Could the SuperHeavy booster be SSTO?

[sevenperforce]

On 11/6/2022 at 11:40 PM, Exoscientist said:

I’m suggesting they should do more than just fold up against the sides to varying degrees. They should also rotate forward and backwards. This would provide better control of the pitching moment.

How?

All-moving control surfaces on aircraft are lift surfaces. By moving in their rotational axis, their angle of attack changes, which changes the amount of lift they generate, producing forces about the center of mass to induce pitch, yaw, or roll.

The flaps on Starship are not lift surfaces. They are drag surfaces; essentially nothing more than airbrakes. They do not produce lift (other than body lift, which is irrelevant here), and so changing their angle of attack doesn't do anything.

The use of airbrakes to control aircraft is not new; flying wings use it for yaw control. The B-2 bomber has split ailerons; if it needs to yaw to the left, it will deploy the ailerons on its left wing in opposite directions, which increases drag on the left wing, which pulls the aircraft to the left. But in that situation, adding another axis of rotation wouldn't do anything, because the ailerons are not producing lift in the yaw plane, only in the pitch/roll planes.

What you are suggesting is that changing the angle of attack on the Starship flaps would alter their control authority. It will not, because changing the angle of attack of a control surface does not produce new control forces unless that control surface was already providing aerodynamic lift.

Here is a CFD simulation showing that the flaps are not producing any lift:

On 11/8/2022 at 8:14 AM, Exoscientist said:

On 11/7/2022 at 4:26 AM, RCgothic said:

The 30t of propellant is needed for landing. Putting it in the nose for stability is a two birds one stone situation. Getting rid of the landing propellant results in a crater regardless of the aerodynamic stability.

 If you do the calculation the amount of propellant used for landing is far less than 30 tons. 

 The 30 tonnes of propellant is used not only for landing, but for the deorbit burn as well. A deorbit burn requires around 100 m/s of Δv.

Starship will need roughly 7 tonnes of propellant residuals. So only 23 tonnes can actually be used.

An empty Starship has a mass of just over 100 tonnes, approximately, plus the 7 tonnes of residuals. For the tanker variant, 27 tonnes of propellant gives around 630 m/s of Δv, which is certainly more than needed. But Starship needs to be able to return downmass as well. With a notional 50-tonne return payload, that's less than 450m/s. So after the deorbit burn, that's less than 350 m/s to execute the flip and come down to a soft landing while also fighting gravity drag. As you yourself pointed out, the lowest you could expect for that whole process is 250 m/s.

Does that settle the controversy?

[tater]

52 minutes ago, sevenperforce said:

An empty Starship has a mass of just over 100 tonnes, approximately, plus the 7 tonnes of residuals. For the tanker variant, 27 tonnes of propellant gives around 630 m/s of Δv, which is certainly more than needed. But Starship needs to be able to return downmass as well. With a notional 50-tonne return payload, that's less than 450m/s. So after the deorbit burn, that's less than 350 m/s to execute the flip and come down to a soft landing while also fighting gravity drag. As you yourself pointed out, the lowest you could expect for that whole process is 250 m/s.

There's also Mars to consider. The header tanks need to be able to accomplish this task as well, even if not an immediate use case.

The SpaceX landing simulations from the various update presentations have suggested something on the order of 600 m/s for the propulsive phase—mach ~2.5 on Mars at ~2km altitude works out to a little over 600 m/s.

[Exoscientist]

On 11/13/2022 at 2:24 PM, sevenperforce said:

How?

All-moving control surfaces on aircraft are lift surfaces. By moving in their rotational axis, their angle of attack changes, which changes the amount of lift they generate, producing forces about the center of mass to induce pitch, yaw, or roll.

The flaps on Starship are not lift surfaces. They are drag surfaces; essentially nothing more than airbrakes. They do not produce lift (other than body lift, which is irrelevant here), and so changing their angle of attack doesn't do anything.

The use of airbrakes to control aircraft is not new; flying wings use it for yaw control. The B-2 bomber has split ailerons; if it needs to yaw to the left, it will deploy the ailerons on its left wing in opposite directions, which increases drag on the left wing, which pulls the aircraft to the left. But in that situation, adding another axis of rotation wouldn't do anything, because the ailerons are not producing lift in the yaw plane, only in the pitch/roll planes.

What you are suggesting is that changing the angle of attack on the Starship flaps would alter their control authority. It will not, because changing the angle of attack of a control surface does not produce new control forces unless that control surface was already providing aerodynamic lift.

Here is a CFD simulation showing that the flaps are not producing any lift:

 The 30 tonnes of propellant is used not only for landing, but for the deorbit burn as well. A deorbit burn requires around 100 m/s of Δv.

Starship will need roughly 7 tonnes of propellant residuals. So only 23 tonnes can actually be used.

