Friday, September 30, 2022

The nature of the true dry mass of the Starship.

 Copyright 2022 Robert Clark


 The earlier version of the SpaceX Mars ship known as the Interplanetary Transport System(ITS) showed high mass ratios both for the first and second stages:


 
For the first stage and upper stage tanker version, the mass ratios were above 25 to 1. In contrast the upper stage passenger version had mass ratio of only about 14 to 1. This is understandable since the passenger version was to contain the food, supplies and accoutrements for ~100 colonists on a 6 month flight to Mars.

 The latest version is the Superheavy/Starship combination but has a much poorer mass ratio for the first stage and tanker upper stage, which I'll refer to as the tanker Starship. It has been reported that the tanker Starship might have a dry mass of 85 tons with the propellant mass of 1,200 tons. However, it may actually be that this comes from an attempt to find a lower mass of the tanker Starship where the passenger Starship has been cited as having a dry mass of 100 to 120 tons. I have not been able to find any cited link from SpaceX or Elon Musk that would give the dry mass of the tanker Starship.

 The closest Elon came to it was in his article “Making Life Multi-Planetary”:

BFR ship overview

The ship is 48 meters in length. Dry mass is expected to be about 85 tons. Technically, our design says 75 tons but inevitably there will be mass growth. The ship will contain 1,100 tons of propellant with an ascent design of 150 tons and return mass of 50 tons. You can think of this as essentially combining the upper stage of the rocket with Dragon—it is as if the Falcon 9 upper stage and Dragon were combined.

 
 He talks about both the passenger and tanker versions of the Starship in this article. It should be noted though in this passage he mentions it’s analogous to having the Dragon capsule combined with the Falcon 9 upper stage which suggests he’s talking here about the passenger version. It is also notable that the Starship is approximately half-size to the passenger upper stage of the prior ITS and 75 tons dry mass would indeed be half-size to the prior dry mass of the ITS passenger upper stage.

 So Elon hasn't said of the current version of the Mars ship what the tanker dry mass would be. It is quite notable that he did make the distinction in dry mass between the tanker and passenger versions for the prior incarnation the ITS, and the proportional size of the tanker version dry mass as shown above was much less than the passenger version.

 An indication of the importance of the distinction in regards to the dry mass of the vehicle is illuminated by this illustration of how the passenger Starship might look:



 In contrast the tanker Starship would just have a big empty space there, as illustrated in this image by Tom Dixon:




 But would that large payload fairing be needed on tanker flights and cargo flights?

 Firstly, note that the Starship is an upper stage. But the weight of the payload fairing is never given as part of the upper stage's dry mass. A reason is you frequently switch out one fairing for another depending on the size of the payload. Here's another way of seeing this: Elon in that quote from the "Making Life Multi-Planetary" article said the Starship was like combining the Falcon 9 upper stage with the Dragon capsule. But the Falcon 9 has a payload capability to LEO of 22.8 tons expendable. But when the  Dragon crew capsule is attached the payload capacity is only 6 tons. Always requiring the full payload section to be attached to the Starship would be like always requiring the Dragon crew capsule to always be attached to the Falcon 9 upper stage even when doing just satellite launches, and with the resulting reduced payload capacity. Note also, if SpaceX only cited the payload capacity with the Dragon attached that would leave the mistaken impression that was the true payload capacity of the Falcon 9.

 This is especially important in regards to the tanker. SpaceX estimates the payload for the reusable Starship as ~150 tons. But the point of the matter is the density of supercooled methane/LOX is  approximately 1,000 tons per cubic meter, giving a volume of about 150 cubic meters. 

 But this only about the volume of a Falcon 9 fairing:



  But the Falcon 9 fairing only weighs in the range of only ~2 tons. We'll see however that the Starship nose cone and fairing barrel weighs in the range of ~17 tons. Then always including the full fairing in the Starship is wasting about 15 tons in payload capacity. 

Starship fairing weighs about 17 tons.

 An estimate for the mass of the payload section can be made based on its height, the density of steel, and the known wall thickness of 4mm. This was done by a member of the NASASpaceflight.com forum:

Starship specs - weight, volumes, etc
« on: 02/04/2020 12:47 am »
...
Rings
Diameter: 9m
Circumference: 28.27m
Internal area (Cross section): 63.6m²
Surface area: 51.71m²
Volume: 0.205m³
Weight: 1621kg

Bulkheads
Surface area: 104.82m²
Volume: 0.419m³
Weight: 3313kg

Fairing (curved nose section only)
Surface area: 287.13m²
Volume: 1.135m³
Weight: 8974kg
...

 There are 5 rings for the barrel section of the payload section. With each ring at 1,621 kg, and the curved nose cone at 8,974 kg the total mass of the payload section is 8,974 + 5*1,621 =  17,079 kg.

