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This topic was inspired by a suggestion of SpaceNut, in another topic ...
SpaceNut wrote:Seems like they should have just copied what worked for the staging separation since the only means to achieve booster separation for escape for crew safety is for the starship to have that ability as early as it can.
We know that Starship even by itself cannot achieve orbit unless it gets close to the altitude required so anything lower means a sub orbital flight and landing that it has shown that it is capable of.
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This post is a follow up to Post #1 of this topic:
For SpaceNut re #42
Your suggestion in this post deserves it's own topic.
When the time comes for passenger certification of Starship, I would expect that the company itself, let alone regulators and insurance companies, will expect to see a reliable escape system for Starship.
Here is something for us all to ponder as we wait for SpaceX to implement a reliable Starship escape system .... How would the Starship control software have known when to escape, in the flight we just witnessed?
If the First Stage is still thrusting, as was definitely the case here, my question is whether Starship could have pulled away from the First Stage even if it wanted to?
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I will inject what I think here. Three early versions of Starship have the most value, I think.
1) Starlink Launches. (Other space machines launched as well).
2) Propellant Ships/Depots.
3) Lunar Starship with crew.
#3 does not involve crew with stages #0 or #1, so safety modes for those stages do not have to be directly human safe.
So, I feel that this is a very good test bed to initiate Starship.
In addition, there could be point to point ships which will not involve the 1st Stage, and even maybe SSTO, but those are very economically questionable.
SSTO is said to be possible, but so far without useful value.
I believe that Lunar Starship interacts with the Orion Capsule to transfer crew to and from the Lunar Starship. So, NASA is responsible for getting Crew transferred to and from the Earth's surface.
So, it is good to have thoughts about how to have thoughts about the protections for missions for humans in passage though the Earth's atmosphere, eventually, for now it could be a low priority and emphasizing it may slow down the needed functions of getting payloads to orbit, including Lunar Starship.
Technically there could even be missions to Mars, without using Starship to move people though the atmosphere. Robotic, Cargo, and even a small crews mission to Mars/Phobos/Deimos. If not Orion, then Crew Dragon, or eventually perhaps Dream Chaser, and maybe even Terran-R.
Done.
Last edited by Void (2023-04-22 11:54:20)
Done.
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I feel that using capsules with Starship, it might be possible to fly to other worlds, and upon return to Earth, use the Capsules for Earth entry.
Perhaps the Starship would even be abandoned, which would then not require propellants to get it back to Earth. For the Moon this could make sense, and is somewhat what NASA has planned, but includes Starship landing on the Moon and launching from the Moon.
A human mission to Mars might include similar methods, but it may not include landing on Mars. I think it would be better to send such a mission as robotic. It would still test the endurance of the ship to transit to Mars. You could even have some testing of human life support, without humans on board. It might scan and sample the Moons of Mars, and who knows, might it pick up a sample from Mars, using some lander device. Probably not.
But good Radar on a Starship to scan the moons and Mars itself would be helpful.
Using Ballistic Capture, no Heat Shield would be needed. Also not landing on Mars, no legs needed. As for maneuvering in orbit, I suppose different header tanks would be wanted.
Done.
Done.
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Ascent abort?
Supposedly, this thing would have 17 million pounds of thrust if all 33 engines were operating. I have heard no one specify whether that is vacuum or sea level, though! That’s about 515,000 pounds of thrust per engine, or about 233 or 234 metric-tons-force.
That’s about what I get for a vacuum thrust at 40:1 expansion with a sea level Raptor-2, and the backpressure force is about 30,200 lb or 13.7 metric tons-force. That puts the sea level thrust per engine at just about 220 metric tons-force. Thus the takeoff sea level total Superheavy thrust with all 33 burning is about 7260 metric tons-force, or about 16 million pounds. Nobody is quoting that number, I noticed.
That’s the sea level Raptor 2. There’s 3 of those in the tail of the Starship upper stage, and 3 of the new Raptor-2 vacuum Raptors that can actually be fired in the open air at sea level without separating.
My best guess is that the backpressure force reduction is atmospheric pressure on about a 2.3 m diameter, for 93,500 lb or 42 metric tons-force. Supposedly the Raptor-2 vacuum engine has about 250 metric tons-force of vacuum thrust, which makes its sea level thrust 208 metric tons.
Now, Superheavy loads 3400 metric tons of propellant into a structure somewhere in the neighborhood of 200 tons inert. That’s a stage mass of 3600 tons without the upper stage as payload.
