Lightweight thermal protection for reentry of upper stages.

 Copyright 2025 Robert Clark

  In the blog post “Reentry of orbital stages without thermal protection, Page 2”, http://exoscientist.blogspot.com/2025/04/reentry-of-orbital-stages-without.html , I discussed some possibilities of thermal protection for the SpaceX Starship. Chief among them was the possibility that lightweight wings added might allow the stainless-steel Starship to survive reentry without added thermal protection at all. 

Other possible methods of thermal protection discussed there were a “parashield” of Dr. David Akin and inflatable conical shield experimented for the Cygnus capsule return.

  The method used there to estimate the temperature reached was calculation of the ballistic coefficient, 

β = (mass)/(drag coefficient*area). In a report by aerospace engineer Dr. David Akin, the estimated ballistic coefficient for the max temperature reached to be 800 C, so as not to need additional thermal protection, was ca. 20 kg/sq.m. 

 However, I calculated the ballistic coefficient for the Starship to be ca. 60 kg/sq.m. Note though this was using a much lower dry mass for the Starship than now obtains. The currently estimated dry mass of the reusable Starship is in the range 160+ tons. I believe this high mass for the reusable Starship is primary reason SpaceX is having difficulty getting effective TPS for it.

 My opinion is that SpaceX should first get an expendable Starship and then proceed to reusability. This approach worked spectacularly well for the Falcon 9.
 

 In this regard it is notable Elon Musk once estimated the dry mass of the expendable Starship as only 40 tons:

Elon Musk @ElonMusk
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.
https://x.com/elonmusk/status/1111798912141017089?s=61

 Then in the following I’ll use the 40 ton value for the Starship dry mass. In this case, there might be an example that would give us a reusable thermal shield for a vehicle the size of Starship. I’m thinking of the X-33/Venturestar.

 The length in meters was 38.7m and width 39m. For the dry mass, the total gross weight was 2,186,000 lbs, propellant weight 1,929,000 lbs, and payload weight 45,000 lbs; giving a dry weight of 212,000 lbs, or 96,400 kg.

 Using a hypersonic drag coefficient of 2, and considering the triangular planform requires multiplying by 1/2 the length*width to get the area, the ballistic coefficient calculates out to be 96,400/(2*1/2*38.7*39) = 64 kg/sq.m.
 

 Remarkably close to the ballistic coefficient of the Starship at the 60,000 kg mass of the expendable’s dry mass + fairing mass.

 But the added weight of the metallic shingle TPS of the X-33/Venturestar can’t be too high to allow the ballistic coefficient to remain close to this value.
The areal density of the metallic shingle TPS was about 10 kg/sq.m:

REUSABLE METALLIC THERMAL PROTECTION SYSTEMS DEVELOPMENT
Max L. Blosser*, Carl J. Martin*, Kamran Daryabeigi*, Carl C. Poteet **
*NASA Langley Research Center, Hampton, VA, USA
** JIAFS, The George Washington University, Hampton, VA, USA
https://ntrs.nasa.gov/api/citations/200 … 095922.pdf

 The metallic tiles had better resistance to impact and rain than the ceramics at about the same weight.

Fig.3 Layered metallic sheeting separated by insulation.

Fig.21 Metallic TPS at same weight of ceramic tiles, ~10kg/sq.m.

At a 10 kg/sq.m. areal density, the added weight covering just the lower half of the Starship would be (1/2)*Pi*9*50*(10 kg/sq.m.) = 7,060 kg, proportionally small enough that the ballistic coefficient would still be ca. 60 kg/sq.m.

This would be advantageous in that you don’t need added wings and you don’t need an additional conical thermal shield.

BUT for this to work SpaceX would have to go back to the smaller, expendable mass of the Starship. SpaceX had tested the X-33 metallic shingles and concluded they were inadequate. But that was with temperatures developed with the higher 160+ ton Starship. With a lighter dry mass, much reduced temperatures result.

Thermal Protection for the Falcon 9 Upper Stage.

