This Aug. 28th tweet from Elon Musk surprised many when it asserted a 20km altitude Starship test hop in October and an orbital flight “shortly thereafter”:
That was surprising because, it is taken, that an orbital flight will require the Super Heavy booster stage. The problem is the Super Heavy is to have 35 Raptor engines. At current production rates it’s not likely you could have 35 Raptor engines for the SH and 6 Raptors for the Starship by the end of, say, October.
The “Everyday Astronaut” who usually has good info on the progress of SpaceX suggests as of Aug. 25th, only the 7th and 8th Raptors have been produced and he estimates a production rate of one Raptor per 2 weeks:
So some began to speculate, again, that at least for the initial test flights the Starship might be flown to orbit as an SSTO. But that’s OK. SSTO is not after all a four-letter word. Elon has said the Starship technically could be SSTO but not reusably as not having enough payload for adding thermal protection systems and landing fuel.
However, it is important to keep in mind the first test flights will not have the passenger quarters for the full operational Starship so will have a quite a bit lighter dry mass. For the first test flights it will more closely resemble the tanker version of the BFR upper stage. Elon has said that a stripped down Starship with no payload fairing and only three Raptor engines will have a dry mass of only 40 tons:
Having only 3 Raptors would work for the application mentioned in that tweet as upper stages commonly have lower thrust than their gross weight, as they don’t have to lift off from ground.
The test version of the Starship to only do the 20 km test hop is supposed to only use three engines, with reduced propellant load. So we can get an idea how accurate that dry mass of only 40 tons is for a 3 engined Starship-version with no passenger quarters in this case as the first test vehicle, if Elon releases the dry mass for it, which is open to doubt.
The Raptor engine might have a mass of 1 ton based on its high estimated T/W ratio and thrust ratings. So adding 3 additional Raptors to have 6 Raptors as planned for the full Starship might only have a dry weight of 43 tons. You would have to add the weight of the fairing but since the fairing is ejected once reaching near vacuum and well before attaining orbit, this should subtract only a proportionally small amount from the payload.
But 6 Raptors at 200 ton sea level thrust each would just barely be able to lift off a 1,200 ton BFR upper stage. You might have to reduce the propellant load somewhat to get a better liftoff T/W ratio. With a vacuum Isp of 356s vacuum Isp and 334s sea level Isp you would still be able to reach orbit with significant payload as long as the propellant is reduced by a proportionally small amount. _______________________________________________________________
That’s the argument why the first test flights might be SSTO. However, it still is possible that the Super Heavy booster will be used. This may have been aspirational on his part, but Elon tweeted back in May they want to ramp up Raptor production to one every three days by the Summer:
If they have reached this production level, then over the next 60 days to the end of October you could have 20 additional Raptors produced. This still will not be enough for the full 41 engined two-staged BFR. But there has been suggestion the initial test Super Heavy might only be given 20 Raptors:
With the 8 Raptors already produced, this would be enough using 6 or 3 Raptors on the upper stage. This could not lift the full propellant loads of the Super Heavy and Starship. So also in this case you would have one or both stages with reduced propellant loads.
But this introduces additional problems for reusability in regards to this test flight because such large amounts of propellant need to be kept on reserve for returning the first stage booster to the launch site.
Then actually the SSTO test might be better because for reusability of a returning upper stage only a proportionally small amount of propellant needs to be kept on reserve to cancel out ca. Mach 0.25 = 80 m/s on landing:
I remember thinking when reading of the debate about reusable vehicles between proponents of horizontal winged and vertical propulsive landing that all this debate was about a measly 100 m/s delta-v. The reason is whether you use wings or not almost all the speed of orbital velocity is going to be killed off aerodynamically on return. For even for vertical landing, the stage entering broadside will be slowed to terminal velocity, ca. 100 m/s. This is only about 1.3% that of orbital velocity of 7,800 m/s.
