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#20 by JEF_300 on 05 Mar, 2021 18:34
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4 engines and a landing engine (based on a beefed up Rutherford?) wouldn’t be terrible.
I think if they use four engines they'll use just one of them gimbaled way the hell over and land at an angle.
Or they'll just use all four and throttle really deeply. This isn't unprecedented; DC-X did it. -
#21 by Lars-J on 05 Mar, 2021 18:34
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I haven't done an real calculations, but from a gut level it seems like Neutron would need 5-6 Merlin 1D's - if using Merlin's
- to get the capability they announced.
So *if* they are looking at 3 or 4 engines, it would need to be significantly more powerful, or significantly more efficient, or both.4 engines and a landing engine (based on a beefed up Rutherford?) wouldn’t be terrible.
I think if they use four engines they'll use just one of them gimbaled way the hell over and land at an angle.
That would have to be a HUGE gimbal angle to accomplish that. -
#22 by JEF_300 on 05 Mar, 2021 18:51
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I haven't done an real calculations, but from a gut level it seems like Neutron would need 5-6 Merlin 1D's - if using Merlin's
- to get the capability they announced.
So this is a good opportunity to have this conversation:
How much thrust does an 8 ton LV need on it's first stage? Let's look at some examples
Soyuz FG: I believe this is the Soyuz variant currently used for crew rotation. It's a ~7t to LEO rocket with a liftoff thrust of ~4,100 kN (I can't figure out if this number is vac or sea level). It's much heavier than it's needs to be, due to being massively overbuilt because it was designed by Soviets in the 1950s.
Atlas III: Atlas III was an 8-10 ton to LEO vehicle, depending on the upper stage. Had an RD-180, and therefore a liftoff thrust of about ~3,800 kN. It's design is about as mass efficient as it gets.
Delta IV Medium: The totally booster-less Delta IV (using post RS-68A specs) puts ~8.5t into LEO and has a lift-off thrust of "only" 3,140 kN. It get's away with this because the whole thing is hydrolox, and hydrogen is light.
Antares 230+: Current version of Antares. Has a payload to LEO of ~8t with a liftoff thrust of 3,844 kN. Keep in mind that Antares has a heavy solid upper stage weighing it down.
Falcon 9 v1.0: This was actually a 10t to LEO vehicle, with 9 Merlin 1Cs providing ~3,800 kN of thrust at liftoff.
So I generally estimate that we should be looking at 3,500 - 3,850 kN of liftoff thrust for a scratch-built modern 8t to LEO vehicle with a liquid upper stage. That's before any budgeting for reusability.
Edit for more: When it comes to thrust level, the Merlin is both the most convenient and most confusing existing option to use in a comparison. You have to remember that while the Falcon 9 started as a medium-lift launcher, it isn't anymore, and it's engine has scaled up accordingly.
Having run back through it like this, my numbers for max thrust for the engines (of a Neutron with 3-4 engines) were low to begin with, and then much lower still because I forgot about propulsive landing. it would be more like 1,000-1,750 kN engines.
Edit: made another edit to correct some numbers. There's actually a pretty strong consensus around 3,800 kN for expendable rocket's in this class. -
#23 by Lars-J on 05 Mar, 2021 18:57
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So I generally estimate that we should be looking at 3,000 - 4,000 kN of liftoff thrust for a scratch-built modern 8t to LEO vehicle with a liquid upper stage. That's before any budgeting for reusability.
...Which in the SpaceX case adds ~30%. Think of Neutron as a 10-12mt expendable launcher instead of comparing with expendable 8 ton launchers. -
#24 by JEF_300 on 05 Mar, 2021 19:13
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So I generally estimate that we should be looking at 3,000 - 4,000 kN of liftoff thrust for a scratch-built modern 8t to LEO vehicle with a liquid upper stage. That's before any budgeting for reusability.
...Which in the SpaceX case adds ~30%. Think of Neutron as a 10-12mt expendable launcher instead of comparing with expendable 8 ton launchers.
