Tuesday, September 23, 2014

A SpaceX Heavy Lift Methane Rocket, Page 2.

Copyright 2014 Robert Clark

 In the blog post A SpaceX Heavy Lift Methane Rocket I discussed some possibilities for a heavy lift rocket using SpaceX's planned methane engine, the Raptor. Those calculations were based on the initial released values for its thrust of 660,000 lbs, 300,000 kilograms-force. However, recently SpaceX has said the Raptor may have a thrust of 1,800,000 lbs in vacuum, 1,600,000 lbs at sea level. So I'll give some revised estimates for its payload. 

 As before I suggest using the same tooling for the core stage as that used for the SLS core to save on development costs. Corresponding to the new higher thrust of the Raptor I'll use the full tank size of the SLS core, which would hold 1,000 metric tons (MT) of hydrogen-lox propellant. Since methane-lox is 2.4 times as dense as hydrogen-lox, the SpaceX methane-lox core will hold 2,400 mT of propellant. The relative densities of methane-lox compared to hydrogen-lox are given in Table 1 of this report:

Alternate Propellants for SSTO Launchers.
Dr. Bruce Dunn
Adapted from a Presentation at:
Space Access 96
Phoenix Arizona
April 25 - 27, 1996

 In regards to the specifications of the methane rocket engine it should be noted the high vacuum Isp of 380 s  cited really would only be expected of a vacuum optimized nozzle. Such engines can not be operated at sea level. However, in an upcoming blog post I'll discuss some altitude compensation methods that will allow the engine to have this vacuum optimized Isp while still being able to operate at sea level. So I'll assume both the first stage and upper stage engines have the vacuum Isp of 380 s.

 As before I'll take the number of engines for the core as five and the propellant-to-dry mass ratio as 20 to 1. For the upper stage I'll take the propellant size as approx. 1/5th that of the core stage, at 500,000 kg with the same mass ratio, and use a single Raptor for the upper stage.

SSTO Case.
 We'll use again Dr. John Schilling's Launch Vehicle Performance Calculator. Input the thrust as the value of the vacuum thrust in kilonewtons of 5 Raptors as 5*8200 kN = 41,000 kN, and the Isp as the vacuum Isp of 380 s. Input the propellant mass as 2,400,000 kg and the dry mass as 1/20th of this at 120,000 kg. For the "Default Propellant Residuals?" option select "Yes", and for the "Restartable Upper Stage?" option select "No". Selecting "Yes" for this last option would reduce the calculated payload.

 Use the default altitude of 185 km. Select Cape Canaveral as the launch site with an orbital inclination of 28.5 degrees to match the latitude of the launch site. Then the Calculator gives the result:

Mission Performance:
Launch Vehicle:  User-Defined Launch Vehicle
Launch Site:  Cape Canaveral / KSC
Destination Orbit:  185 x 185 km, 28 deg
Estimated Payload:  110395 kg
95% Confidence Interval:  75079 - 152433 kg

 So this larger version could give 110,000 kg payload as an SSTO.

Two Stage Case
For the two stage case, in the column for the 2nd stage, input the propellant mass as 500,000 kg and the dry mass as 25,000 kg. Input the thrust as the vacuum thrust of 8,200 kN, and the Isp as the vacuum Isp of 380 s. Then the calculator gives the result:

Mission Performance:
Launch Vehicle:  User-Defined Launch Vehicle
Launch Site:  Cape Canaveral / KSC
Destination Orbit:  185 x 185 km, 28 deg
Estimated Payload:  196089 kg
95% Confidence Interval:  162727 - 236334 kg

  Close to 200 metric tons payload to LEO. Actually since the mass ratio of stages improve as you scale them up, quite likely the mass ratio will be better than just ca. 20 to 1, perhaps in the range of 25 to 30 to 1. This would then improve our payload to above 200 metric tons.

