Thursday, December 28, 2023

SpaceX should withdraw its application for the Starship as an Artemis lunar lander, Page 2: The Raptor is an unreliable engine.

 Copyright 2023 Robert Clark

 I had earlier argued that SpaceX should withdraw the Starship as a lunar lander. The primary basis for this was for safety of the surrounding population in case of an explosion on launch, SpaceX should withdraw its application for the Starship as an Artemis lunar lander.

 However, an additional reason why the Starship should not be used for a lunar lander is for safety of the crew. In the blog post, Did SpaceX throttle down the booster engines on the IFT-2 test launch to prevent engine failures?, I noted two separate methods of calculation suggest the SuperHeavy booster was throttled down to <75%. I also suggested the Starship upper stage was fired at ~90%. Given this difference in thrust power levels, I suggested the booster completed its portion of the ascent because it was throttled down and the upper stage did not because it was at close to full thrust. 

 Even though the booster engines successfully fired during the ascent, the booster exploded during the attempted return. One explanation offered was the engines were damaged by fuel slosh during flip of the booster. However, it should be noted the Starship during tests of the landing procedure, that at least one Raptor always leaked fuel and caught fire.

 Note even in the last two shown here, SN10 and SN15, there were engine fires on landing. For SN10 the engine fire led to the vehicle exploding a few minutes after landing. For SN15 the fire was extinguished before it caused an explosion. SN15 was called  a “successful” landing test because it did not explode. But that a Raptor still caught fire during this test gives further evidence the Raptor is still not a reliable engine. 

 And SN11 experienced a catastrophic explosion after a fuel leak and engine fire: 

 Since relighting the Raptors in flight always resulted in an engine fire, that is the most likely explanation for the IFT-2 booster explosion as well.

SpaceX Misleadingly Characterizes Raptor's Qualification for Flight.

 SpaceX has been using the term "full duration" for their Raptor static fire tests when they might only last 5 seconds. In the rest of the industry other than SpaceX, a full duration static test means firing for the full duration of an actual launch. 

280 seconds of glorious hot fire! 🔥 We are incredibly proud to be the 1st private company in #Europe (🤯) to hot fire a staged-combustion upper stage for its full duration. This qualifies our upper stage and Helix engine for flight 🚀 Enjoy the video and read more in our press release ➡️

And for the four SSME's on the SLS core stage:

 SpaceX calling their 5 second long test fires "full duration" misleadingly gives the impression that is sufficient to qualify the engines for full mission flight time.

No estimates for Raptor engine reliability publicly provided.

 For engines for a craft intended to carry astronauts and for which billions of dollars of public funds are earmarked there should be provided some indication about the safety and reliability of such engines. For instance this report provides estimates of the reliability of the different components of the SLS:

National Aeronautics and Space Administration

 But no such estimates for the Raptor have been provided. That so many engines have consistently failed in actual flights suggest they have quite low reliability.

 In the scenario of the Merlin engines used for crewed flight, over 80 missions of the Falcon 9 were successfully flown before the first crewed flight. That means over 800 successful firings of the Merlins during that time. And added on after that the many launches since then, over one thousand successful firings of the Merlins have been made.

  Robert Clark

Friday, December 15, 2023

Did SpaceX throttle down the booster engines on the IFT-2 test launch to prevent engine failures?

 Copyright 2023 Robert Clark

Given the Raptors repeated history of leaking fuel and catching fire I was surprised the booster was able to complete its portion of the ascent with no engine failures.
Hypothesis: the booster flew without engine failures because it throttled down to < 75%. The Starship had engine failures because it ran at ~90%, like the booster did on the first test flight with its multiple engine failures.