There aren't a lot of 10-12mt expendable launchers to compare to. The best example is going to be the Falcon 9 v1.0, because it's also all kerolox and modern. Sure, stainless steel tanks are going to be a little heavier, but the wider diameter of Neutron will make it's tanks more mass efficient anyway.
I think 5,000+ kN is a good ballpark for us to all shoot for when speculating. -
#25 by ZachF on 05 Mar, 2021 20:37
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4 engines and a landing engine (based on a beefed up Rutherford?) wouldn’t be terrible.
I think if they use four engines they'll use just one of them gimbaled way the hell over and land at an angle.
Or they'll just use all four and throttle really deeply. This isn't unprecedented; DC-X did it.
DC-X only had a mass ratio of ~2, so it could land on ~50% total throttle. This stage will probably have a mass ratio of somewhere between 10-20, and that doesn't include the upper stage mass... It will need to land on ~5% of liftoff thrust.
You could probably use electric pumps for a landing engine (s)... It would only need thrust in the 15-20 tonnes range. -
#26 by bstrong on 05 Mar, 2021 20:59
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My guess on engine layout: Four equally-sized engines, one in the center and three evenly-space around the perimeter (which would be unique, as Beck implied). As a future upgrade, they stretch the tanks and fill in the gaps around the perimeter with an additional three engines.
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#27 by gin455res on 05 Mar, 2021 21:14
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4 engines and a landing engine (based on a beefed up Rutherford?) wouldn’t be terrible.
I think if they use four engines they'll use just one of them gimbaled way the hell over and land at an angle.
Or they'll just use all four and throttle really deeply. This isn't unprecedented; DC-X did it.
DC-X only had a mass ratio of ~2, so it could land on ~50% total throttle. This stage will probably have a mass ratio of somewhere between 10-20, and that doesn't include the upper stage mass... It will need to land on ~5% of liftoff thrust.
You could probably use electric pumps for a landing engine (s)... It would only need thrust in the 15-20 tonnes range.
Is there any reason the landing engine couldn't be pressure-fed?
But, if it is electrically pumped, can the chamber pressure be reduced to improve thrust per kg of electric pump?
Could 3 dual-bell engines be arranged in a hexagon leaving space for a central landing engine? -
#28 by ArbitraryConstant on 05 Mar, 2021 21:25
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Is there any reason the landing engine couldn't be pressure-fed?
Yes, poor performance. It would need a dedicated pressurized tank, because the structural mass to make the whole stage tolerant of the needed pressure would be prohibitive.But, if it is electrically pumped, can the chamber pressure be reduced to improve thrust per kg of electric pump?
Probably but not that much, it needs to work efficiently at sea level.Could 3 dual-bell engines be arranged in a hexagon leaving space for a central landing engine?
Maybe, but why would they want to? This would mean developing and qualifying two new engines. If the hex+central configuration were preferable they would probably do 7 independent engines, and I don't think they'll do that either.
If they're going to develop two new engines it would probably be so they could have a smaller upper stage engine, since a meganewton-class engine on the upper stage is probably too much. Which suggests, if they need a different central engine, why not make that a sea level version of the upper stage engine. -
#29 by JEF_300 on 05 Mar, 2021 22:01
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So my Falcon 9 v1.0 thrust numbers are WAY off. I don't where I got ~5000 kN from, but the Merlin 1C has a thrust of about 422 kN according to b14643 (http://www.b14643.de/Spacerockets_2/United_States_1/Falcon-9/Merlin/index.htm), which lines up with the ~420 kN that Wikipedia lists. So that would put the Falcon 9 v1.0 at (422x9=) 3,798 kN of thrust.