Estimate of the bare, expendable Starship mass w/o fairing.
 There are several reasons for concluding the bare Starship upper stage without reusability systems and without fairing is far less than the 85 tons frequently cited. The prior ITS tanker upper stage at a propellant mass of 2,500 tons and dry mass of 90 tons had a mass ratio of 28.77. This would correspond to a dry mass of the Starship at 1,200 ton propellant mass of 43.2 tons. Also Elon famously cited a dry mass of only 40 tons in discussing such a stage:

Probably no fairing either & just 3 Raptor Vacuum engines. Mass ratio of ~30 (1200 tons full, 40 tons empty) with Isp of 380. Then drop a few dozen modified Starlink satellites from empty engine bays with ~1600 Isp, MR 2. Spread out, see what’s there. Not impossible.
 Also a Nasaspaceflight.com video allows us to estimate the weight of the tank section:

SpaceX Boca Chica - Starship SN1 Aft/Fwd Tank Section Stacking.


 The video gives the mass of the upper portion of the Starship tank section as 46,200 pounds, 21,000 kg. From this schematic of SN1 by Rafael Adamy we see the upper tank section had 9 rings and two bulkheads, i.e., tank domes, and the lower section 5 rings and 1 bulkhead:


 So take the lower tank section as about half that of the upper section, 10,500 kg, plus half the weight of another ring, 800 kg, so 11,300 kg, giving a total mass of the tank section as 32,300 kg. Notable also though is that SpaceX believes they can shave down the the tank wall thickness from 4 mm to 3mm, cutting 25% off the tank mass, or saving in the range of ~8 tons.

 The original mass of the Raptor was 2,000kg, then adding on 3 Raptor engines would add 6,000 kg to the dry mass so to 38,300 kg, remarkably close to the 40,000 kg dry mass estimate.

 There are likely some ancillary weights not included in this such as thrust structure, avionics, wiring, etc. But not much considering that SpaceX originally had been planning on doing a high altitude test flight set for March, 2020 with the SN1 vehicle:

SpaceX filed a request to conduct a Starship SN1 test flight above Boca Chica Texas
by Evelyn Arevalo February 04, 2020

 Emphasis on the had been. The SN1 test vehicle failed during cryo pressure testing. However, reportedly this was due to welds failing in a part of the thrust structure Elon refers to the "thrust puck" at the base of the rear dome of the tank where the engines are attached:

What is a thrust puck on Starship?

Finally found a source, in this article Musk is interviews and says:

“Well, I just had a lot of talks with the team about that today,” he said of the SN1 failure. “It’s what you might call the thrust puck—there’s an inverted cone where we mount the three sea-level engines. In fact, it’s drawn on that whiteboard over there.”

The Puck looks different on the MK1 build (Starhopper, welds failed on the pad during a pressure test) than the SN1 (lower weld failed on the pad during a pressure test).

...

 (The thrust puck for SN3)
https://space.stackexchange.com/questions/41761/what-is-a-thrust-puck-on-starship


 So this suggests thicker walls for the tanks would not be required, which is important since that would suggest significantly greater weight. Just improved welding would not incur much increased weight. 


Ballast weight for Starship return.
  I had thought that was it for the weight the bare Starship had to carry to orbit. I was therefore quite surprised to read that for the reusable version the Starship has to carry quite a large amount of ballast weight in the nose for balancing during the return flight. The size of the ballast weight is 30 tons, nearly the mass of the entire stage!


 The reason SpaceX uses header tanks is to ensure a steady flow of propellant to the engines even when the vehicle is horizontal just before the flip to vertical ahead of landing. However, the size of the propellant being used for ballast is excessive.

 The aerodynamic concept at play here is that for stability reasons you want the center of gravity(CG) of a vehicle to be ahead of the center of pressure(CP). This page explains it very well:

Stability and Control

The motion of an aircraft takes place in three dimensions. To a reasonable approximation it can be treated as a rigid body. In addition to the three translation components of the center of mass the plane may rotate about its center of mass. The diagram shows the names for the component rotations: roll about the longitudinal axis, yaw about the vertical axis, and pitch about the horizontal axis. For stable controlled flight the pilot must be able to control all of these motions.

The resultant of the aerodynamic forces acts on a different point, the center of pressure. The design of the aircraft will determine the relative location of the center of pressure and the center of gravity. The lower diagram shows two cases that yield different stability behavior. In (a) the center of gravity is in front of the center of pressure and small yaw or pitch motions produce an aerodynamic force that tends to restore the system to its initial orientation - an aerodynamically stable situation. When the center of pressure is in front of the center of mass, the aerodynamic forces due to small yaw or pitch deflections increase these deflections and the system is aerodynamically unstable.

 However, most military aircraft put the the CP ahead of the CG. The reason is the usual CG ahead of CP position makes the craft too stable. Military craft need to make quick changes of attitude, and the overly stable attitude would limit that. In their case, their computerized control surfaces are constantly making changes to enable stability in level flight.

 Then my suggestion of SpaceX is to take input from military aircraft designers on how deal with a CP ahead of CG scenario for their vehicle. Another possibility is to add additional fins at the end of the craft to bring the CP rearward. This would add weight but at a fraction of the weight for ballast now used.