This was confirmed by a graphic just released by SpaceX about the BFR’s Starship upper stage reentry:
Here is Elon also discussing how the Starship will land on Earth, including the low terminal velocity, and low final landing burn:
How SpaceX's BFR Rocket Will Land - Elon Musk Explains.
This shows for the Starship it only has to fire the engines at about Mach 0.25, 80 m/s. So it only has to kill off 80 m/s propulsively. But with the Starship just needing to kill off a 80 m/s velocity with a 3,300 m/s Raptor sea level exhaust velocity, about 330s Isp, by the rocket equation the mass ratio to do this is e[80/3300] = 1.025. Subtracting 1 from this is the ratio of the propellant required to the dry mass. The tanker version of the BFR upper stage, i.e., without the passenger quarters, will have a dry mass of ca. 50 tons (compared to the 85 tons of the Mars Colonial Starship version.) This means you only lose 1.25 tons for the landing.
But a rocket equation estimate using a 356 s vacuum Isp for the sea level raptors gives a ca. 40 ton payload for the tanker version as an SSTO.
Then the 1.25 tons lost due to propellant kept on reserve loses ca. 3% of the payload due just to the reserve propellant required. You also lose a proportion due to thermal protection system and landing legs but that’s also true for the TSTO.
However, because of the huge amount that needs to be kept on reserve for a first stage booster of a TSTO for slowing the booster down for reentry and for boost back to the launch site as well as for final landing approach, you can lose 40% of the payload for full reusability as indicated by the Falcon 9. Actually, for the BFR for full reusability Elon has said it will lose 50% payload off the expendable version.
Because of this huge loss for reusability for the TSTO, on a percentage of the rocket size basis, and therefore also on a cost basis, the SSTO is more cost effective.
The advantage of the SSTO becomes even greater when you add altitude compensation nozzles. Used on a TSTO this can improve the payload ca. 25%. But on a SSTO it can improve the payload 100% or even more.
Elon has said the current plan is for the BFR first stage, now called the Super Heavy, to have 35 engines, with 6 engines on the Starship for 41 engines total:
It may be possible to accomplish the same payload of a super-heavy lift launcher with a fewer number of engines, and significantly lower cost. But first ...
A Heavy-Lift Launcher.
A 100 ton launcher is commonly taken as a requirement for manned lunar landing mission. Running the numbers, the BFR’s upper stage in the tanker version, i.e., without the passenger quarters, being used as a first stage booster with an additional StarHopper-sized stage added could form a 100 ton launcher.
Note there is a tanker version of the BFR upper stage that will not have the passenger quarters and provisions for 100 colonists for a six month flight to Mars, but only an empty fairing. This is the version being discussed here. The Starship version, i.e., the one that does have the passenger quarters, will have significantly greater dry mass than the tanker version.
The term "Starship" is used in the title only for its current familiarity. It is actually only the tanker version of the upper stage being discussed here.
BFR tanker on left refueling the BFR Starship on the right.
The tanker version of the upper stage cuts nearly half off the dry mass of the Starship version which has the passenger quarters for 100 colonists on a six month flight to Mars. Then the tanker version would have a dry mass in the range of ca. 45 to 50 tons, at a propellant load of 1,100 tons.
The Starhopper from its size appears to be about in the 400 ton propellant load range. However, the actual Starhopper itself is not weight optimized as it is only intended to make short, low altitude hops. What is needed for the upper stage of this new launcher is a weight optimized stage intended to reach orbit in a TSTO.
Assume we can get this weight optimized Starhopper-sized stage at a ca. 25 to 1 mass ratio, similar to the BFR tanker. Then it would have a dry mass of ca. 16 tons. Then with the 356 s vacuum Isp of the tanker as first stage, and the 382 s vacuum Isp of the Starhopper-sized upper stage, it could get 107 tons to LEO:
SpaceX expects to test launch the BFR’s upper stage, likely in tanker or cargo version, i.e., without the passenger quarters, next year. Since the Starhopper is being built in parallel, SpaceX could probably have the weight-optimized Starhopper-sized additional stage ready in the same time frame. Then you could have a manned lunar mission class launcher by next year, in 2020.