 

 SpaceX had originally intended to make the Falcon 9 upper stage reusable as well as the first stage but decided it was too difficult and chose to only make the first stage reusable. They also engaged in attempts to recover the separated fairing half’s, but decided not to continue implementing this. 

 However, the metallic shingles of the X-33/VentureStar may provide a method to recover the upper stage and fairing.

 This page gives the F9 upper stage as 12.6m long, 3.66m wide at a dry mass of ~4,000 kg, and the fairing as 13.1m long, 5.2m wide, at ~ 1,750 kg:

Falcon 9 FT (Falcon 9 v1.2)
https://web.archive.org/web/20230710234357/https://spaceflight101.com/spacerockets/falcon-9-ft/

 The interstage has been estimated as weighing 1,000kg. Then using again a cylinder’s hypersonic drag coefficient of 2, the ballistic coefficient calculates out to be:

(4,000 + 1,750 + 1,000)/(2*(12.6*3.66 + 13.1*5.2)) = 29.5 kg/sq.m.

 This is well less than the desired 60 kg/sq.m point for metallic shingle TPS. But we have to make sure the added weight of the TPS still allows the ballistic coefficient to stay below this point.

 The weight of this added metallic shingle TPS would be (1/2)*Pi*(12.6*3.66 + 13.1*5.2)*10 kg/sq.m. = 1,800 kg. Adding this on, the ballistic coefficient would still only be 36 kg/sq.m.

 Another possibility though arises from the low ballistic coefficient of 29.5 kg/sq.m from the bare upper stage+fairing without TPS. This is close enough to the 20 kg/sq.m ballistic coefficient point for a stainless steel spacecraft not needing TPS, that should be investigated for the F9 upper stage.

 The tankage, fairing, and interstage would have to be replaced by stainless-steel. The tanks are aluminum-lithium. The specialty high-strength stainless-steel as used on Starship saves about 1/3rd the weight off aluminum-lithium tanks. But the fairing and interstage are composite. The stainless-steel alloys are about the same weight as the carbon-composites.

 Doing some rough estimates it will be approx. at the 20 kg/sq.m point if the tanks and fairing are converted to stainless-steel but the interstage is jettisoned and just use a lightweight steel plate to block the engine from the high temperature air stream during reentry. 

 

Clamshell wings for hypersonic reentry of rocket stages. UPDATED, May 4, 2023.

 Copyright 2023 Robert Clark

(Patent pending)

 It is known that large wings can reduce the speed and therefore aerodynamic heating a stage can experience during reentry. But such wings would induce high drag on ascent in addition to their high weight.

 A proposal to solve both of these issues: wings that open up from the stage sides or from the fairing for the upper stage, clamshell wings.

An overview of research on waverider design methodology

August 2017

Acta Astronautica 140

• DOI: 
• 10.1016/j.actaastro.2017.08.027

• Project: 
• Aerodynamic design theory and methodology of hypersonic waverider vehicle

• Feng Ding
• Jun Liu
• Chi-bing Shen et al. 

   The curved shape of the wings around the cylindrical rocket when opened up would provide both high lift and drags, important for the hypersonic reentry.

 For a reusable lower stage, though it would be difficult to maintain the structural integrity of the tanks for reuse when they open up to be wings, especially for maintaining a leakproof seal for the next flight. If this is to be used for a lower stage, the clamshell wings would have to be added around the stage. This would reduce the weight efficiency of the stage. However, quite likely they would still weigh quite a bit less than the propellant that has to be kept on reserve, unused, during ascent, for use for return to launch site.

 For the SuperHeavy it’s to be 7% of the propellant mass or 250 tons being kept on reserve for return to launch site. This high amount of propellant kept on reserve is a large part of the reason why the reusable versions of the Starship/SuperHeavy, just as with the Falcon 9 lose so much on reusability, 30% for partial reusability, and 50% for full reusability. 