Throttle Down Calculated by Propellant vs. Time Graph.
Two separate observers, u/jobo555 and @space_josiah found fairly constant propellant flow rate, and therefore throttle, before where the booster begins to prepare for stage separation. Rocket thrust is given by (thrust) = (exhaust speed)*(propellant flow rate). So can get degree of throttle by propellant flow rate.
The graphs give the percentage of propellant remaining vs time. From this we can calculate the percentage change rate as the slope. For the booster it’s about 0.5%/s, 0.005/s as a decimal. Then given the total propellant load of 3,400 tons, in absolute term that propellant flow rate is 17 tons per second.
But the full thrust propellant flow rate for each Raptor v2 can be calculated as:
props flow rate = thrust/exhaust speed = 230,000*9.81/(327*9.81) = 700 kg/s. Then for all 33 engines on the booster that’s 33*700 kg/s = 23,100 kg/s, 23.1 tons/s. Then the throttle down for the booster amounted to: 17/23.1 = .736, less than 75%.
For the Starship, from the first image below, in its second graph we see from 4 minutes to 8 minutes, 240 seconds, the propellant level dropped from ~80% to ~5%, for a percentage rate drop of 75/240, 0.313%/s. Then the absolute flow rate for a 1,200 ton prop load is 3.756 tons per second. But for the 6 engines the flow rate at full thrust would be 6*700 = 4,200 kg/s, 4.2 tons/s. Then the throttle is .894, ~90%.
Note that throttling down to 75% also correspondingly drops the combustion chamber pressure from 300 bar to about 225 bar, allowing the Raptor to operate without leaks.
But this reduced thrust would also mean the SuperHeavy/Starship could carry less payload. I estimate a drop in payload to ca. 100 tons reusable. In such a scenario, the 16 refueling launches needed for a Starship HLS would be increased to 24 launches.

Throttle Down Calculated by Acceleration Graph.
A completely separate argument allows us to conclude the thrust was throttled down to less than 75%. This observer @meithan42 looked at the velocity and altitude data and derived the acceleration data.

On the acceleration graph I marked where the horizontal acceleration visually appears about 10 m/s2. The vertical acceleration there visually appears as about 6 m/s2. Visually this occurs at about the 90 second point.
Note that gravity subtracts ~10 m/s2 from the vertical acceleration, the actual vertical acceleration produced by the engines thrust is about 16 m/s2. Then the actual acceleration generated by the engines thrust is SQRT(102 + 162) = 18.87 m/s2 .

But now lets calculate the actual acceleration that should be produced by the engines assuming they were running at full throttle at the 90 second point. The thrust is, (thrust) = (exhaust speed) * (flow rate). Since we are near vacuum the Isp will be 363 s and the exhaust speed 363*9.81 = 3,560 m/s. Then the thrust at full throttle with a total prop flow rate of 23,000 kg/s, should be thrust = 3,560*23,000 = 81,880,000 N.

We'll take the total mass of the rocket as 4,850,000 considering the tanks are filled slightly less than 100%. If the engines are at full throttle then the mass after 90 seconds is 4,850,000 - 90*23,000 = 2,780,000, and the actual acceleration generated would be 81,880,000/2,780,000 = 29.45 m/s2. This is well beyond amount observed.

In contrast, if we take the throttled down propellant flow rate as 17,000 kg/s, then we calculate the actual acceleration as:

363*9.81*17,000/(4,850,000 - 90*17,000) = 18.23 m/s2 ,a value much closer to what is actually observed.

Robert Clark

All reactions

Wednesday, October 25, 2023

Towards return of Europe to dominance of the launch market, Page 2: ESA needs an independent oversight agency.

 Copyright 2023 Robert Clark

 Recent news reports are the Ariane 6 will not be able to compete with the SpaceX Falcon 9, requiring an increase in subsidies to ArianeSpace resulting in a total of €350 million($380 million) per year:

Oops—It looks like the Ariane 6 rocket may not offer Europe any launch savings
Europe is subsidizing the launch of Internet satellites for Jeff Bezos.
ERIC BERGER - 10/12/2023, 11:26 AM

 At a launch cadence of 6 launches per year this is a subsidy of nearly €60 million per launch. This means European tax payers will be paying over a billion euros for the contracted 18 launches on the Ariane 6 of the commercial venture the Kuiper satellite system of Jeff Bezos. In effect, European tax payers will be paying a billion euro subsidy to Jeff Bezos, the 2nd richest man in the world.

European Space Agency mulls extra Ariane 6 cash.