A loaded operational Starship upper stage would load around 170 tons payload and 1200 tons propellant into around 120 tons inert. That’s 1490 metric tons of loaded Starship upper stage as the payload for a Superheavy. Liftoff mass should be near 5090 metric tons. Liftoff thrust-to-weight would be near 7260/5090 = 1.42 or 1.43. Musk said a while back he wanted liftoff thrust-to-weight to be near 1.5. Looks like he just about got that, with his Raptor-2 performances and his stage designs.
This one had a prototype Starship with no payload, inert near 120 tons, and likely a full propellant load of 1200 tons, for a loaded mass of 1320 tons. Total vehicle mass at liftoff would have been near 4920. It took off with maybe 3 engines unlit, maybe more. Call it 4. Total liftoff thrust would be 29*220 = 6380 metric tons-force. That’s T/W = 1.30, which is adequate, but less than desired. See why in part that he wanted 1.5? About the minimum that was ever successful is around 1.1 or 1.2. Kinematics requires that as a minimum.
Now look at the upper stage Starship. Nominal stage separation would call for lighting the 3 vacuum engines for the trip in quite-thin air our into vacuum toward orbit, at a very low trajectory angle. That would be 3*250 (in near-vacuum) = 750 metric tons-force thrust on a 1320 to 1490-ton vehicle. Thrust is less than weight, and if the trajectory angle above local horizontal exceeds about 30 to 35 degrees, the vehicle cannot even accelerate at all against its weight component along the trajectory.
In an emergency abort, you will need to fire up the sea level engines to get the vectored thrust needed to flip and to land. That’s 3x220 (down in the thick air) = 660 metric tons-force. You also need to light up the vacuum engines to help carry the weight of the propellant still aboard as you land. That’s 3x208 (down in the thick air) = 624 metric tons-force. Summed for both types of engines: 1284 metric tons force.
Initially, you still have 1200 tons of propellant aboard, and a total mass in the 1320-1490 ton range. You have less thrust than weight initially, even with all 6 engines burning! The TW=1 “breakeven point” occurs at a vehicle mass of 1284 metric tons. That means you have to burn off some 36-to-206 metric tons of propellant just to reach the breakeven at T/W = 1.
You will need T/W > 1 by considerable margin to effect a landing, since you need considerable deceleration to kill lots of downward velocity. We’ve already seen it. 2 sea level engines burning in a 120 ton vehicle with perhaps 30 tons propellant aboard, in the Starship-alone flight tests. 2*220 = 440 metric tons-force of thrust, in a vehicle mass of 150 to 320 tons, depending upon whether there is payload aboard (in the flight tests, there was not). That’s a landing T/W = 2.93 (empty) to 1.38 (payload aboard) range. Switch that to all three sea level engines burning for 660 tons thrust, and T/W at landing is in the 4.40 (empty) to 2.06 (with payload) range. Looks to me like the design landing T/W is very probably near 2.
For abort purposes with a crew or a valuable payload aboard, you are looking at the heavier loaded Starship masses, and the lower landing T/W data. Which is why I said about T/W = 2. With all 6 engines burning for 1284 tons thrust, your vehicle should mass near 642 tons to achieve that landing T/W = 2 figure. 642-170 payload-120 inert = 352 metric tons of propellant still aboard at landing, max! That means you must burn off some 1200-352 = 848 tons of propellant on the way down, or you will NOT be able to stick a survivable landing!
Each engine is burning something like 600 kg/s = 0.6 tons/s. With 6 burning, that’s in the neighborhood of 3.6 tons/s. To burn off 848 tons of propellant at a rate like that, will take something like 236 s = 3.9 minutes. Achieving that before smacking the surface will be problematical at best! The problem is even worse if you try to do a low-altitude abort! There needs to be a way to dump propellant fast, something we have not seen before in these prototypes.
As for stage separation, if the booster fails to shut down, and you release the Starship, the worst-case booster thrust is 33 engines at full vacuum thrust way up in the very thin air: 33x233 = 7689 tons-force. The booster-alone mass is 200 tons inert + maybe 50 tons unused propellant = 250 tons. Without the Starship upper stage, this thing wants to accelerate at some 7689/250 = 30.8 gees along the trajectory! If those engines do not shut down, the lower stage will rapidly break up as separation is attempted.