The clamshell wings around the tanks likely can be designed to be well less than the mass of the tanks, which for the SuperHeavy is in the range of 80 tons. Wings in general commonly weigh in the range of 5% to 10% of the aircraft weight to be carried. This would be quite heavy if they had to support the fully fueled weight of the vehicle, as is normally the case with aircraft. Note though the wings would be closed around the tanks on ascent and would only open up to support the dry mass on return. Elon has estimated the dry mass of the SuperHeavy as less than 200 tons, so only at most 10 to 20 tons would be added to the stage weight, far less than the 250 tons of propellant needing to be carried now as “deadweight” during ascent. 

 For this to work you would want the clamshell wings to have high lift and drag at hypersonic speeds. The Space Shuttle for example has been described as a flying brick at hypersonic speeds having a hypersonic L/D at about 1, though its subsonic L/D was better at about 4.5. The clamshell wings will quite likely have high hypersonic L/D because of the prior research done on caret-wing hypersonic waveriders:

 The clamshell wings would be analogous in shape to the caret-shaped waveriders able to achieve high hypersonic L/D. During the return flight, we can also imagine achieving high levels of control by varying the angle on each side of the wing.

Falcon 9 opened up fairing as clam-shell wings.
Renders Credit Caspar Stanley 








Starship fairing opened up as clam-shell wings.
Renders Credit Caspar Stanley


 Note that SpaceX is returning the fairings for reuse now:




https://twitter.com/Erdayastronaut/status/1653510335582617600?s=20
 In this case though the fairings are returned, in separate halves, with the convex outer side downwards facing the airstream. We are proposing instead having the concave inner side facing the airstream. This will provide greater L/D drag ratio and also greater drag in that at high altitude hypersonic speed the clam-shell wings will be analogous to hypersonic waverider caret-shaped wings and then at low altitude, slow speed they can act as a parachute.


 This second mode is rather analogous to the Rogallo wing concept that had been proposed for capsule return from space:
High Wing Area per Weight Gives Lower Reentry Heating.


 If you can make this extra surface be lightweight then you would get low wing loading. The importance of low wing loading for reentry for spaceplanes is discussed here:


Wings in space.

by James C. McLane III
Monday, July 11, 2011
http://www.thespacereview.com/article/1880/1

At the end of the article there is this passage:

Wing loading (the vehicle’s weight divided by its wing surface area) is a prime parameter affecting flight. The antique aluminum Douglas DC-3 airliner had a big wing with a low loading of about 25 psf (pounds per square foot of wing surface). At the other end of the spectrum, the Space Shuttle orbiter has a high wing loading of about 120 psf. This loading, combined with an inefficient delta-shaped wing, makes the orbiter glide like a brick. A little Cessna 152 private plane features a wing loading of about 11 psf and modern gliders operate down around 7 psf. A space plane with huge lifting surfaces and a very low wing loading might not require any external thermal insulation at all. Building a space plane with a wing loading of, say, 10 psf should not be an impossible proposition. Perhaps some day it will be done.

{emphasis added}

Moreover, because of their curved shape they should be even more effective at slowing down the descent during reentry at high angles of attack, like a parachute.

I estimated the wing loading using this clamshell wing idea for the new Falcon 9 FT first stage, assuming they added a proportionally small amount to the weight. I used the specifications here:

Falcon 9 FT (Falcon 9 v1.2).
http://spaceflight101.com/spacerockets/falcon-9-ft

The dimensions given there are listed as 42.6 meters long and 3.66 meters in diameter, at a dry mass of 22,200 kg.

  Regarding the stage horizontally, you would have to put the swing points along the sides, rather than at the top, so that the clamshell wing on each side could open without blocking the opening of the clamshell wing on the other side. This means the wing area would be half that of the full surface area. So the surface area is (1/2)*Pi*3.66*42.6 = 244.9 m^2, 244.9*3.28^2 = 2634.86 ft^2.

The dry mass is 22,200 kg, 22,200*2.2 = 48,840 lbs. So the wing loading is 48,840/ 2634.86 = 18.5 pounds per square foot(psf). This is not 10 psf, but it is significantly better than the shuttle, and with the reduction in descent due to the curved surface this might still be enough to require minimal thermal shielding.