OCTOBER 20, 2023 7:00 AM CET

Strategic autonomy? Ariane 6 is in danger of turning into a fiasco for the ESA and France’s aerospace industry. Since being commissioned a decade ago, Ariane 6 has been surpassed by Elon Musk's SpaceX which has slashed the cost of launches with its partially reusable rocket technology. Because its predecessor Ariane 5, a super reliable commercial launcher, has already been retired, the European Commission is even having to look to SpaceX to get its satellites into orbit. That's exactly the kind of outsourcing Paris wants to avoid in its focus on building strategic autonomy.

 The fiasco has come to pass as Europe was forced to ask SpaceX to launch its vaunted Galileo GPS system on the Falcon 9, when it was intended to be launched on the Ariane 6:

Europe Reluctantly Chooses SpaceX to Launch Its GPS Satellites.
Elon Musk's SpaceX is set to undertake its first launch of European satellites equipped with classified technology, specifically for the Galileo system.

 Reports are the Vega-C is in a similar bad position with respect to the SpaceX Falcon 9 using rideshare for small payloads:

The Accidental Monopoly
How SpaceX became (just about) the only game in town
Jeff Foust
October 13, 2023
SpaceX came with these Transporter missions, which have been really disrupting,” said Marino Fragnito, senior vice president of the Vega business unit at Arianespace. They have been a boon for smallsat developers, he acknowledged, offering low-cost access to space. “But at the same time, they have created a big problem in terms of the business case for all of the other players.”
He accused SpaceX of, in effect, predatory pricing, willing to lose money on Transporter missions to drive out competition. He noted that past Vega smallsat rideshare missions sold payloads at $25,000 per kilogram, whereas SpaceX has sold Transporter launches for one-fifth that price. “It’s crazy.”

 This has been warned about for several years now:

Europe is starting to freak out about the launch dominance of SpaceX
The Falcon 9 has come to dominate commercial satellite launches.
ERIC BERGER - 3/22/2021, 11:24 AM
However, there now appears to be increasing concern in Europe that the Ariane 6 and Vega-C rockets will not be competitive in the launch market of the near future. This is important, because while member states of the European Space Agency pay for development of the rockets, after reaching operational status, these launch programs are expected to become self-sufficient by attracting commercial satellite launches to help pay the bills.
Economic ministers in France and Italy have now concluded that the launch market has changed dramatically since 2014, when the Ariane 6 and Vega-C rockets were first designed. According to a report in Le Figaro newspaper, the ministers believe the ability of these new European rockets to compete for commercial launch contracts has significantly deteriorated since then.

 European tax payers have the right to ask where the great expense of the Ariane 6 launcher is deriving from.

 No one in European space community is willing to ask or answer the question, “How much just to add a second Vulcain to the Ariane 5/6 core?”

 Then can someone, anyone in the European space community at least ask the question, “Does a single P120 solid rocket used for the Ariane 6 SRB’s and the Vega-C first stage really cost €20 million?”
 “So that the two 
SRB’s on the Ariane 62 cost €40 million, and the four on the Ariane 64 cost €80 million?”

“So that out of the €115 recommended cost of the Ariane 64, €80 is just for the 4 solid side boosters?”

 It is important to recognize that the high price of the Ariane 6 and the Vega-C is coming solely from the large solid rockets they use.


 It is common to think of solid side boosters as only adding a small amount to the price of a launcher, like with the small solids used on the Delta IV or Atlas V. But it is quite important to realize these are for small side boosters that might be only ca. 1/10th the mass of the core. But for the Ariane 6 the large side boosters are the size of the core in mass, and for the Vega the large solid booster is the core.

 Then the concept of the low cost solid booster is no longer valid; indeed these solid rockets boosters are the cause of the high cost of these launchers. To give an illustrative example, imagine the size of the side boosters on the Delta IV and Atlas V were 10 times larger than they are. I think you can see that would mean their cost would be radically higher than they are now.

 So how bad is the pricing of the P120 solids used for the Ariane 6 solids and the Vega-C first stage? Three separate and independent arguments suggest the P120 solid booster costs ca. €20 million each.

