LEO. definitely more efficient in terms of mass.

6
ICBMFixer•

So what you’re saying is if we go full Kerbal, toss a third stage on the Heavy, then we’re looking at about 150 tonnes to LEO with the FH? Sweet, if I’ve learned anything from KSP, just throw on enough struts and it will hold together.

In all seriousness though, a Raptor based upper stage would probably surpass SLS in just about every way and allow enough payload to have a lunar lander or detachable landing legs with enough fuel on the Dragon 2 to do a lunar landing with Super Draco’s. Not that this is a serious consideration with BFR on the horizon, just pointing out the absurdity of the SLS because you basically already have something just as capable at an order of magnitude cheaper.

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SwGustav•

third stage for FH would actually be amazing, it would be more fit for GTO/GEO launches than LEO though

not 150 tonnes since it would weight significantly less than S-IVB, but closer than current config

3
ICBMFixer•

I agree, and with modern navigation, in orbit docking could make up a huge difference in launch capabilities as well. Basically do two launches with FH, dock and perform TLI for a moon mission with landing. Obviously this would be on par with an Apollo era mission in size and capability, BFR takes things to an all new level. I just laugh when I keep seeing that the SLS is more powerful than the Saturn V, it’s not. Some future and currently undeveloped version is projected to be more powerful, in maybe 15 years or so. There’s not even proof that the SLS will put perform the FH yet. So if Elon truly goes for the jugular and does a Grey Dragon mission to the moon, it could go a long way to killing off future SLS funding.

1
sarahlizzy•

Did a back of the envelope calculation. FH should be able to heft an Apollo CSM and Lunar Module to TLI. You could go to the moon with it.

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SwGustav•

current FH can launch Apollo stack into LEO provided it's properly structurally strong and can handle large payload on top

1
FuckCSS•

there's no such thing as "1969" or "2018" tonnes either

Yeah, progress in spaceflight is not in any way comparable to stuff like electronics. There are pretty hard physical limits for ISP and tank mass ratios and they were mostly reached back in the 60s. It's possible to improve performance but only by margins like ~10%.

One very generic measure of performance would be LEO payload mass / liftoff mass: Falcon Heavy gets ~4.5% while Saturn V gets 4.7%, though that is mostly because of hydrolox upper stages. The Saturn V is also about twice the size of FH (by mass).

What makes SpaceX interesting is that they are pushing down costs and one of the ways they are doing it is by using a single fuel and very similar engine/tanks for both stages. This is cheap but less efficient that hydrolox and has pretty bad performance for high orbits.

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SwGustav•

i think what OP meant is that with modern tech we could send something to space that would weigh more in the past and hence require a bigger launcher. it's true to some extent, but only applies to unmanned sats/probes that have good budget, and usually the gap is filled in with more antennas, instruments or fuel, and sats generally have been getting heavier since 60s

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Shrike99•

The Falcon heavy doesn't have more efficient engines than the Saturn V, overall, because the Saturn V used Hydrolox on it's second and third stages, while Falcon Heavy is all Kerolox. However, the first stage engines on Falcon Heavy are significantly more efficient and higher TWR than the also kerolox first stage engines on Saturn V.

But overall, Rocket technology hasn't improved a lot beyond the 60's, save for the electronics, because they were already pretty close to the upper limit back then.

For example, the NK-33 engine was built in 1968-1969. It has a TWR of 137 versus 180 for Merlin, and a specific impulse of 297/331 versus 282/311 on Merlin. Since TWR gives diminishing returns, while specific impulse doesn't(for exhaust velocities less than intended Delta-V anyway), one could quite rightly argue that the NK-33 is the better engine, despite being nearly 50 years old.

For another example, the super lightweight shuttle external tank flew in 1998, and had a mass fraction of 3.5% The Atlas LV-3B first stage however had a mass fraction of only 2% in 1961. Even the Saturn's S-IVB which first flew in 1966 was only about 5% and that had to support 45 tonnes of payload above it and include mounting for an engine, while the shuttle tank only had to support it's own weight and no engines.

Point is, the nuts and bolts of rockets haven't changed much. The limit is the energy stored in chemical fuel, the technology itself doesn't have much room left to improve, save for things like reuse.

BFR is planned to use the most cutting edge technology yet only has a payload fraction of 5.7% in expendable mode, versus Saturn V's 4.7%, a whopping 21% improvement. But the difference is that it has a reusable payload fraction of 3.4%.

