GW Johnson Postings and @Exrocketman1 YouTube videos

GW Johnson · 2024-11-09 08:41:40

To fill the LOX tank before the vehicle topples would take an opening about the same size as the diameter. I don't think there's any practical way to do that. You only have a single handful of seconds to get the job done.

GW

tahanson43206 · 2024-11-09 09:29:36

For GW Johnson re #451

Thank you for considering the question of admitting water to the vehicle to stabilize it while keeping it afloat.

It appears that trying to fill the oxygen tank, or even just admit ** some ** water into the tank is unlikely to succeed,

The rate of release of air inside the body of the vehicle would be a factor, if an opening at the top of the vehicle were provided.

I'm back from looking at several video's, and there was no evidence of an opening at the top of Starship in any of them.

There ** is ** another strategy that might allow the Starship to survive rotation from vertical to horizontal at landing... If the vehicle had some horizontal velocity, then as the ship's stern enters the water, it would drag, and the ship would tip in the direction of movement. If the direction were controlled so that the back of the ship hits the water, that part of the ship would be least elevated in temperature.

(th)

tahanson43206 · 2024-11-17 11:24:12

For GW Johnson re Void idea/suggestion in post: http://newmars.com/forums/viewtopic.php … 69#p227869

In one of his creative modes, Void explored the possibility that a Starship might be able to achieve LEO without a payload. His proposal seems to be that the ship itself would be (or could be) a useful "payload".

The material used to construct the ship is of very high quality. Void's idea seems to be to remove the engine compartment and return that to Earth. That reminds me of the Vulcan idea, of parachuting the engines back to Earth. In Void's case (if I understand correctly), the engines would be brought back by ships designed for that purpose.

My question for you is: Can a Starship deliver itself to LEO, if there is no payload other than itself?

How low would the orbit be, if it is even possible?

Void has hinted that a Space Tug of some kind might retrieve the empty rocket. The fuel for that operation would need to be delivered by a separate vehicle.

Is it simply more efficient to use a Super Heavy to give the Starship the boost it needs and call it a day?

Fuel and oxidizer are needed to place the vehicle in a higher orbit. Is this a situation where the fuel consumption is the same, or does one system have an advantage over the other?

The challenge of the Space Tug matching orbit with the single stage is non trivial. That problem is solved by just using the Super Heavy.

The ship to bring the engine compartment back to Earth would necessarily be larger than Starship, because the engine compartment is contained in a cylindrical section that is the same diameter as Starship. Fuel would have to be invested in delivering the return ship to orbit, and maneuvers would be needed to collect the engine compartment to be returned.

By the time you add up all the fuel expenses, I'm wondering if the two stage method works out as the most cost effective.

If Void's basic idea (as I understand it) is to use the Starship that reaches orbit as a component of a structure, then the engines would ** still ** need to be returned even if a Super Heavy is used to boost the ship to where it needs to go. So in that case, the special return vehicle would be needed, and it would need fuel.

If we consider Void's basic idea as delivery of components to LEO to build a large structure, than the cylindrical shape is not obligatory. In other words, the girders to make a square structure in space could be welded to the cylindrical walls of the Starship, so that the entire system could be simply bolted into place.

If we have anyone in the group with the skills and software needed to make drawings of what such a configuration might look like, it sure would be interesting to see it.

(th)

GW Johnson · 2024-11-18 12:31:23

Question: could a “Starship” without any payload reach LEO without a “Superheavy” booster?

Answer: it’s complicated. But it is very likely not possible.

Details:

As being currently flown, which is not yet fully fitted out, the inert mass of “Starship” is somewhere in the vicinity of 120 metric tons. These notions of getting that down to something in the 50-90 ton range are utter bullshit! That inert will grow, if anything. But let’s use it anyway. That same configuration has tanks that can hold a max of 1200 metric tons of propellant. That puts the vehicle at just about 1320 metric tons mass, ready for ignition with no payload.

To get efficient launch kinematics and not waste propellant by climbing too slowly, the very-well-verified rule-of-thumb is that you need thrust/weight = 1.5 at launch (basically half a standard gee above the local pull of gravity). For 1320 metric tons mass, that is 1980 metric tons-force of thrust that you need at liftoff on Earth.

As currently flown, “Starship” has a total of 6 Raptor engines, those being 3 sea level and 3 vacuum variants. These are Raptor 2’s for which the sea level variant has a sea level thrust somewhere in the vicinity of only about 200 metric tons-force. The vacuum variants cannot be used at sea level; they are on the verge of bell separation and with a much-reduced thrust. Raptor-3 is reputed to be nearer 250 metric tons-force sea level thrust in the sea level variant. There might be room to mount 9 engines in the “Starship” engine bay, if all 9 were sea level variants. The inner 3 would gimbal, but the outer ring of 6 could not gimbal.

If these 9 sea level engines were Raptor-2, total takeoff thrust would be 9*200 = 1800 metric tons-force, only a little bit short of the rule-of-thumb. If they were instead Raptor-3’s, total thrust would be nearer 9*250 = 2250 metric tons-force, more than enough. So you could successfully launch with either Raptor model, but you would get significantly better overall performance results at the higher thrust level of the Raptor-3 sea level design. Especially if inert mass grows, as I think it will.

As you burn off propellant on the way up, you will need to shut some engines down, or else overstress the structure with too much acceleration gee. 9 Raptor-3’s at 1/3 thrust on a dry-tanks 120 metric tons is still too high at 6.25 gees thrust-induced acceleration, near-horizontal and exo-atmospheric. The 3 gimballing center engines, at half-thrust, would produce about 3.1 gees, which is much more realistic. And don’t forget: at reduced thrust, Isp is always somewhat lower. You really have to look at this thrust stuff, otherwise, the rocket equation will lie to you, because of a GIGO problem!