 ArianeSpace suggested a price of €75 million for the Ariane 62 with two SRB's and €115 million for the Ariane 64 with four SRB's. The €40 million increase in price for the two additional SRB's on the Ariane 64 suggests that is the price for two, or €20 million each.

 A second argument for the high cost of the P120 solid rocket comes from comparing it to the cost of the GEM 63 SRB used on American launchers. The GEM 63 is estimated to cost in the range of $5 million to $7 million each. But the P120 is three times the size of the GEM 63. So based on that we expect the price to be in the range $15 million to $21 million each.

 A third argument comes from looking at the price of the Vega-C. It's in the range of €35 million. Elon Musk has estimated the first stage of the Falcon 9 is 60% of the price of the rocket, with the upper stage, fairing, and range costs making up the rest of the cost. 

 The Vega-C is a 4-stager instead of the 2-stage Falcon 9, but the salient point still remains: the much larger size of the first stage than the other stages means it makes up the largest proportion of the cost. 

 Using the 60% Elon Musk estimate for the cost of the first stage would give a €21 million cost for the P120 first stage of the Vega-C.

 So there is abundant evidence the large side boosters used on the ArianeSpace rockets are quite expensive. But is there an alternative? Yes! The price of a single Vulcain is only €10 million. So eliminate the Ariane 6 SRB's entirely and use two to three Vulcains on the core instead. Not using the SRB's results in a greatly reduced price.

 For a two Vulcain Ariane 6 sans SRB's, use a smaller upper stage of ca. 10 ton size so it would be loftable by the two Vulcains. Then it could get ca. 12 tons to LEO.

 For a three Vulcain version without SRB's, it could get ca. 20 tons to LEO if you use a larger 40 ton to 50 ton upper stage made possible by the higher take-off thrust of the three Vulcains

 And for the replacement of the Vega-C? Use an approx. half-size Ariane 6 core and again a small ca. 10 ton upper stage so as to be loftable on a single Vulcain. This could get ca. 5 tons to LEO. This compared to the 2 tons LEO payload of the Vega-C. And it would be much cheaper than the Vega-C in not using the large SRB's.

 These LEO payload numbers can be easily confirmed by a rocket equation calculation.

 See discussion here:

Towards return of Europe to dominance of the launch market.

 In looking at how it is determined the path ESA will take in selecting it’s future launchers, what missing is an independent review authority tasked with reviewing the finances and architectures chosen.

 This is what is done with NASA. NASA has an Office of Inspector General independent of the NASA leadership tasked with reviewing the finances and architectures for the space programs NASA selects. It serves as an independent oversight agency:

NASA should consider commercial alternatives to SLS, inspector general says
"NASA’s aspirational goal to achieve a cost savings of 50 percent is highly unrealistic."
ERIC BERGER -  10/13/2023, 3:07 PM

 Note the report breaks down the costs of the different components of the Artemis program. This is a necessity for analysing the cost effectiveness of the different parts of the program.

 But such is lacking at ESA. For instance in that article “European Space Agency mulls extra Ariane 6 cash”, it is ESA that is effectively providing oversight of itself.

 With this arrangement ESA won’t question whether the architectures it chooses to begin with are the right ones or cost effective ones. Thus THE major question that must be asked remains unasked: is use of large solid side boosters cost effective? Would a cheaper architecture be obtained by using all-liquid propulsion?


  Robert Clark

Monday, October 9, 2023

Towards return of Europe to dominance of the launch market.

 Copyright 2023 Robert Clark

ESA delays Vega C return to flight to late 2024
Jeff Foust
October 2, 2023

 Large solids like on the Vega and as used on the Ariane 5 and 6 are not price competitive. Note this is true for large solids. Small solid side boosters like used on the Atlas V and Delta IV might be only 1/8th the size of the core stage, with a concomitant small increase in cost. But when the solids are large size such as being as much or more than the size of the core such as on the Ariane 5 and 6 or actually being the core like on the Vega, the bulk of the high expense of the rocket comes from the solids.