Cross-Feed Fueling for Multiple Cores.
 We'll just look at the 3 core case here. As described in the A SpaceX Heavy Lift Methane Rocket post, we emulate cross-feed fueling in the Schilling calculator by inputting for the two side boosters 2/3rds the actual  propellant mass, and also increase the first stage propellant mass by an additional 2/3rds. The rest of the specifications, dry mass, thrust, Isp remain the same. 

 So enter 2 as the number of side boosters. Then in the column for the boosters, input 1,600,000 kg for the propellant mass and 120,000 kg for the dry mass of the boosters. Enter the actual vacuum thrust of 41,000 kN and Isp as 380 s. Then the calculator gives the result:

Mission Performance:
Launch Vehicle:  User-Defined Launch Vehicle
Launch Site:  Cape Canaveral / KSC
Destination Orbit:  185 x 185 km, 28 deg
Estimated Payload:  536113 kg
95% Confidence Interval:  449806 - 639526 kg
 Over 500 metric tons of payload! This rocket however would be truly massive at 3 times the mass of the Saturn V. This would likely require a new,expensive launch pad to handle a rocket this size.

  Bob Clark

UPDATE, October 25, 2014:

 Some methods of accomplishing the altitude compensation mentioned are discussed here:

Altitude compensation attachments for standard rocket engines, and applications.


Rick Boozer said...

Interesting and enlightening article, Robert.

One question though. Why did you use 5 Raptor engines for the vehicle instead of the 9 that SpaceX say they plan to use?

Robert Clark said...

Thanks for responding. I am not a fan of the nine engines on the F9, and even less so of the 27 proposed to be on the Falcon Heavy. Space industry scientists and engineers became turned off to large numbers of engines on rockets by the experience of the Soviet N-1 Moon rocket that had 30 engines on the first stage. All 4 test flights failed:

This Insane Rocket Is Why The Soviet Union Never Made It To The Moon

Bob Clark

Rick Boozer said...

Yes, Bob. I'm already familiar with the N-1 case. The Russian's took some slipshod short cuts on that program to try to get to the moon in the shortest time possible (to beat the Americans), along with a lack of funding for extensive testing. Though individual rocket engines were fired in static ground tests, all were not static fired together as an assembled unit. Furthermore, stages had to be reassembled from the ground up from constituent parts after they were sent to Baikonur (because of lack of a barge, large train platform or other means of hauling large assembled units) increasing the likelihood of mistakes in the finally assembled rocket. As a result, it's severe pogo vibration was unanticipated and that caused vibrations that hammered away at propellant lines and turbines. While the NK-33 engine that was it's primary power plant is today one of the most reliable rocket engines in existence, in the late 60's and 70's it was an immature technology without all of the bugs worked out of it.


Because of extensive precautionary testing that SpaceX is almost anal about, it just doesn't make sense that the oversights that occurred with N-1 will happen with either Falcon Heavy or the future BFR. Also, the strategy behind 9 rockets is to only have to rely on 6 or 7 to get the payload to its destination orbit. That way one, two or three can malfunction and the mission be a success.

Jeff Findley said...

The devil is in the details. If anyone asserts that the N-1 failed solely because of the high number of engines in the first stage, I personally think they're just scratching the surface of that program. The devil is in the details, and there was more "wrong" with the N-1 than can be directly attributed to the number of engines used in the first stage.

We have the luxury today of vast computing power in small packages that require very little power. These days, it's not uncommon, nor particularly expensive, to dedicate several computers (running in a redundant configuration) for each engine. There is no need to resort to the primitive analog control systems the Soviet Union used for the first flights of the N-1. Errors like shutting down the wrong engine in response to a problem are therefore far less likely today than they were in the days of the N-1.

Anonymous said...

The big difference between the 30 Engine disign of the N1 and the 9 engine one of the Falcon9 is the massiv improvement in material science. This allows one to bulid rockets which seems to be impossible 50 years ago...