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merlinfireOP•

put that way it seems like the biggest improvements have simply been finding ways to do the same thing cheaper

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Shrike99•

This is correct.

Falcon 9 isn't even very efficient by 1960's standards, let alone modern ones. But it's still the best rocket pound for pound because it's cheaper and simpler than it's peers.

The most common example of this is that Falcon 9 uses practically the same engine for the first and second stages, just with a bigger nozzle on the second stage.

It's more efficient to design a sea level engine and a vacuum engine each optimized for their roles, but having to design and build two completely different engines is a lot more expensive than 1.

Think of it this way, if you needed to do a one-off trip of 500 miles for some reason and didn't have a car and couldn't use any alternative method of transport, would you buy a 2017 Honda Accord, or a second hand 2003 Toyota corolla that wasn't quite as efficient but was only a fraction of the price?

The Falcon 9 is the Toyota corolla of rockets, and that's a good thing.

Now if you plan to use the car multiple times, then fuel efficiency might become more relevant, see BFR. But there are currently no rockets that can fly more than once, with the partial exception of Falcon 9 of course.

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ICBMFixer•

Super cooled fuel actually has been a huge improvement for fuel density, so even with the same fuel, you’re getting more to LEO.

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Shrike99•

It's only really a significant improvement if you can't make the tanks any bigger like on Falcon 9. If you could just build the rocket slightly bigger, all other things being the same, it wouldn't be much different.

Higher fuel density on it's own doesn't represent more than a slight performance gain, unless it's pretty major like LH2 vs RP-1 where the improvement is a factor of 12 or so. But the improvement for supercooled lox is like a factor of 1.1

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ICBMFixer•

But if you can improve by a factor of 1.1, I doesn’t mean a 10% increase in payload though, it can have a much bigger increase due to the 10% more fuel with minimal increase in dry weight. So you can see a 20% improvement, and as the Merlin improved thrust, you can really see how SpaceX put it all together and took a decent rocket and turned it into a beast. It’s one of the big reasons Falcon Heavy kept getting pushed, there was no hurry because the Falcon 9 can just about do what the original Heavy was planned to be capable of. It’s funny to think that a Falcon 9 can almost do what a Delta IV Heavy can to LEO in expendable mode. Granted the IV Heavy blows it away to GTO due to the different fuel, but almost on par at LEO.

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Shrike99•

Building a tank to hold 10% more volume doesn't increase it's mass by 10% thanks to the square cube law. And increasing tank mass doesn't increase rocket dry mass proportionally either, since engines and such are a large fraction of the weight too. A 10% tank volume increase is only liable to increase the rockets dry mass to the order of 1%.

You're also forgetting increased gravity losses from the lower TWR, since improved thrust is a separate matter from increasing propellant by either method. If your TWR is already low, improving density or larger tanks 10% will give significantly less than 10% improvement in payload. Indeed, the only reason Falcon benefits so much is that is TWR is far above optimal.

For Falcon 9, a straight 10% increase in density is about 18% more payload. For a stretched tank variant, that would be about 17% more payload.

The point i was trying to make was not that improved propellant density doesn't give impressive gains, it's that the gains relative to what you would get from instead scaling the tank up is not very impressive.

Indeed, SpaceX did use the stretch route to much the same effect, look at the height difference between F9 V1.0 and F9 FT . They only started looking into supercooled propellants once they reached the point where stretching the tanks further would give aerodynamic problems.

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TheMightyKutKu•

Per Wikipedia, FH can lift (in fully expendable launch profile) about 70t to LEO, guessing somewhere between 50-60t in a reusable profile.

64 t to LEO, between 50-55 (landing the side boosters on barges, which is unlikely to ever happen) and <25 t (landing all three cores at the cap).

So in terms of capability I'm guessing 150 "1969 tons" would be equivalent to somewhat less (130? 120? 100?) of "2018 tons", but surely not 70 tons.

I don't understand this, yes Falcon Heavy is made of lighter materials, alloy that weren't theorised in the 60s, yes Merlin has two time the Thrust to weight ratio of a F1, (individually it is smaller though), but FH is also much smaller (1400 t vs 3000 t) so it's normal it has a lower payload. and a "1969 ton" is the same as a "2018 ton", i'm sure you meant something different but i don't get it.

either Saturn V could have done crewed Mars missions

It kinda could, with some upgrades

or Falcon Heavy can't.