SSTO launcher.
That the BFR tanker is significantly lighter in dry mass than the Starship version is important. This means the BFR tanker in expendable mode can carry significant payload as an SSTO:
The BFR’s 35 engine Super Heavy first stage will likely take longer and be more expensive to develop than the BFR upper stage. I suggest instead that SpaceX develop a triple-cored launcher using the BFR tanker stages as the cores. Judging from the expendable versions of the Falcon Heavy in comparison to the Falcon 9, this could launch about 3 times the payload of the TSTO, so to about 300 tons. This is about what the planned LEO payload of the BFR expendable is expected to be:
100mT to 125mT for true useful load to useful orbit (eg Starlink mission), including propellant reserves. 150mT for reference payload compared to other rockets. This is in fully reusable config. About double in fully expendable config, which is hopefully never.
Elon Musk has given the development cost of the Falcon heavy as $500 million. This is a little more than 50% above the $300 million development cost of the Falcon 9, while being able to launch 3 times as much. Elon on the other hand estimated the development cost of the full BFR as $5 billion. Likely, the triple core version would be significantly cheaper than this. For one thing the triple cores would only take 27 engines plus 3 for the Starhopper upper stage for 30 engines, much fewer than the 41 engines for the planned BFR. Advantage with Altitude Compensation. Another advantage of the triple cores is the increase in payload with altitude compensation. With a typical TSTO, alt.comp. might increase payload 25%. However, with a parallel staged launcher, alt.comp. typically can improve payload 40%. This improves even more so when cross-feed fueling is used in conjunction with alt.comp, typically by 100%. So this triple-cored version with the addition of alt.comp. and cross-feed could have a payload of 600 tons to LEO(!) Altitude Compensation Improves Payload for All Launchers. http://exoscientist.blogspot.com/2016/01/altitude-compensation-improves-payload.html
Bob Clark UPDATED, 8/13/2019 Another advantage of having a third stage for the BFR at a ca. 250 ton to 300 ton propellant range is that this stage could be launched fully fueled to orbit by the BFR expendable, whether it uses the current plan of a superheavy booster or the triple-cored option. Then a smaller mission size of ca. 25 colonists could be launched to Mars in a single launch, no multiple refueling flights required. Judging from the fact the Falcon 9 reusable only reduced a proportionally small amount on the price, this one expendable launch would be cheaper than using 5 to 8 refueling flights. Also based on the long lag time between flights of the Falcon Heavy it could be launched much faster than the refueling, reusable version.
Note also this small mini-Starship if you will could serve as small SSTO launcher to LEO: A Small Raptor Spaceship. https://exoscientist.blogspot.com/2017/10/a-small-raptor-spaceship.html I argue this small, reusable SSTO would go a long way toward making spaceflight routine since it would be low cost and could be purchased and operated by independent owners. It gets even better. Once orbital propellant depots are in place in LEO, then that one single SSTO once refueled in orbit can make the full round-trip flight from Earth to the Moon and back again. And if orbital propellant depots are in place at both Earth and Mars then that one single SSTO can make the full round-trip flight from Earth to Mars and back again. Then private, independent owners can make their own manned interplanetary flights. See here also for the argument a reusable SSTO can actually be more cost effective than a reusable TSTO because the two-stage loses 50% of its payload on reusability while a SSTO only loses a proportionally small amount: Case proven: SSTO's are better than two-stage launchers. https://exoscientist.blogspot.com/2019/08/case-proven-sstos-are-better-than-two.html
UPDATED, 9/2/2019 The recent successful test hop of the SpaceX Starhopper raises again the possibility of it being used or more accurately a Starhopper-sized stage being used as a lander for the Moon and Mars.