And, you have to admit that it can't be that bad, because the Falcon 9 is launched 11(12?) times without of bigger prroblems.


DougSpace said...

I'm counting 32 engines in the following picture. How many would you come up with?


Is it really the number that's the issue?

Steven Rappolee said...

Extend the nozzle of a few raptors on the first stage after entering vacuum, i think the RL-10 does this

Anonymous said...

Not so sure about the materials science ... the US has not succeeded in producing the RD-180, let alone an equivalent of the NK-33. I understand it's the material science which is deficient. So we see much of the US launch capability dependent on Russian and Ukrainian technology ...

Kent said...

I remember, but have yet to be able to find again, a shuttle-era proposal for a massive rocket (picture two shuttles ET's and 4+ SRBs bolted together for the first stage, etc.). The item of interest was that they would have to budget something like $50 million to replace every window in Titusville, FL with one that would not blow out with every launch.

Anonymous said...

The NK-33's suffered some sort of failure, so it wasn't just the KORD system,

Robert Clark said...

That's like two space shuttles. The three core case I considered would be like 3 Saturn V's so would likely be even worse.

Bob Clark

Robert Clark said...

I like this. In an upcoming blog post I'll consider this as well as other methods of accomplishing altitude compensation with engines.

Bob Clark

Robert Clark said...

Thanks for that link. I like that they are considering a methane version of the SSME's, the RS-25. This should have a lower development cost than developing one from scratch. Moreover, it would have high reusability as the SSME's.

Bob Clark

Rick Boozer said...

People need to quit coming up with these modification schemes for SLS. All it can possibly serve is to give the porkonauts in Congress more excuses to continue this weight around the neck of NASA that is inhibiting the space agency’s ability to get astronauts BLEO (beyond low earth orbit) and is negatively affecting Commercial Crew. Had Congress allowed NASA to pursue the alternatives as laid out in the agency’s 2010 plan, Commercial Crew would be farther along and closer to flying astronauts to ISS. Furthermore, NASA would have been a lot closer to going deeper into space because the budget now consumed by SLS would have allowed them to simultaneously pursue more practical strategies to that end. It’s still not too late to stop the massive waste and actually have NASA accomplish something significant. But continuing SLS is just, as the saying goes, “Throwing good money after bad”.

Robert Clark said...

I agree that a commercial space can get us to the Moon or Mars much more cheaply than NASA. Dennis Wingo has suggested a return to the Moon can be financed for only $5 billion:

Site Selection for Lunar Industrialization, Economic Development, and Settlement.
Posted on October 16, 2014

Bob Clark

Reggie said...

(From "Page 1") "There have been some estimates proposed for this launcher that use 7 copies of Raptor engine on the core. This many probably would be needed when you take into account the reduction in thrust at sea level if using a 1,700 mT sized tank. However, I wanted to keep the maximum number of engines on a core to be at most what was used on the Saturn V at 5 engines."

Why not run the numbers for the 7x Raptor engine variation? If they can run 9 Falcon engines on a stage successfully (and they have been), running 7 engines successfully seems a reasonable possibility. You could even take a page from the old Atlas 1½ stage rocket, and drop 2, 4, or all 6 of the the outer 6 engines; leaving a vacuum-nozzled Raptor engine to push the core to orbit.

Robert Clark said...

I'm not a fan of the 9 engines used on the Falcon 9. Today there was an explosion on the first stage of the F9 during flight. Likely this was an engine failure:

Live coverage: Falcon 9 disintegrates after launch.

Bob Clark

Rick Boozer said...

Robert, it appears it was due to an overpressurization of the second stage engine. So probably not one of the nine engines on the first stage. Best to wait to see the details of what happened before any speculation.

A route to aircraft-like reusability for rocket engines.

  Copyright 2024 Robert Clark   A general fact about aircraft jet engines may offer a route to achieve aircraft-like reusability for rockets...