Are aerospikes better than LV-T30/45 for rocket first stages?

[Pipcard]

I thought they would be better due to low specific impulse loss at low altitudes.

[MockKnizzle]

Ehhh, if they're on a first stage they're not burning for very long, so you're not really saving all that much fuel. They'd be better suited to an asparagus core stage where they'd be burning for the entire duration of the flight. For first stages, TWR is more important so you're likely better off with the LV-30.

[Pipcard]

Unless the TWR at full thrust is too high, resulting in exceeding terminal velocity.

[MockKnizzle]

Well yeah, but that means you've overdesigned your lifter.

[koshelenkovv]

Aerospikes are heavier and have lower thrust, but higher Isp even in vacuum.

Only way they can be more effective than LV-T30s is when they burning for long enough, like inner layers of asparagus staging or single engine of whole small interplanetary rocket.

[bsalis]

They are just better. For any stage. Not as OP as they once were.

[CaptRobau]

The lack of stacking removes a lot of the advantages.

[Nao]

Aerospikes are heavier and have lower thrust, but higher Isp even in vacuum.

Only way they can be more effective than LV-T30s is when they burning for long enough, like inner layers of asparagus staging or single engine of whole small interplanetary rocket.

Agree 100%

@ OP From TWR of 1,2+ (or was it 1,4) the LV-T30 beat aerospikes in stages shorter than 4000-5000m/s Dv in mass efficiency. (the single LV-T30 will burn more fuel but the overall launcher weight will be lower, in a span of several stages it nets a decrease in total fuel consumed compared to all aerospike launcher)

Also , is that a Kerbaloid ?

...wait, wait what if ... what if we could mod kerbals into Nendroids OMG. <Nao imagines having [rin][miku][luka] in 3man pod, x_X dies from blood loss.>

[capi3101]

@OP: Aerospikes offer higher Isp (and therefore more delta-V) than LV-30s, but less thrust (though the amount if comparable). The other nice thing about them is that the Isp doesn't change much as you go into space. That said, a lower stage engine is more about thrust than efficiency, so if you have to choose between the two, I'd go with the LV-30/45.

I mainly use aerospikes for interplanetary landers; the fact you can't stage anything under them is a big negative, but I usually circumvent that simply by launching the whole thing upside down...

[T10]

A good way to take advantage of the Aerospike on an upper stage is to use two mounted on cubic struts underneath the fuel tank. If you leave a small gap between the two engines, you can then use a probe decoupler and run more cubic struts between the attachment nodes of your upper and lower stage. Essentially, instead of attaching your stages with rocket motors, you are attaching them with a central metal pole.

[UmbralRaptor]

Aerospikes are of painfully limited use for getting off Kerbin. Initially their Isp helps more than the LV-T30's TWR, but that's gone by the time you're 3.5 up. Very late in ascents, TWR doesn't really matter, and again their Isp is an advantage. But for the several km/s in between, the LV-T30 outperforms the aerospike. (Greater TWR at the same ÃŽâ€V)

[foamyesque]

We need thrust-varying Isp.

[UmbralRaptor]

We need thrust-varying Isp.

ôð. Especially if it extends down to the bottom of Jool's atmosphere.

[Pipcard]

Well yeah, but that means you've overdesigned your lifter.

What if you're trying to launch a small interplanetary probe? I think I_sp > TWR in this case.

Edited June 19, 2013 by Pipcard

[MBobrik]

I did the maths and aerospikes save the weight(scaled by thrust ratio) difference's worth of fuel mass after cca 26 sec at sea level. so they are better over LV-30 on any stages that burn longer than 26 sec. Which most of my first/second stages are.

EDIT : When I've done the same calculation again, I came with different a number 49.46 s at sea level.

For different heights it's 74.61 s at 10000 m and 169.66 s in orbit

The eq I used is t = (Mspike*Thrlv30-Mlv30*Thrspike)/(Fflowlv30.1atm*Thrspike-Fflowspike.1atm*Thrlv30-(1-EXP(-LOG(2)/5000*Height))*((Fflowlv30.1atm-Fflowlv30.vac)*Thrspike-(Fflowspike.1atm-Fflowspike.vac)*Thrlv30))

.

note that the engines have different thrust so I scaled everything by their thrust ratio to make a comparison

Edited June 20, 2013 by MBobrik

[M5000]

A good way to take advantage of the Aerospike on an upper stage is to use two mounted on cubic struts underneath the fuel tank. If you leave a small gap between the two engines, you can then use a probe decoupler and run more cubic struts between the attachment nodes of your upper and lower stage. Essentially, instead of attaching your stages with rocket motors, you are attaching them with a central metal pole.

Allow me to illustrate the concept:

See that part on the right hand side? That stick basically went between the four rocket engines on the left hand side module, strutted around the outside, of course.

[Nao]

I did the maths and aerospikes save the weight(scaled by thrust ratio) difference's worth of fuel mass after cca 26 sec at sea level. so they are better over LV-30 on any stages that burn longer than 26 sec. Which most of my first/second stages are.

I don't know about that. Lets do Science!

1) make a mini LV-T30 engine. Same thrust as Aerospike, with adjusted mass, 1,25*(175/215) = 1,0174 [t] (instead of using mini LV-T30 engine the same calculations can be made for something like: 35LVT30 vs 43Aerospikes which give same thrust)

2) count fuel flow for both LV-T30mini and Aerospike

175/388/9,81 = 0,04597 [t/s] for aerospike

175/320/9,81 = 0,05574 [t/s] for LV-T30mini

difference: 0,00977 [t/s] adding cost of additional fuel tank weight needed 0,00977*1,125 = 0,01099

3) with the mass difference between engines being: 1,5 - 1,0174 = 0,4825 [t]

It takes 0,4825 / 0,01099 = 43,9036 seconds for the LV-T30mini to burn enough fuel to match the difference in mass between the two engines. And that does not count the fact that after 30s of ascent LV-T30 would have gained 20s of ISP. If we to factor it, the time to get equal performance can go above 70s.

Comparing actual performance is much more complicated, with many things that affect the launch profile thus changing the actual amount of Dv needed for launch as well as design specific problems.

[UmbralRaptor]

I did the maths and aerospikes save the weight(scaled by thrust ratio) difference's worth of fuel mass after cca 26 sec at sea level. so they are better over LV-30 on any stages that burn longer than 26 sec. Which most of my first/second stages are.

Every time I run the numbers on SSTO and 2STO (4500 m/s) rockets, I get somewhat larger payload fractions on the LV-T30 powered ones.