See discussion here:

Friday, May 19, 2023
Who in European space will ask the impertinent question: How much would it cost to add a second Vulcain to the Ariane 5/6?

 The cost of the two SRB’s on the Ariane 62 cost €40 million out of the €75 million cost. So the rest of the two-stage rocket is only €35 million. Then those two large SRB’s cost more than the entire rest of the rocket.

 As I argued there it would be cheaper just to put additional Vulcain(s) on the core and dispense with theSRB’s entirely. An additional Vulcain would add €10 million to the price to bring it to €45 million.

 Using all liquid propulsion also results in a cheaper rocket than the Vega. To see what such an all-liquid replacement for the Vega would look like see discussion here:

Saturday, November 29, 2014
A half-size Ariane for manned spaceflight.

 By cutting down the core’s propellant size to a bit less than half and using a smaller ca. 10 ton upper stage, so it could be launched by a single Vulcain, you get an all-liquid two-stage rocket capable of about 5,000 kg to LEO. This compares to the 2,000 kg payload to LEO of the Vega.

 Quite important is the better cost per kilo for the all-liquid case. The Vega costs about €35 million for that 2,000 kg to LEO. But taking into account our all-liquid replacement to the Vega is half-size to the all-liquid Ariane 6, the cost conceivably could be in the range of only half the €45 million estimate of the all-liquid Ariane 6, so only ca. €22 million for a 5,000 kg to LEO launcher(!)

 And what about reusability? The Space Shuttle abundantly showed you don’t save on reusing solids. But SpaceX has abundantly showed you do save significantly on reusing a liquid-fueled booster. SpaceX reduces the price on the Falcon 9 from $60 million to $40 million, by reusing the booster only, so a price reduction of about one-third. If the same price reduction would apply for reusing the booster only for our half-sized Ariane, that would be a price of only €15 million for a 5,000 kg launcher(!)

 Europe could then dominate the market by offering rockets of differing sizes. For small payloads at 5,000 kg or less, a reused half-sized Ariane at €15 million. For larger payloads at ca. 10 tons, €30 million for the reused two-Vulcan Ariane. And for payloads in the Falcon 9 range of 20 tons, €36 million for the reused three-Vulcain Ariane 6.

  Bob Clark


Thursday, September 14, 2023

Could ‘Oumuamua be a spent rocket stage?

Copyright 2023 Robert Clark

 I was interested to read of a supposed asteroid passing near Earth turning out to be a Centaur upper stage launched in 1966. Because of the oddities of 'Oumuamua I wondered if the same could be true of 'Oumuamua as well.

 This occurred to me in reading Avi Loeb discussing 'Oumuamua in his book, ExtraterrestrialHe noted it had some small changing acceleration as if by outgassing. This would be the case if say a rocket stage developed a small hole and was outgassing the residual propellant left in the rocket after burnout even years later.

 Because the Centaur rocket stage is bright white I wondered if such a stage could be visible in asteroid surveys at such a distance as 'Oumuamua was seen, about 25 million km at closest approach to Earth. The Centaur is only 3 meters wide by 12 meters long. 'Oumuamua's cylindrical shape is also notable. Also notable is Spitzer space telescope could not detect it in the infrared. The explanation given was a bright, i.e., reflective, object would heat up less, as it reflects much of the incident light, and so it was concluded Oumuamua was among the brightest of comets, assumed a comet because of the outgassing:

NEWS | November 14, 2018
NASA Learns More About Interstellar Visitor 'Oumuamua
Small but Reflective
The new study also suggests that 'Oumuamua may be up to 10 times more reflective than the comets that reside in our solar system — a surprising result, according to the paper's authors. Because infrared light is largely heat radiation produced by "warm" objects, it can be used to determine the temperature of a comet or asteroid; in turn, this can be used to determine the reflectivity of the object's surface — what scientists call albedo. Just as a dark T-shirt in sunlight heats up more quickly than a light one, an object with low reflectivity retains more heat than an object with high reflectivity. So a lower temperature means a higher albedo.

But this would also be the case for a rocket stage painted white. Could the Centaur stage be visible at 25 million km distance by optical telescopes in being bright white?