I'm gonna go back an edit the post I made before to reflect this, because otherwise it's a decent reference. -
#30 by trimeta on 05 Mar, 2021 22:08
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So my Falcon 9 v1.0 thrust numbers are WAY off. I don't where I got ~5000 kN from, but the Merlin 1C has a thrust of about 422 kN according to b14643 (http://www.b14643.de/Spacerockets_2/United_States_1/Falcon-9/Merlin/index.htm), which lines up with the ~420 kN that Wikipedia lists. So that would put the Falcon 9 v1.0 at (422x9=) 3,798 kN of thrust.
I'm gonna go back an edit the post I made before to reflect this, because otherwise it's a decent reference.
Oddly enough, Launcher Space's competitive intelligence calculator does estimate Neutron's first-stage thrust as 5,166.851 kN. So whatever intuition you were using for that, you weren't alone. -
#31 by JEF_300 on 05 Mar, 2021 22:26
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So my Falcon 9 v1.0 thrust numbers are WAY off. I don't where I got ~5000 kN from, but the Merlin 1C has a thrust of about 422 kN according to b14643 (http://www.b14643.de/Spacerockets_2/United_States_1/Falcon-9/Merlin/index.htm), which lines up with the ~420 kN that Wikipedia lists. So that would put the Falcon 9 v1.0 at (422x9=) 3,798 kN of thrust.
I'm gonna go back an edit the post I made before to reflect this, because otherwise it's a decent reference.
Oddly enough, Launcher Space's competitive intelligence calculator does estimate Neutron's first-stage thrust as 5,166.851 kN. So whatever intuition you were using for that, you weren't alone.
I can see why. The good examples we have of rockets that put 8-10t into LEO and have a kerolox first stage (Antares, Atlas III, Atlas V, and Falcon 9 v1.0; we're excluding Soyuz, it's a unique case) all have a liftoff thrust of ~3,800 kN ; granted, two of them have the same engine and the third has two halves of the same engine.
So if you take 3,800 kN, and assume that propulsive landing will require a SpaceX-like amount of additional performance (30%), you end up with (3,800x1.3=) 4940 kN. So around 5,000 kN is good for estimating.
I'd argue it's actually on the high-end though, since Falcon 9 v1.0 and the Atlases can actually get close to 10t to LEO, and the Antares is weighed down. You could probably build a totally expendable 8t to LEO kerolox launch vehicle with as little as 3,100 kN of liftoff thrust. -
#32 by Pueo on 06 Mar, 2021 00:49
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Based on the height dimensions of the mock-up, I estimate the height of the first stage propellant tank to be 23.5 m, and the height of the second stage propellant tank to be ~6.6 m. Modeling the propellant tanks as cylinders, using a kerolox density of 1026 kg/m3, and using a dry mass fraction of 8% I end up with a lift-off mass of ~540 tonnes. That's a pretty heavy rocket, and makes the 8 tonnes to LEO seem quite conservative.
The following table assumes a configuration with a central engine, and the throttleability is set to the throttle needed to achieve a TWR of 1.9 for the dry first stage with a single engine (similar to the early F9 recoveries) with a TWR of 1.2 at lift off. The comparable engines are those for which liftoff TWR is in the range 1.1 - 1.4Number of Engines Required Throttleability Engines with Comparable Max Thrust 7
_68%
_Merlin 1D FT (US)
RD-107a (RU)
_6
_58%
_RD-801 (UKR)
YF-100 (CN)
_5
_49%
_RD-801 (UKR)
NK-33A (RU)
YF-100 (CN)
_4
_39%
_RD-810 (UKR)
NK-33a (RU)
RD-181 (RU)
RD-151 (RU, used by ROK)
SCE-200 (IN)
_3
_29%
_AR-1 (US)
RD-181 (RU)
_
Nearly all these engines are on an ORSC cycle, though to be fair most of the engines are descendants of the NK-33. Developing such an engine would seem to be a pretty big challenge, even without the constrained timeline. If only there was a fuel that was low coking so a FRSC cycle could be used and when subcooled had similar impulse density to RP-1... -
#33 by JEF_300 on 06 Mar, 2021 01:10
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Based on the height dimensions of the mock-up, I estimate the height of the first stage propellant tank to be 23.5 m, and the height of the second stage propellant tank to be ~6.6 m. Modeling the propellant tanks as cylinders, using a kerolox density of 1026 kg/m3, and using a dry mass fraction of 8% I end up with a lift-off mass of ~540 tonnes. That's a pretty heavy rocket, and makes the 8 tonnes to LEO seem quite conservative.