Also, we might be able to get additional wing area by putting clamshell wings on the upper surface, though not the same size as the lower ones so that all can open fully. This would essentially be a hypersonic biplane. It is known biplanes increase left at subsonic speeds. Recent research shows this also happens at hypersonic speeds.



The hypersonic I Plane has a unique biplane configuration to increase its payload and reduce drag. China Science Press




 For attitude control we allow the swing points to be moved up or down.

For making upper stages reusable.

 SpaceX has wanted to make the Falcon 9 upper stage reusable but has been unable to do so. This method can allow the upper stage to be reusable as well as providing a simpler approach to recovering the fairing.

 The upper stage dry mass of the Falcon 9 is about 4 tons. Then a 5% wing mass using the clamshell approach would only be an additional 200 kg, a small reduction in the payload mass. Note also the high wing area afforded by a clamshell wing approach would reduce the reentry heating as well.

 For the fairing we could allow the fairing itself to open up to form the clamshell wings. Unlike the case for the propellant tanks, you don’t have the need for the high degree precision and accuracy for closing up the wings for reuse for just the fairing.

 Another possibility might not detach the fairing at all. The fairing would be carried to orbit along with the upper stage. It would open up like a clamshell to release the payload. Then it would remain in the open position for fly back to the launch site, serving as the wings for the upper stage as well. This has the advantage of not having to recover and reintegrate the fairing and upper stage separately, but more importantly it's a simpler task than attaching variable clamshell wings to propellant tanks without damaging the structural integrity of the tanks. At a mass of the fairing at about 2 tons for the Falcon 9, this would subtract about 300 kg from the payload instead of just 200 kg for the case where the fairing was detached and clamshell wings applied to the upper stage. Actually, it would be a little better than this since the fairing being jettisoned so high in the flight, it subtracts nearly it’s weight from the payload anyway.

 Aerobraking is the proposal to slow down at Mars aerodynamically only, not using thrusters which would require carrying extra propellant at arrival. For several years hypersonic waveriders have been proposed to accomplish it:

Hypersonic waveriders for planetary atmospheres
• December 1989
• Journal of Spacecraft and Rockets -1(4)
• DOI: 
• 10.2514/3.26259
• Project: 
• Hypersonic Waveriders
• Anderson, John D., Jr
• MARK J. LEWIS
• Ajay Kothari
• Stephen Corda
•  This would be especially important if we want to reduce the travel time to Mars to limit the health effects due to high energy cosmic rays and long exposure to zero gravity, rather than the commonly proposed 6 to 8 months.
  Interestingly the SpaceX Starship upper stage if fully refueled in orbit could achieve a 12 km/s delta-v. This would be sufficient to get a small habitat for a small exploration team to Mars in 35 days.

Elon Musk

@elonmusk

·Mar 29, 2019

Replying to

@Erdayastronaut and @DiscoverMag

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.

Robert Clark

@RGregoryClark

Replying to

@elonmusk @Erdayastronaut and @DiscoverMag

Holy Criminy! A 380s Isp and 30 mass ratio means over 12,000 m/s delta-v in a single stage. It could do flight to Mars in less than 35 days, IF you have orbital propellant depots and full aerocapture at Mars. See calculations here: http://exoscientist.blogspot.com/2015/08/propellant-depots-for-interplanetary.html…

3:12 AM · Mar 30, 2019

https://twitter.com/rgregoryclark/status/1111888828577574913?s=61&t=pHTwgnohxv2QCsZngqS2Iw 




Fully aerodynamic landing at Mars, aerobraking.

 Such high departure speeds would result in high arrival speeds at Mars as well, in the range of 20 km/s. I’m proposing clamshell wings emulating caret-shaped waveriders perhaps in biplane format, by approaching at low altitude, “skimming the tree-tops” so to speak, can accomplish aerobraking at Mars to land without propellant burn.
 Further modeling needs to be done to confirm this.
  Robert Clark