 But there is also the issue of the fact 'Oumuamua was on a trajectory suggesting escape velocity. In Loebs book he discusses that asteroids can be ejected from their parent systems by interactions with other planets in their systems. Then what occurred to me is whether a spent rocket stage could reach solar system escape velocity by the slingshot effect used on some of our spacecraft to get additional speed.

 The possibility I was thinking about was the New Horizons mission. The spacecraft was given high initial speed by the stages sending it to the outer solar system but then additionally to that it was first directed towards Jupiter for a Jupiter slingshot effect.

 The NH spacecraft was given escape velocity at Jupiter after the gravity assist though actually aimed at Pluto.

 But suppose the Centaur, which was the upper stage to the Atlas V launcher, was also aimed at Jupiter in order to get NH there. There was an additional solid 3rd stage carrying the NH that got a slingshot effect from Jupiter. But it may have been the Centaur was aimed at Jupiter as well.

 Now suppose the Centaur on reaching Jupiter happened to be so aimed, unintentionally perhaps, that Jupiter's orbital motion around the Sun plus the slingshot effect would be sufficient to give the Centaur solar system escape velocity. 

 But the aiming, whether unintentional or not, would have to be that rather than heading directly out of the solar system it would be headed towards the inner solar system.

 But that’s where idea encounters problems. On calculating ‘Oumuamua’s trajectory backwards from its sighting near Earth, it appears to have had an incoming trajectory nearly perpendicular to the plane of the Solar System, at 123 degrees, as described by Adam Hibberd

 It doesn’t seem possible that the Centaur after it’s gravity assist at Jupiter would come into the inner solar system at such a high angle. 

 Another problem is the closeness of the flyby’s of Mercury, Venus, and Earth by ‘Oumuamua. 

 See discussion here:

Further strangeness of 'Oumuamua.

 It doesn’t seem reasonable that a rocket  stage after doing a close flyby of Jupiter would subsequently also do ones of Mercury, Venus, and Earth. 

 So here’s another possibility. The Messenger spacecraft did multiple flybys of Mercury, Venus, and Earth on its trip to Mercury.

 Then the upper stage that delivered Messenger on its trajectory to escape Earth may also have followed Messenger in its encounters with the inner solar system planets. The rocket that launched Messenger was the Delta II. 

 The upper stage had white fairings around it. But it seems likely these fairings are jettisoned like are the fairings making up the nose cone around the spacecraft. Then we’ll look at the second stage under the fairing.

 It was the Delta K stage. It was smaller than the Centaur stage at 2.4 meters wide and 6 meters long. There is also the issue of it, likely, not having bright white sides like the Centaur. So the question of whether it would be visible at the 25 million km closest approach of ‘Oumuamua becomes even more relevant. 

The Color of ‘Oumuamua.

 The color of ‘Oumuamua has been described as red. This was taken as supportive of it being like the outer solar system objects that are also red. The inference was of it either acquiring a similar coating from long periods in the outer solar system or of it being a similar object in the outer stellar system of its origin.

 On the issue of ‘Oumuamua being uniformly red however there is some debate. At least one observational report describes it as “neutral” colored:

Col-OSSOS: Colors of the Interstellar Planetesimal 1I/'Oumuamua

Michele T. Bannister, et al.

The Astrophysical Journal LettersVolume 851Number 2

Focus on the First Interstellar Small Body `Oumuamua

The grJ colors of 1I/'Oumuamua are at the neutral end of the solar system populations (Figure 3). About 15% of the TNOs have colors consistent with 1I/'Oumuamua, all in dynamically excited populations. 1I/'Oumuamua's color is also consistent with that of the less-red Jupiter Trojans, which are P type (Emery et al. 2011), and with Bus & Binzel (2002a) and DeMeo et al. (2009) X type in the asteroids, which encompasses the Tholen (1984) E, M, and P classifications. As its albedo is unknown, we do not describe 1I/'Oumuamua as consistent with the Tholen (1984) P type.