The following table assumes a configuration with a central engine, and the throttleability is set to the throttle needed to achieve a TWR of 1.9 for the dry first stage with a single engine (similar to the early F9 recoveries) with a TWR of 1.2 at lift off. The comparable engines are those for which liftoff TWR is in the range 1.1 - 1.4
This table is awesome, so thanks for making it.
Apparently the RD-181, or at least the RD-191 can throttle down to 27%, so the RD-181 might actually work. I don't think Rocket Lab would buy RD-181s, but it amuses me that it might work. -
#34 by Pueo on 06 Mar, 2021 01:18
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Apparently the RD-181, or at least the RD-191 can throttle down to 27%, so the RD-181 might actually work. I don't think Rocket Lab would buy RD-181s, but it amuses me that it might work.
I would also find that really funny, it would really reinforce the idea that the Neutron is just a chonky Antares. -
#35 by matthewkantar on 06 Mar, 2021 02:27
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Maybe Neutron is a little fat to allow a cluster of Rutherford style engines in the center for landing? Would provide engine out landing capability and make the boost engines simpler from lack of deep throttling. Maybe 3/3 or 4/4?
Edit: This idea would put them back up to 6-8 engines, which Beck wants to avoid.
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#36 by ArbitraryConstant on 06 Mar, 2021 02:30
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Realistically I don't think it can be anything other than an in-house engine. Most of the other engines, whether or not they're even on the market, would either cost far too much or incur too many concerns about supply sustainability, relying on a direct competitor, etc. Having to re-engine Neutron down the road would be calamitous.
And given RP-1 as the fuel and given the timeline I don't see how it can be anything other than electric, GG, or a combination. And if as seems the case another new engine is needed for the upper stage and potentially as a landing engine for the first stage, electric or a combination seems even more likely. -
#37 by jbenton on 06 Mar, 2021 02:43
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This is starting to sound like the Falcon 5 (but with 1 or 2 fewer engines)
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#38 by gin455res on 06 Mar, 2021 04:05
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How about scaling up an rd-107. Give it 6 chambers in a hexagon. Add electro-pumping to the central peroxide turbo-pump and an afterburner. For landing extra peroxide plumbing bypasses the turbine and leads directly to the afterburner/combustion chamber for an electro-pumped landing engine. Perhaps afterburner and landing combustion chamber need to be separated and concentric.
One engine with an afterburning peroxide turbopump/landing engine hybrid.
Maybe design in sky-diver entry and stainless once SN11 sticks a landing.
Alternate geometries and pump propellant choices might also work; 4 main chambers, afterburner in the center, 4 smaller chambers outside the main chambers for landing; 2-3 x 2-chambered engines; Fuel rich ethanol turbopump with oxygen injected afterburner. -
#39 by JEF_300 on 06 Mar, 2021 04:28
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How about scaling up an rd-107. Give it 6 chambers in a hexagon. Add electro-pumping to the central peroxide turbo-pump and an afterburner. For landing extra peroxide plumbing bypasses the turbine and leads directly to the afterburner/combustion chamber for an electro-pumped landing engine. Perhaps afterburner and landing combustion chamber need to be separated and concentric.
I hadn't thought about 1 engine with multiple chambers, but that's actually a really good point. That provides most of the production and testing benefits of having 1 engine, but should make development and throttling easier.
I can imagine a 7 chamber engine, with a "landing mode" where the pumps are throttled down and the flow to all but the center chamber is cut off.
- to get the capability they announced.