It could have, would have needed the development of plenty of other modules, including a large lander that would be completly unrelated to the red dragon, a single BFR is more powerful and efficient than all of this.

To me it seems like, in its current form, Falcon heavy would only really be suitable for robotic or pre-supply Mars missions, or at best, a manned flyby.

It's simple, Falcon Heavy won't send anyone to mars, and with the red dragon's cancellation, it likely won't send any spacex proprietary probe to mars' surface.

It may be selected for the launch of a future NASA mars lander or rover, as well as orbiters. But it won't send a crewed mission to its surface or even for a flyby (unless someone want to spend several billions for a mission that wouldn't have a lot of scientific interest).

It will however launch people on a fly by trajectory to the moon.

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merlinfireOP•

and a "1969 ton" is the same as a "2018 ton", i'm sure you meant something different but i don't get it.

if a rocket's overall power is measured by its LEO lift capacity, then making the payload for a given weight more capable means that, in effect, the rocket is "lifting more" in terms of capability than the numbers otherwise show

IOW, if it is made of lighter materials, then a 70t leo payload might have the capability of what would have been, in 1969, something even heavier. i would bet money that a theoretical 140t 2018 SpaceX payload with modern materials and electronics would be able to go further and do more than a 1969 150t NASA payload. but the difference is more than just 10 tons

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fishdump•

So your point is more energy dense batteries, stronger lightweight materials, more efficient and powerful electronics, and lighter heat shields the 'effective' weight of a 1969 lunar mission for the same lander/capsule/crew size would be lower correct? The problem is that only about 7% of the total launch weight is structure, 1.4% is payload structure (5.6% was just first and second stage) and the other 93% is fuel. Without finding a way to burn the fuel more efficiently even making the 1.4% into weightless unobtanium wouldn't be able to make much difference in total payload capacity.

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ICBMFixer•

But that’s the Saturn V numbers, I don’t have the numbers on the Heavy, but it should actually be better than the Falcon 9 FT, which I do have numbers for. On fully expendable mode, it can put up a max of 22,800 kg to LEO and it weighs in at 549,054 kg fully fueled. That works out to a 4.15% vs 1.4%, so vastly improved. More efficient engines, super cooled fuel and lighter materials have made up most of these gains.

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merlinfireOP•

That's why I said I could believe maybe a 10 ton savings but not a 70 ton savings. As you say, most of that is going to be fuel (though I didn't know the percentages). That inability to find further "savings" is the crux of my argument - either Saturn V could have done Mars missions (in terms of life capability) and just didn't due to budget constraints/political winds or Falcon Heavy really couldn't (as its real lift capability is significantly lower) or possibly even both.

the big difference is that falcon heavy costs about 1/10th of what a saturn v launch would cost in inflation-adjusted 1969 dollars so, that's a big deal

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latenightcessna•

Yeah makes sense now but you really need to work on your explanation skills.

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merlinfireOP•

I thought it was pretty clear, since the initial statement had some preceding explanation, but comprehension and explanation are a two-way street

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latenightcessna•

I thought it was pretty clear (...)

It wasn’t.

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cjc4096•

Was clear to me. But my explanations aren't clear to most people.

Interesting post.

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paul_wi11iams•

IIUC the big things about FH are:

  1. access to more orbits, getting S2 into those orbits with more remaining fuel to play with, so late relights, circularization and other manouvers that no longer have to be done by the satellite.
  2. avoids most S1 expendable mode launches.
  3. alongside F9, FH gives a better progressivity of mass-to-orbit options. This should argue well against Ariane 5 which lacks a light payload option. More ability to avoid rideshares between different customers.

If anyone can answer these follow-on questions:

  • Is a triple RTLS (still) on the cards ? (third landing pad)
  • Could a triple ASDS be envisaged with three barges? (Two barges or one barge would be very exciting but maybe not a good idea, well...)
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[deleted]•

[deleted]

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LAMapNerd•

Actually, that plan of Von Braun's was concocted in 1948, based on 1948 tech, then written up and first published in German in 1952; in translation to English in 1953.

Reading the introduction to the 1962 edition, (which you linked), is fascinating as Von Braun talks about all the improvements in tech between 1948 when he first created the plan, and 1962, when we were actually sending men on day-long (!) missions to orbit.

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andygates2323•

Heavy isn't a people-to-Mars rocket. Saturn could have been with the scrapped upgrades. But the big difference apart from gross mass is cost efficiency; Heavy is about an order of magnitude cheaper to launch than Saturn, so with a huge mission budget we could do multiple launches.