Notably, 1I/'Oumuamua does not share the distinctly redder colors of the cold classical TNOs (Tegler et al. 2003; Pike et al. 2017), which may be on primordial orbits. Nor is its color among the red or "ultra-red" colors of the larger TNOs on orbits that cross or are well exterior to the heliopause (Sheppard 2010; Trujillo & Sheppard 2014; Bannister et al. 2017). The cause of ultra-red coloration of these TNOs is unknown, but has been attributed to long-term cosmic-ray alteration of organic-rich surfaces (Jewitt 2002), such as would be expected during the long duration of interstellar travel.

 Still the detected red coloration of some observations could be due to the red engine nozzle of the Delta K stage.

 Escape velocity by gravity assists from inner planets? 

 The high velocity of ‘Oumuamua, well above Solar System escape velocity, has been much remarked upon, and it is the reason why it was taken to be an interstellar object. However, the New Horizons spacecraft was able to achieve escape velocity from a gravity assist, a “gravitational slingshot”, from Jupiter. And actually a surprising fact is that a close flyby of any of the planets can result in escape velocity being achieved by a spacecraft.

 See the discussion here:

Gravitational Slingshot

An extreme form of the maneuver would be to approach a planet head-on at a speed v while the planet is moving directly toward us at a speed U (both speeds defined relative to the "fixed" Solar frame). If we aim just right we can loop around behind the planet in an extremely eccentric hyperbolic orbit, making a virtual 180-degree turn, as illustrated below.



The net effect is almost as if we "bounced" off the front of the planet. From the planet's perspective we approached at the speed U+v, and therefore we will also recede at the speed U+v relative to the planet, but the planet is still moving at (virtually) the speed U, so we will be moving at speed 2U+v. This is just like a very small billiard ball bouncing off a very large one.

 You see the maximum added velocity due to the gravity assist is about twice the speed of the planets orbital velocity around the Sun. But the escape velocity at a planets distance from the Sun is square-root of 2 times its orbital velocity. So a spacecraft encountering the planet at sufficiently close distance and at the right angle could achieve escape velocity from a gravity assist.

 Then the Messenger launchers upper stage following Messenger’s trajectory, during one of the Messenger flyby’s, or even one years later, could due to small variations in the flight path be sent on an even closer encounter to the planet than Messenger and thus achieve escape velocity. 

 But if this is true how is it the simulations projecting backwards ‘Oumuamua’s trajectory show the closest approach to the inner planets millions of kilometers away not the hundreds to thousands of kilometers away needed for a gravitational slingshot? 
 Note that when you project back a trajectory you assume the object has not at some point flipped the direction it is traveling. But in the most extreme case that is exactly what can happen with a gravitational slingshot, as shown in the diagram above.

 This can be taken account in such trajectory simulations if you know a gravity assist did take place. That is to say, an alteration in the flight path in such a case can be taken into account, even if not necessarily to the most extreme case of reversing direction. 

 But there is a scenario where you did not know it. You see an object have a certain speed and a certain direction at a certain time, say, when it passes closest to Earth. You project backwards and note the distance at closest distance to, say, Venus is millions of kilometers away from that planet

 But IF that object at some earlier time than the time of the supposed Venus closest approach the object had a higher speed then the object could still have made a gravitational slingshot flyby of the planet with Venus being at a different location.

 Then to see if this did happen in the ‘Oumuamua case you would have to project back further then the supposed closest approaches of Venus and Mercury to see if at earlier times gravitational slingshots by these planets could have resulted in it appearing at the same location, speed, and direction as observed when it made its closest approach to Earth. The calculations are made more complicated though by the possibility of it having multiple flybys.

Detections by radio telescopes?

 The possibility that spent rocket stages can be mistaken for asteroids suggest additional observational techniques should be made when they are discovered. ‘Oumuamua was scanned to see if it was emanating radio transmissions, and none were found. But it still might have been detectable by radio telescopes or by radar. It might have been determined for example if it were more reflective than usual suggesting it was made of metal.

  Robert Clark

Could asteroidal impacts be the cause of the coronal heating problem?

 Copyright 2024 Robert Clark   A puzzle in solar science that has existed for 150 years is the corona heating problem: Why is the sun’s coro...