Starship for human lunar lander.
 The estimate of an approximate 100 to 120 ton dry mass for the Starship led to some estimates of ~16 launches being required to use the Starship as a lunar lander. Elon, stung by these criticism, argued a human lunar lander would weigh less without reusability systems:


Replying to
16 flights is extremely unlikely. Starship payload to orbit is ~150 tons , so max of 8 to fill 1200 ton tanks of lunar Starship. Without flaps & heat shield, Starship is much lighter. Lunar landing legs don’t add much (1/6 gravity). May only need 1/2 full, ie 4 tanker flights.

 But in the discussions on the Starship used as the lander, it still does have the nosecone and payload section still attached, as in these illustrations:


Image credit Tony Bela.

 Frankly, it's a little surprising in their proposed plan for using the Starship as a lander, that NASA allowed SpaceX to keep the payload section attached. In a Moon mission shaving off every single unnecessary kilogram from the lander is paramount. Then it is surprising they allowed the unneeded payload section to remain attached at a 17 ton weight penalty. 

 The actual lander in the SpaceX plan sans the lunar crew module should look like the second figure here:

(Image adapted from Tom Dixon art.)

  Note also the 17 tons is only for the payload section. It does not even include weight for life support, astronauts suits, food, water, etc. 

 You wouldn't want to use the Orion capsule itself for this purpose due to its high mass, about 10 tons for the capsule itself without the service module. And the current plan is to keep the Orion in orbit.

 So you do still need a lightweight crew lunar module for the lander. You could use the Dragon 2 at 7,700 dry weight. The addition needed reportedly would just be a more robust communication system from the Moon.
  
 For this purpose, the Dragon 2 would have reduced dry weight. The thermal protection, reaction control system(RCS), Superdraco abort thrusters and propellant tanks, parachutes and associated recovery equipment would be removed.

 A more lightweight lunar crew module than that though probably can be derived from the Orbital Sciences Cygnus unmanned ISS cargo supply capsule. Orbital Sciences is considering given it life support for crewed capability. Based on the Cygnus small size likely a lunar crew module could be derived from it at about the same weight as the Apollo lunar crew module dry weight, about ~2 tons. 

 The Cygnus is actually built in Italy. Then this would be another way the European partners in the Artemis program could contribute to Artemis by giving the Cygnus life support systems sufficient for a lunar mission.

Importance for accurate financial estimates of the lunar missions.

 The issue of the true dry mass is a serious one. For instance SpaceX in its lunar lander proposal said it could take as much as 16 launches including refuelings:

SpaceX CEO Elon Musk details orbital refueling plans for Starship Moon lander. By Eric Ralph Posted on August 12, 2021
First, SpaceX will launch a custom variant of Starship that was redacted in the GAO decision document but confirmed by NASA to be a propellant storage (or depot) ship last year. Second, after the depot Starship is in a stable orbit, SpaceX’s NASA HLS proposal reportedly states that the company would begin a series of 14 tanker launches spread over almost six months – each of which would dock with the depot and gradually fill its tanks.

In response to GAO revealing that SpaceX proposed as many as 16 launches – including 14 refuelings – spaced ~12 days apart for every Starship Moon lander mission, Musk says that a need for “16 flights is extremely unlikely.” Instead, assuming each Starship tanker is able to deliver a full 150 tons of payload (propellant) into orbit after a few years of design maturation, Musk believes that it’s unlikely to take more than eight tanker launches to refuel the depot ship – or a total of ten launches including the depot and lander.
https://www.teslarati.com/spacex-elon-musk-starship-orbital-refueling-details/

 The problem is in their proposal to NASA granting them ~$3 billion for development and two missions, we don’t know if SpaceX was allowing for as many as 16 launches per mission. It seems likely it was since that was the original proposal. Consider again though that tweet where Elon said it might be only 4 launches with a stripped down Starship, with no reusability systems, since the lander was not planned to return:


Replying to
16 flights is extremely unlikely. Starship payload to orbit is ~150 tons , so max of 8 to fill 1200 ton tanks of lunar Starship. Without flaps & heat shield, Starship is much lighter. Lunar landing legs don’t add much (1/6 gravity). May only need 1/2 full, ie 4 tanker flights.

 Such a stripped down Starship might mass only 40 tons dry. If that would result in a 1/4th as many launches for the lunar missions then conceivably it might be only 1/4th of the $1.5 billion cost for each of the two missions, or only $375 million per mission. That is such a radically reduced price NASA needs to know if the dry mass of the stripped down Starship would really mass only 40 tons.

 Note also for the lunar lander mission, the 30 ton ballast/header tanks for landing on Earth also would not be needed. For a program to be funded by the U.S. government SpaceX needs to open about the real dry mass of the Starship in this configuration. The difference in size of the lander from 40 tons to 120 tons would mean a difference in number of launches needed from 4 launches to 16 launches, where each of those launches would be paid for by the U.S. tax payers. 

 Here's an analogy, would it be imaginable that a company that provided the lander or stages for the Apollo lunar missions would not provide the weights for those stages?
  


   Robert Clark


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