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merlinfireOP•

excellent point. i mean, if you can launch 10 heavies for one saturn for the same money, the 1 to 1 comparison sort of starts to break down.

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KSPSpaceWhaleRescue•

And that's not including when they start pushing for 10 reuses and succeed in fairing recovery.

1
Glaucus_Blue•

On a similar note could the falcon heavy launch a Hubble sized telescope to the outer planets type of orbit.

And any idea if it would be capable of sending a small probe in combination with light sail or ion engine to a reasonable time to say alpha alpha centauri, suppose the biggest part of the probe would be communication dish and power supply. Or what sort of rocket we would need for such a system. Or would we still need a rocket many times larger.

Am I right in thinking heavy is capable of Europa clipper mission?

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merlinfireOP•

in a non-reusable configuration, FH can send ~ 13mt to Mars transfer orbit IIRC. Mars capture would be about another 900m/s so let's say ~11-12mt to capture orbit. Someone who knows the math can correct me or give exact amounts.

Hubble weighs about 11mt, so technically it ships. Though given that an orbital planetary observing satellite could be much lighter because it is looking at something much closer, putting observation satellites in low orbit of Mars would be a piece of cake.

In fact, if they wanted to basically land a small rover, they could probably get to the surface of Deimos or Mars. I don't know about Europa but it seems like it could work.

The problem is that this is all based on a non-reusable launch profile. What makes SpaceX so cheap relative to others is the ability to reuse and refurbish. Based on what I can find, its re-usable "3 rockets land" configuration puts about 1/3 of the payload to GTO.

That said, with the Tesla roadster weighing in at about 2900lbs and plus the extra cameras and the mounting hardware and such the payload for the test flight was minimum 3500lbs. cut that payload in half and it might have been able to do a Mars capture with full reusability. I am not sure about the difficulty of adjusting the launch and throttling profile based on a different payload mass, but I'm sure there's something to that. We need to figure out what the maximum theoretical payload to Mars orbit in a fully-reusable configuration, and then start launching shit like every other month.

2
fishdump•

For reference the actual lunar hardware was 45.3mt wet, with the third stage was just for the dV. I don't think 20mt will be enough for that but I'd love to be wrong.

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merlinfireOP•

i'm with you there. the good news is (I think) the things they learn from FH will help towards development of the BFR, which will finally open up new possibilities we never had before instead of trying to mimic past successes

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fishdump•

I had a chance to sit back down and look over the numbers some more. 37.3mt looks to be the lightest you could do the manned section of the vehicle assuming a 50% cut in dry mass. However in more practical terms, the ACM was 5,560kg and D2 comes in at 6,400kg (subject to changes until flying). So keeping costs low you wouldn't be saving much weight, but you might be able to eek out a limited mission. Fundamentally though Falcon Heavy was a stopgap measure to supplement Falcon 9 for some of the heavier payloads currently seeking launches. It has little to no bearing on the development of the BFR beyond really driving home the idea to do one big rocket and to avoid multistick. Personally I don't think we will see Falcon Heavy fly more than a handful of times and looking back most people will probably consider it a waste of resources, but that's the benefit of 2020 vision.

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merlinfireOP•

I think you're right. It does more for the satellite and LEO market but doesn't really break any new ground for scientific ventures except in the respect that it can take more than a Delta 4 to LEO for less money....

Maybe some probes or robotic missions could be sent up on SpaceX rockets?

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fishdump•

Maybe but I think it's biggest selling point will be heavy lifts for large constellations, fuel depots, or space stations. The low isp upper stage loses a lot of payload compared to Delta 4H. Robotic missions would be nice, but so far NASA hasn't planned for any that I know of. The 2020 rover (and MSL) is/was launching on Atlas variants which put as much into LEO as Falcon Heavy does to Mars.

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HeadsUpHigh•

beyond really driving home the idea to do one big rocket and to avoid multistick.

I think at some point it might make sense to make a 3 or 4 stick modified bfr specifically for Mars, but with specialized side boosters instead of just 3 bfrs strapped together. That would probably be way ahead in the 30s or so though.

1
fishdump•

Why not just make a bigger one? Surface area to volume is a powerful tool and why cargo ships keep getting bigger. Doesn't make a lot of sense to redesign the BFR just to allow for side boosters when the same effort could be used to increase diameter to 12 or 15m.

1