Monday, September 2, 2019

The prospect of SSTO once more rears its ugly head.

Copyright 2019 Robert Clark
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:

Bob Clark


  1. Myself, I think tweets have less relevance to truth and fact than actions. We'll see, soon enough. GW

  2. I'm probably overconservative with gravity and drag losses for ascent (5% each), and otherwise just conservative regarding sea-level Raptor performance flying into vacuum (340 sec avg), and the safety factor required upon the landing burn (1.5, applied to slowing from Mach 1). I did NOT include any deorbit burn, to compensate for that overconservatism.

    What I got for the total deliverable dV out of just the "Starship" without its booster at ZERO payload was 8.79 km/s. That's for 85 metric tons inert (which I tend to disbelieve), and 1100 tons loadable propellant, and an average Isp = 340 sec.

    The factored required mass ratio-effective dV is the sum of 1.10 *7.8 to reach orbit, factor 1 x zero for the deorbit burn, and factor 1.5 x 0.33 km/s (Mach 1-ish) for the landing. That totals some 9.08 km/s required to do the SSTO mission single stage at zero payload.

    As I said, I'm likely too conservative. This thing might just well actually reach LEO at zero payload. But what is the point of an SSTO that cannot carry payload?

    As I said, by the time they make the heat shield and the landing pad areas work right, I really, really doubt the vehicle inert weight will only be 85 metric tons. That's the REAL kicker here.


    1. Thanks for that. Keep in mind though there are two versions of the upper stage, the passenger and tanker versions. The passenger version now called the Starship with have the passenger quarters and provisions for 100 colonists on a 6 month flight to Mars. The tanker version in contrast will just have a big empty fairing.

      The tanker will therefore have a much lighter dry mass. It is this version we are considering for the SSTO use. As a point of comparison, the earlier Interplanetary Transport System (ITS) had a passenger upper stage dry mass of 160 tons, while the tanker version's dry mass was 90 tons:

      Assuming a similar savings in dry mass for the current BFR, the dry mass for the tanker version might be ca. 45 to 50 tons.

      Bob Clark

  3. I did some reverse-engineering of the Raptor engine, and raised my average Isp a bit. "Starship" alone (no booster) now makes orbit unladen, in spite of fully-conservative assumptions. Takeoff thrust/weight is very low, sort of similar to Saturn-5. -- GW


    Test version weighs near 200t dry, final maybe 120t.

    1. That increase of 120 tons from the 85 ton SpaceX first estimate is puzzling. Having weight growth for rockets is common of course, but increasing nearly 50% is quite unusual.

      The fins/mini-wings are larger in the latest version but even full-size wings typically make up only 5% to 10% of an aircraft weight.

      Elon has spoken often of trying to save weight on the BFR design, such as for example the switch to stainless steel. But it must be said SpaceX has not succeeded at that goal.

      Bob Clark

  5. Given the wildly varying numbers for inert mass, I think we can conclude this "Starship" design is very far from complete. That is usually the case in development testing. The "x-plane" prototypes bear little resemblance to the final product. -- GW