For brevity, this Starhopper-derived stage weight optimized to have a comparable mass ratio as the Starship cargo/tanker version of ca. 25:1 to 30:1, I'll refer to just as the Starhopper stage. For Moon missions, I had originally just wanted the Starship to be used as a first stage and a Starhopper stage to be used as a second stage, i.e., without a Superheavy booster, for a launcher for lunar missions. This would be 100-ton class launcher. It would be cheaper than using the Superheavy booster. In this case, though you would still need a service module propulsive stage and a lander propulsive stage, which would still needed to be designed and constructed. NASA has contracted with SpaceX and other space companies for proposals for a lunar lander: Blue Origin and SpaceX among winners of NASA technology agreements for lunar landers and launch vehicles by Jeff Foust — July 31, 2019
https://spacenews.com/blue-origin-and-spacex-among-winners-of-nasa-technology-agreements-for-lunar-landers-and-launch-vehicles/ SpaceX also is rapidly progressing with the Raptor engine so that vacuum optimized engines will be available for near term lunar and Mars missions: SpaceX’s space-optimized Starship engine could be ready sooner than later. By Eric Ralph Posted on May 23, 2019 SpaceX CEO Elon Musk says that there is now a chance that a vacuum-optimized version of the Raptor engine will be ready for near-term Starship launches, indicating that development has either been re-prioritized or is going more smoothly than expected.
https://www.teslarati.com/spacex-speeds-up-starship-vacuum-engine-development/ Then if SpaceX were to use a Starhopper stage it would already have the lander for a lunar mission. With a full BFR being ready by 2020, with a Superheavy booster or using triple Starship cores, and the Starhopper only needing to be weight optimized, SpaceX would have a full lunar landing capable rocket already in 2020. Moreover, with the Starhopper stage being able to be delivered to LEO fully-fueled by the BFR in expendable mode, the Starhopper would then have sufficient capability to carry the Dragon 2 capsule from LEO to the Moon's surface and back to Earth in a single stage. No refueling flights required. In fact, it would have the capability to carry the Orion capsule and its service module at ca. 16 tons dry mass to the Moon and back, as long as the service module was unfueled, with all propulsion at the Moon being done by the Starhopper stage. Both the Dragon and Orion capsules are rather small in diameter compared to the 9 meter diameter of the BFR though. You would need a rather large adapter for either of them. Then instead we could use a large hab for the purpose. Being able to deliver and return ca. 16 tons to the lunar surface, we could use a Transhab-sized crew/passeger module. The Transhab was designed to be a habitat for a several months long Mars mission. It was to carry a crew of 6 at a mass of 13 tons and a 340 cubic meter volume. The Transhab was inflatable but at an inflated diameter of 8.2 meters it could be launched fully inflated on the BFR.
The Transhab could be transported to the lunar surface for longer stays a la the 6 month crew rotations on the ISS. The Starhopper would have enough capability so that the same stage could also return it to Earth without needing refueling.
Additionally, the Starhopper could deliver 35 tons of cargo to the Moon in a reusable mode where it returned to Earth after dropping off the cargo, or 55 tons to the Moon in a one-way, expendable mode. This would go a long way towards constructing the Moon base.
Being able to deliver 55 tons to the lunar surface becomes quite important when you consider the size of the Starship's passenger quarters. Since the Starship with the passenger quarters masses 85 tons, but the cargo/tanker version without the passenger quarters masses at 45 to 50 tons, we can estimate the Starship passenger quarters for 100 colonists on a months long flight to Mars as 35 to 40 tons. Then this can be transported to the Moon with 100 passengers for long stays on the Moon by the Starhopper lander. In fact, it could form the basis for a lunar base, or colony.
Image of the Starship with passenger quarters. For the Starhopper version the tankage section would be 1/4th the size.
https://twitter.com/elonmusk/status/1166860032052539392?s=21
https://twitter.com/Erdayastronaut/status/1165630118603251712?s=20
https://twitter.com/rdstrick777/status/1167458646579564544?s=21
