For this construct to lift,
MgT−(M+m)g>0
We assume a linear temperature profile,
T=Tmax(1−xd)
and a log squared space density profile,
ds(T)=(dTmax−dn)ln(1T2max)ln(1T2(x))+dn
ds(T)=(dTmax−dn)ln(1T2max)ln(1T2max(1−xd)2)+dn
ds(T)=(dTmax−dn)ln(T2max)ln(T2max(1−xd)2)+dn
ds(T)=(dTmax−dn)ln(T2max)(ln(T2max)+ln((1−xd)2))+dn}
d(ds(T))dx=(dTmax−dn)ln(T2max)(−1(1−xd)22d(1−xd))
d(ds(T))dx=(dTmax−dn)ln(T2max)(−2(d−x))
And we have,
gT=GTxo(dn−dTmax)(dTmax−dn)ln(T2max)(−2(d−x))
gT=2GTxoln(T2max)1(d−x)
M2GTxoln(T2max)1d−(M+m)g>0
From this equation we see that small xo (radius of body) and small d provide for greater lift. We shall now optimize Tmax for greater lift.
It seem odd that gT should be inversely proportional to Tmax. In free space where Tmax is allowed to fall over a distance x=L to reach T=0 increasing Tmax increases L but ds cannot fall below zero. Δds=dn remains fixed, as such,
d(ds(x))dx=dnL when we use a linear approximation, decreases with increasing Tmax. And so, gT decreases.
We can fix L by placing a cold body at distance d but still Δds=dn remains the same.
So in practice Tmax should be increased until gT and so lift is at a maximum and then no further. At this point Δds=dn and cannot be increased further.