Monday, February 27, 2023

The Missed Lesson of the Falcon Heavy

  Copyright 2023 Robert Clark

  Every time Gwen Shotwell or Elon Musk are interviewed about the Superheavy launch they are always fretting about a possible explosion on the launch pad, possibly damaging the launch tower.

 Shotwell and Musk have even said the test launch will be considered a success just clearing the tower without damaging it. Presumably then, the test launch will still be considered a success even if it does explode during the flight, as long as it first clears the tower. This hardly instills confidence in the reliability of the flight. Indeed it begins to look like the approach of the Russian N-1 engineers who tested the N-1 by launching it multiple times without sufficient ground testing first, resulting in the  rocket exploding on each test flight.

 One wonders, if the SuperHeavy does explode during flight, would SpaceX like the N-1 engineers before them do the next test launch again without full length test firing of all engines together, as long as the launch tower is undamaged? Suppose the launch tower is damaged, would they still take this same approach?

 What should have been done in regards to the SuperHeavy booster is to construct a separate test stand to test all 33 engines at the same time for the full, true flight burn time of the engines. The static test fire done so far was barely more than 5 seconds long, hardly a true shake out of the complete engine package at once. Plus, it was only at half thrust. During that short test, 2 engines failed. Without further information it can just as well be every 5 seconds or so another 2 engines would fail.

 Constructing a separate test stand will allow the engines all together to gradually be ramped up to full thrust and to full, true burn length duration. The automatic and manual shutoff of the two engines in the last test is encouraging. If might mean in a gradual testing program, flaws could be detected and the test curtailed if one or more engines failed. Then the flaws in those engines could be corrected and the tests conducted again.

 Note this was how it was done for the five F-1 engines of the Saturn V, conducting true, full duration tests of all five engines at once. The engines were not certified for flight until all five engines successfully completed true, full duration test firings all together, for multiple test firings.

 However, more importantly SpaceX missed major advantages of the Falcon Heavy approach of using three cores to form a heavy lift vehicle. They incorrectly concluded the Falcon Heavy was not a good approach because it cost something(!) The SpaceX engineers should have noted that for the Delta IV Heavy, also a triple-cored vehicle from existing cores, the development cost was in the range of $500 million. The correct conclusion they should have drawn is how much cheaper it was than building an entire new booster three times as big. The FH development cost also turned out about $500 million. This is only about 50% more than that of developing the original Falcon 9 at $300 million but at 3 times the payload of the current Falcon 9. 

 Actually the advantage may be even greater than that. The original Falcon 9 was only about 10 tons to LEO. So the Falcon Heavy is at 6 times the payload of the original Falcon 9. On the other hand the total development cost for all the Falcon 9 versions up to the current Falcon 9 FT has been estimated in the billion dollar range. So the Falcon Heavy increased the payload by a factor 3 over the current F9, but at a development cost less than half that of the current version 

 Likewise to the Falcon Heavy, a triple-cored Starship could have formed a launcher at 3 times the payload of a two-stage launcher based on the Starship being the booster with a smaller mini-Starship as the second stage.This was discussed here:

Starhopper+Starship as a heavy-lift launcher. Triple-cored Starship for super-heavy lift. 2nd UPDATE, 9/2/2019: Starhopper as a lunar lander.

 Quite importantly the two-stage to orbit vehicle, TSTO, would be able heavy lift 100+ tons to LEO. This is important because a 100 ton launcher is regarded as a requirement for a manned lunar mission in a single launch architecture. So already in 2021 with the Starship performing its test launches then we would already have had a manned lunar mission capable launcher. This is assuming the "Starhopper" as a small upper stage would also have had its development continued.

 This is for a single launch architecture, no 4 to 16 launches needed to refuel the Starship in orbit as a lunar lander. Note also the triple-cored version also could do a manned Mars mission in a single launch.

 And before the Falcon Heavy flew, there were over a 100 flights of the Falcon 9. That's over 1,000 actual full, operational burns of the Merlin engines. The equivalent of more than 30 full flights of the Falcon Heavy. 

  This brings up another major advantage of this approach, in regards to safety. Gwen Shotwell has said ideally Starship would have 100 launches before launching people. This is actually a logical disconnect to the Artemis missions with the Starship intending to carry people to the Moon as a lander by 2025:

Shotwell says SpaceX ready for Starship static-fire test
Jeff Foust
February 8, 2023
She said she expected Starship to fly at least 100 times before it carries people for the first time, a challenge as the company prepares a lunar lander version of Starship for NASA’s Artemis 3 mission, currently scheduled for as soon as 2025.

In her later conversation with reporters, she called that 100-flight milestone a “great goal” but suggested it was not a requirement. “I would love to do hundreds before. I think that would be a great goal and it’s quite possible that we could do that,” she said.

She noted the company has a goal of 100 Falcon launches this year. “If we can do 100 flights of Falcon this year, I’d love to be able to do 100 flights of Starship next year. I don’t think we will do 100 flights of Starship next year, but maybe 2025 we will do 100 flights.”

 But the Starship making 100 flights would mean the SuperHeavy making 100 flights by 2025. This is highly unlikely with the Superheavy not having made a single launch yet.

 Note the Falcon 9 made 85 unmanned flights before it launched crew to orbit. With, instead, a Starship  TSTO making its first flight in 2021 at over 5 times the payload as the Falcon 9, it very well could have already superseded Falcon 9 at that role and have been making ~25 flights per year over the 4 years from 2021 to 2025.

Single Stage to Orbit(SSTO) possibility.  

 The accepted interpretation of the SSTO as infeasible stems from the earliest days of the Space Age where ground launch engines only had ca. ~300 s vacuum Isp. Having to fire from the ground put severe limits on the engine efficiency as measured by Isp of engines. Because of that, it was argued an SSTO would need some major technical advance to be feasible, such as nuclear engines with ca. 900 s Isp.

 It is unfortunate that the paradigm for making a SSTO feasible was by assuming nuclear thermal propulsion. In point of fact for a kerosene-fueled engine only a ~330 s vacuum Isp was needed and for hydrogen fueled only ~440 s vacuum Isp for the ground-launch engines. Both of these became possible by the 1970's with the Russian RD-180 for kerosene-fueled at 338 s vacuum Isp and the American SSME at 452 s vacuum Isp. 

And now, with the SpaceX Raptor as a ground-launch capable engine reaching 370+ s vacuum Isp, quite significant payload becomes possible as an SSTO.

 With the Starship and mini-Starship as SSTO's radical increases in orbital flight especially for point-to-point transport would have been possible.

  Robert Clark


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