The ascent-averaged Isp of the Raptor-2 (or -3) would be in the 350-360 sec Isp class. Call it 360 s just to be optimistic about what Raptor-3 will eventually be capable of! That puts the effective ascent-averaged exhaust velocity pretty near 3.53 km/s. For the rocket equation, that puts the no-payload mass ratio at 1320/120 = 11.00, producing a deliverable dV = 8.46 km/s. That has to cover the theoretical energy needed, plus at least the drag and gravity losses. (There’s also rendezvous and deorbit to worry about, plus maybe a landing burn.)

Now, estimate the dV and losses: conveniently, Earth circular orbit speed at the surface is 7.913 km/s. That’s a good measure of the energy we have to achieve going to low circular orbit (say near 200-300 km altitude), in a low-inclination eastward direction. It is also a good basis for figuring the losses. If we achieve launch thrust/weight near or above 1.5, gravity losses will be only on the order of 5% of surface circular, or 0.396 km/s. The “Starship” shape is aerodynamically pretty clean and of a nice L/D ratio, so the drag losses can be low, near 5% of surface circular, also 0.396 km/s. So the energy and losses total so far to 8.705 km/s, already greater than what the stage design can deliver (which is only 8.46 km/s)!

Conclusion: it won’t work!!! It cannot work!!! But even if it did, you would reach orbit with dry tanks. You would be unable to rendezvous with anything, which budgets somewhere around another 0.1 km/s as a minimum, so what good would that be as something delivered on-orbit? And you would also be incapable of doing a de-orbit burn, to dispose of this massive launched hardware safely, which is just about another 0.1 km/s from low circular orbit. How unethical would not being able to safely dispose of this thing be?

The real velocity requirement with losses would include both the rendezvous and deorbit burns, for a total of just about 8.905 km/s (or maybe a bit more) that your rocket equation delivery must meet. And that would be for a crash landing after deorbiting. You need even more, if you actually intend to land the thing and reuse it. Say, at or just above 9.1-9.3 km/s.

Only if you believe in the bullshit numbers for incredibly low inert mass, can you reach a mass ratio that could deliver near 9.2 km/s dV (or even only 8.9!!) at an ascent-averaged Isp of 360 s. I do not believe in crap like that! And with my background, I ought to know. I have never, not in all my life, seen a vehicle inert mass that did not grow during development and testing. They ALWAYS grow!

GW

Last edited by GW Johnson (2024-11-18 12:33:10)

GW Johnson · 2024-11-19 12:09:57

Here's one from AIAA's "Daily Launch" for Tuesday 11-19-2024 that is not worth the price of the paper it was written on.

The New York Times
SpaceX Starship’s Sonic Boom Creates Risk of Structural Damage, Test Finds

SpaceX’s new Starship rocket far exceeds projected maximum noise levels, generating a sonic boom so powerful it risks property damage in the densely populated residential community near its South Texas launch site, new data suggests. The measurements — of the actual sound and air pressure generated by the rocket during its fifth test launch last month — are the most comprehensive publicly released to date for Starship, the largest and most powerful rocket ever constructed.

My take:

I went and looked at the linked NYT article. Whoever wrote it was totally technically ignorant and confused the thrust noise made at launch with the sonic booms heard when the booster comes back. The sonic booms are never that loud, but the launch thrust noise is! There are no sonic booms heard at launch, period! The vehicle is subsonic until a handful of miles up and a handful of miles downrange eastward, out over the Gulf.

Coming back, the booster approaches some handful of miles up on 13 engines, and at much lower altitude on only three engines. That's all the thrust it needs to decelerate subsonic at a mile or two up, and then come down to land at roughly thrust equal to weight, with 3 engines, running throttled. You might hear a weak double boom for the sonic boom, and a stronger noise signal from the 13 engines as it reaches subsonic about a mile or so up, unless the thrust noise covers up the sonic boom. The 3 engines at touchdown are no noisier than any of the earlier Starship-only tests landing, which is also only thrust equal to weight on 3 engines, throttled.

It's the launch on 33 engines at full thrust, that is the real noise generator! I warned about this being a problem before they ever got started trying to test things there, if you will recall. At around 12-15 million pounds of thrust, about twice the Saturn-5, this thing is about as "thrusty" as the smaller end of the old "Nova" paper designs Von Braun brought with himself when he went to NASA in 1958 from the Army at Huntsville, AL. NASA could not use any of the "Nova" designs, as the launch noise level was thought to be too dangerous, for the 3-mile clearance they had to populated areas. Note that the launch pad at Boca Chica is only 5 miles from the Brownsville, TX, city limits. And there's many houses closer than that, and I am not referring to those few that were right next to the office building at Starbase. Not to mention civilians in Mexico, too.

Why would anybody be surprised when someone finally flagged the noise risk? I warned of this years ago! The writeup breaking this news is a piece of crap, but the data it refers to are quite real! I totally expected this! All I wondered was why this did not come up sooner!

GW

Last edited by GW Johnson (2024-11-19 12:15:11)

tahanson43206 · Yesterday 07:32:42

For GW Johnson re testing of new FluBB web site...

Please log into the new system at http://newmars.com/new/

Please post a message there to confirm you were able to log in, and to report your observations.

(th)

tahanson43206 · Today 07:20:39

For GW Johnson re new web site...

Thanks for logging in and confirming the new site looks "normal" to you!

(th)

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#451 2024-11-09 08:41:40

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