A loaded Starship with all 6 engines burning has 3X233 + 3*250 = 1449 tons-force in the really thin air, acting on 1490 tons of fully loaded vehicle. At best it can accelerate at 1449/1490 = 0.97 gees along a fully-horizontal trajectory! The near-empty booster on only one engine burning has 1.0 gee capability along a flat trajectory! Thus there is no way this Starship upper stage can pull itself away from a booster that is still thrusting! Period! So, achieving full engine shutdown on the booster in order to achieve stage separation is an utterly critical requirement! And it only works at very low trajectory angles, well under 30 degrees! That full booster shutdown did NOT happen in the first test! It was spinning end-over-end out of control with most of the engines still burning!
I do not know whether SpaceX has yet really looked at the ascent abort problem. But these numbers I have run tell quite the tale! Saving a crew or a valuable cargo by doing an ascent abort with this vehicle is going to be a really tough problem to solve! It can be done, but will likely require adding fast propellant dump features to the Starship upper stage. But it needs doing, before this vehicle is man-rated!
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Starship seating is 100 and the Orion capsule while it can transfer 7 it's not able to repeat and cannot land on the moon so it's useless, for use with starship.
If this was a normal crewed launch, we would be burying 100 souls due to oversite of manned taxi launching abort. All other flavors for cargo we would not care about.
Also, for the engine shut down stopping the flow of fuel and oxidizer will do that.
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For SpaceNut re #6
Your suggestion of shutdown of all 33 engines if a malfunction occurs in the first stage seems reasonable (to me at least) at first reading. All engines need to be able to shut down at any time as it is, so the hardware to accomplish that must be installed now.
However, GW's post #5 appears to show that more is involved than just shutting down the engines on the first stage.
If I understand his post correctly, it would appear that Starship cannot land with full tanks, if it has to disconnect from the first stage in an emergency. If fact, if I understand GW's post correctly, Starship cannot even maintain altitude with full tanks.
One option is to add more engines to Starship.
If I understand GW's post correctly, there would need to be enough engines to support the entire fully loaded Starship while the onboard controller burns through propellant to deliver Starship back to the launch point.
It seems to me highly likely no one has thought that far ahead, in the initial flurry of activity to get the system to launch at all.
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For SpaceNut re Escape system.... this is a follow up to #7
It might be possible to achieve the needed safety objectives without modifying Starship by adding new engines.
A piece of hardware mounted between the first stage and the Starship could provide additional engines for an Escape procedure.
If the flight proceeds normally and stage separation occurs as expected, then the middle section could stay with the First Stage and land back at the launch point. If escape is necessary, then the middle section would stay with Starship and provide the needed additional thrust to burn off propellant while Starship maneuvers back to launch point or to a safe landing site such as in Florida, depending upon where the launch failure occurs.
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Escape system: I wouldn't have designed crew vehicle the way Starship is designed. I would design with a detachable capsule that uses a parachute to land, like New Shepard or Falcon 9 / Dragon. Or a winged orbiter like Shuttle, or lifting body like HL-20 / DreamChaser. All these allow landing without power. A propulsive landing requires high precision. But propulsive landing is the only option on the Moon or Mars. So, Starship is really designed for the Moon or Mars. A radical re-design is not an option, so the only valid alternative is what SpaceNut suggested. Ability for Starship to separate from the SuperHeavy booster at any point in flight. Use Starship has the escape pod. Commercial aircraft do not have parachutes or other escape system; the aircraft either lands or all passengers and crew lose their lives. Starship is designed with this principle. The F-111 Aardvark aircraft was designed with a detachable cockpit pod. Rather than ejection seats and parachutes, the whole pod separates and parachutes down. The reason was to allow escape during supersonic flight. You could think of Starship as the escape pod. Again, conclusion is Starship must be able to separate at any point in flight.
20:20 hindsight. But this is why SpaceX tests early and often. To learn lessons like this.
The Saturn V used explosive bolts to perform the trick of separating stages as they became empty. The falcon made use of an interstage where it was the throttle down of the first that allow for a change of forward momentum.
https://www.spacex.com/vehicles/falcon-9/
This of course would add mass that would need to be removed from the possible payload to orbit.
https://ufl-rocket-team.org/rocket-separation-basics/
https://www.reddit.com/r/rocketry/comme … _separate/
The first stage already has the grid fins locked into place unlike the Falcon 9.
The Dragon Capsule also has the engines which could be built into an inter stage to perform this same function after a booster stage is shut down.
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