An empty Starship has a mass of just over 100 tonnes, approximately, plus the 7 tonnes of residuals. For the tanker variant, 27 tonnes of propellant gives around 630 m/s of Δv, which is certainly more than needed. But Starship needs to be able to return downmass as well. With a notional 50-tonne return payload, that's less than 450m/s. So after the deorbit burn, that's less than 350 m/s to execute the flip and come down to a soft landing while also fighting gravity drag. As you yourself pointed out, the lowest you could expect for that whole process is 250 m/s.

Does that settle the controversy?

  Your mentioning of air brakes brings up another advantage of the rotatable rear flaps: in the fully rotated position they would create a great deal of drag to further slow down the descent. See the image displayed on the video start up screen here:

That would be like a parachute in regards to slowing the spacecraft down.

 It might be SpaceX wants to give the Starship the ability to also land payload, but that amount of reserve propellant shouldn’t be used for all launches when it is not needed. That is unnecessarily subtracting from its normal payload.

 By the way, the “dry mass” Elon has been quoting for the reusable Starship also seems excessive. The latest is 120 tons(!)  That’s nearly 3 times the bare dry mass of the expendable version, i.e., no reusability systems, of only 45 tons.

  That seems an excessive weight added for reusability.

     Bob Clark

Edited November 17, 2022 by Exoscientist
Typo.

[sevenperforce]

1 hour ago, Exoscientist said:

  Your mentioning of air brakes brings up another advantage of the rotatable rear flaps. . . .

There was no advantage to rotatable rear flaps on Starship in the first place, so the word "another" does not seem appropriate here.

1 hour ago, Exoscientist said:

. . . in the fully rotated position they would create a great deal of drag to further slow down the descent. See the image displayed on the video start up screen here:

In which part of the descent envelope? The belly-flop, or the landing burn?

During the belly-flop, the rear flaps are at maximum drag when they are fully deployed. Rotating them around the transverse axis of the vehicle would reduce their exposure to the airstream and thus reduce drag.

During the landing burn, the rear flaps need to be as the lowest-drag position to allow the vehicle to pitch up and achieve a vertical landing.

1 hour ago, Exoscientist said:

That would be like a parachute in regards to slowing the spacecraft down.

It would be like a drogue parachute in regards to ensuring that the vehicle slams straight into the ground nose-first like a lawn dart.

1 hour ago, Exoscientist said:

 It might be SpaceX wants to give the Starship the ability to also land payload, but that amount of reserve propellant shouldn’t be used for all launches when it is not needed.

I'm sure it will not be.

1 hour ago, Exoscientist said:

 By the way, the “dry mass” Elon has been quoting for the reusable Starship also seems excessive. The latest is 120 tons(!)  That’s nearly 3 times the bare dry mass of the expendable version, i.e., no reusability systems, of only 45 tons.

Yes, because reusability is hard.

[sevenperforce]

5 hours ago, sevenperforce said:

6 hours ago, Exoscientist said:

 By the way, the “dry mass” Elon has been quoting for the reusable Starship also seems excessive. The latest is 120 tons(!)  That’s nearly 3 times the bare dry mass of the expendable version, i.e., no reusability systems, of only 45 tons.

Yes, because reusability is hard.

Just emphasizing something here.

The “bare dry mass” of the Shuttle system — the external tank and the engines — had a combined mass of ~36 tonnes.

The orbiter, engines, and tank combined had an empty mass of 114 tonnes.

So “reusability systems” for the Shuttle drove up weight to well over 3X the “bare dry mass” of the vehicle. Without even recovering the main tank.

[Terwin]

19 hours ago, Exoscientist said:

  Your mentioning of air brakes brings up another advantage of the rotatable rear flaps: in the fully rotated position they would create a great deal of drag to further slow down the descent. See the image displayed on the video start up screen here:

That would be like a parachute in regards to slowing the spacecraft down.

You seem to be suffering from the misapprehension that we want drag when SS is traveling nose-first.  We do not.

We want drag when SS is falling sideways.  In this case, the SS flaps are ALREADY the equivalent of a fully rotated flap on the F22, because the only thing we want from those flaps is drag, so they do not have a 'non-rotated' position, just 'deployed' and 'not-deployed'.

Sort of like the stock air-brakes in KSP, just with better control so that they can fine-tune the drag if needed. 

Edited November 18, 2022 by Terwin

[tater]

120t is ~2.4% of the total mass of SS/SH on the pad. SH is maybe between that and 4%, so 5-6% of total mass with both stages.

Phil Bono's not-really-an-SSTO, ROMBUS (the concept had drop tanks) has a vehicle dry mass of >5% to total mass on the pad.

The huge difference is that SpaceX actually knows how to build reusable space vehicles, and the vehicle in question is literally sitting on the pad.